livinlifeinBKK
5000 Club Member
This thread is for the general discussion of the Article Linckia laevigata: An Analysis of Plausible Causes of Mortality in Captivity. Please add to the discussion here.
The indisputable beauty of Linckia laevigata has led to this fragile organism becoming an icon of the global aquarium trade. However, survival rates in captivity are tragically low. Epitomizing beauty with an exotic appearance and arms as blue as the ocean itself, Linckia laevigata, (commonly referred to as the “Blue Linckia”), seduces thousands of marine aquarium hobbyists into an ill-informed purchase year after year despite their dismal survival rate in captivity. Records reveal that L. laevigata accounts for an astounding 3% of all wild- collected invertebrates for the global marine aquarium trade (Alcazar & Kochzius, 2015). Why then, if so many attempts have been made to maintain them in captivity, have there been so few success stories? Is there something drastically different about this asteroid species which has led many hobbyists to consider them impossible to maintain long term? While most hobbyists would agree that many species of starfish (and particularly certain species such as those within the Linckia genus) are prone to nutritional deficiencies when housed in captivity and ultimately succumb to starvation generally within 6-8 months, it should be acknowledged that this widespread belief is predominantly based upon assumptions drawn from personal anecdotes and not necessarily supported by documented research. Furthermore, this fails to offer an explanation as to why many starfish of this particular species tend to simply “disintegrate” sometimes within only a week after introduction into the aquarium. Many explanations have been proposed and widely circulated ranging from the outrageous to highly plausible. However, often even these plausible explanations fail to take into account the unique and complex anatomy of asteroid species which is required to logically unravel the cause(s) of mortality or ill health. Thankfully, recent studies as well as a review of older research into the Asteroidea class of Phylum Echinodermata can aid in shedding light upon possible causes of death. To further advance the husbandry of every hobbyists’ favorite 5-armed friend and to understand why success has remained dismally low, we must first turn to what is known regarding the biology of this particularly irresistible Linckia species in order to identify what may lie behind the shockingly high mortality rate.
1. Nutrition
A common misperception of many asteroids is the assumption that all non-carnivorous species of starfish feed exclusively on microalgae or can survive long-term by consuming the scant amount of uneaten fish food and fish excrement present in the typical aquarium. Although this is a false belief, it continues to be spread repeatedly by store employees who are either motivated by sales figures or are misinformed themselves. While we (as humans) aren’t entirely sure what the specific nutritional requirements of L. laevigata may be, we do know that in their natural habitat they feed on biofilm. Biofilm is a highly complex assemblage of microalgae, bacteria, archaea, protozoans, and countless other microscopic organisms which forms on rocks and other surfaces in marine ecosystems. While some reports have documented that Linckia species consume carrion and certain other foods in the wild and are therefore considered generalist feeders, biofilm continues to be accepted as their primary source of nutrition. This isn’t to say that no other foods will be accepted, only that there is an absence of scientific data documenting alternative food sources which are known to sufficiently meet their nutritional requirements well enough to completely replace their natural diet and support good health. This offers an answer as to why starvation is frequently cited as the likely cause of death when hobbyists report that an individual of this species had been “doing well” for months before it suddenly ceased to actively forage in the aquarium and succumbed to death shortly thereafter. It is important to note that “doing well” is a subjective term generally used to indicate that no obvious signs of ill health have been observed. In the absence of any assessment to accurately measure health quantitatively, one should not blindly assume that the starfish in question is truly healthy or far from a swift decline likely to culminate in death. In fact, it’s common for asteroid species to appear physically healthy just days before a rapid decline in health becomes evident. Unfortunately, once symptoms indicating a decreased state of health become visible or pronounced enough to elicit concern, little is known regarding what can be done to aid in recovery. It may initially seem to be impossible to explain why these mortalities are so commonly observed in this particular species, but an examination of the biological requirements and sensitivity to both biotic and abiotic factors can lend insight.
1.1. Biofilm Formation in Captivity
As mentioned previously, biofilm is extremely complex, dynamic in nature, and astoundingly diverse in its composition. Not only is it exceedingly difficult to accurately identify and/or culture the bacterial strains present even under laboratory conditions, but studies also show that enormous variability exists among biofilms geographically--largely due to the great influence environmental factors have in the determination of which bacterial species will thrive and become most prominent. Researchers are frequently making discoveries which reveal just how little we actually know regarding biofilm formation. One such study, published in 2019, found great variation between biofilm formation on artificial substrates and biofilm formation on natural rocks. The biofilm composition on each substrate deviated to such a great extent that new bacterial species were discovered on the artificial panels (Zhang et al., 2019). This clearly leads to the conclusion that the surface of the substrate plays a role in selection of bacterial species and other microfauna, raising the question of whether natural rock originally sourced from the ocean would form biofilm even remotely close to that of synthetic rock alternatives. Furthermore, biofilm formation occurs successionally, meaning that it passes through distinguishable phases prior to maturing. During these phases,the bacteria, microfauna, and microalgae species dominating the film shifts dramatically (Remple et al., 2021). There may be partial merit to the common claim suggesting that the maturity of a captive system can increase the likelihood of food availability for L. laevigata and similar biofilm-feeding starfish. However, given the unpredictable variability and uncertainty regarding bacterial strains present in any given biofilm, it would not appear likely that the biofilm formed on surfaces of an artificial habitat would remotely mimic those of any natural habitat. Due in part to this variability, it seems particularly likely an aquarium started with artificial rock and dry sand would lack the sheer bacterial diversity which constitutes natural biofilms, and, since the presence of different bacterial species can affect the formation of the biofilm through competition as well as signaling, biofilms which form in aquariums started through this method would likely vary from that of the natural environment to a greater extent than those started with live rock derived from the ocean. This is not to say, however, that simply because an aquarium was built with any amount of live rock (regardless of origin) that these specialist feeders are guaranteed to fare better. We, as humans, simply cannot condense one of the most complex (and least understood) ecosystems on planet Earth into a tiny, artificial environment and expect life to carry on as it would in nature.
L. laevigata spends much of its time slowly crawling through the rocks of its benthic habitat in search of food.
2. Internal Biological Changes
Similar to other wild-harvested aquarium fauna, after being placed into a captive environment differing parameters and environmental conditions are highly likely to cause internal biological changes in L. laevigata. However, the vast majority of research has been limited to the study of acute biological effects induced by captive conditions, leaving a knowledge gap regarding chronic and semi-chronic effects caused by environmental changes. This leaves us with many unknowns regarding the extent to which these internal changes may alter the overall fitness and health of the organism. As the anatomy of echinoderms is strikingly unique due to the adoption of some rather bewildering adaptations to aid in survival--including radial symmetry (referred to as penta-radial symmetry in class Asteroidea) and even an alternative to a traditional circulatory system, diagnosing ill-health or attempting to determine the cause of death can be exceedingly difficult. (Some of the internal changes which may be of particular relevance to the high mortality rate will be discussed following a brief overview of the internal anatomy.)
2.1. The Internal Anatomy and Effects Due to Environmental Stress Factors
Unbeknownst to many, the anatomy of members of the Phylum Echinodermata is as highly complex as those of many invertebrate species. Asteroid species such as Linckia laevigata are no exception, and at least a rudimentary understanding of the internal anatomy is imperative to gain a basic understanding of the physiological changes induced by common stressors in captive environments. While complex vertebrates (including humans) possess a circulatory system through which blood cells carry required nutrients throughout the body, deliver oxygen required for cellular respiration, and remove waste material, asteroids have developed a water vascular system to serve a similar purpose. Coelomic fluid (analogous to blood) flows through the water vascular system, bathing the internal organs and performing many of the same functions as our blood while also supporting other essential life functions such as reproduction (Aquino et al., 2021). Only one of many components of this system, coelomocytes perform the crucial functions of fighting infections and internal parasites as well as preventing fluid loss following injury. These cells are an integral part of the immune system of starfish and are very similar to the leukocytes (white blood cells) of vertebrates. Just as inhibition of leukocytes leads to extensive and even life-threatening conditions such as HIV in humans, any interference with the normal functions of coelomocytes within the water vascular system--such as clumping or a reduction in coelomocyte count--can result in great disruptions in the organism’s ability to fight foreign or harmful pathogens, hinder recovery rates following injury, and prevent the efficient transport of vital nutrients to internal organs.
The importance of a healthy and naturally functioning immune system cannot be overexaggerated in any organism and although this may seem obvious, few understand how the anatomy of sea stars increases their susceptibility to alterations in the coelomocytes, which subsequently affects the immune system. While relatively few scientific studies have detailed the importance and factors which affect the concentration and function of coelomocytes, the subject has recently gained attention, revealing the effects of various environmental stressors which are present in typical hobbyist aquariums. Although many environmental conditions are likely to have detrimental effects on the immune system of asteroid species (very few studies have been conducted using L. laevigata as test organisms and therefore, it is probable different species are not affected equally), a few specific conditions which have been studied extensively have been shown to have significant effects on the immune system. pH levels varying from that of natural habitats, hypoxic conditions, and temperatures exceeding those naturally encountered have each been shown to affect coelomocytes, lead to variations in the natural volume of coelomic fluid, and affect bacterial population density which can have detrimental effects in relation to respiration.
2.1.1. Effects of decreased pH on Skeletal Development and the Immune System
The rapidly increasing levels of atmospheric carbon dioxide which have been well documented to cause ocean acidification is an obvious cause for concern for any marine organisms possessing calcified skeletons such as echinoderms. However, we simply cannot make the generalization that all organisms with calcified skeletons will experience equivalent or even similar detrimental health effects without first thoroughly examining the class or genus in question and even applying results obtained from carefully performed studies cannot predict the effects an equivalent decrease in pH will have on separate species with high accuracy. This conclusion has been drawn from numerous studies and continues to be accepted. Shockingly, thorough analysis of scientific literature reveals asteroid species such as L. laevigata are surprisingly resilient to inhibition of skeletal formation due to ocean acidification. Some echinoderms such as echinoids (urchins) are far more sensitive than other classes within Phylum Echinodermata such as crinoids (feather stars), holothurians (sea cucumbers), asteroids, and ophiuroids (brittle stars) with varying sensitivity existing within each class. The most obvious cause for the observed disparities in sensitivity is due to differences in the amount of calcium carbonate required for skeletal formation and characteristics of the skeleton itself. Due to the fact that asteroids possess somewhat flexible skeletons embedded in connective tissue, they appear to be far less sensitive than other classes (Dupont et al., 2010).
Though research pertaining to the effects of ocean acidification on calcification may alleviate some concerns relating to the long term survival of many asteroid species in their natural habitat, pH is a parameter of importance for hobbyists desiring to maintain starfish since significant swings are likely to occur more rapidly in closed artificial environments. Two separate studies conducted with the aim of providing an analysis of the effects of decreased pH on coelomocyte counts support the hypothesis that pH is a particularly important parameter and clearly affects the internal functioning of the immune system. Both studies tested the effects on Asterias rubens (the common sea star). The first of the two studies lasted for a period of just 14 days during which the starfish were housed in seawater at a pH of 7.7 which resulted in a 37% decrease in coelomocytes (Aquino et al., 2021). Results from the second study which lasted for 6 months, during which each specimen was housed in seawater at the same pH of 7.7 (the projected pH of the ocean by the year 2100), revealed the occurrence of immunosuppressive effects. It was determined coelomocyte count decreased by 50%, coelomic fluid pH was reduced, and heat shock proteins which indicate stress dramatically increased in number (Mayer & Bukau, 2005). Due to the fact that both studies which were performed independent of the other are in agreement, pH appears to be of great importance to health as decreasing coelomocyte counts can serve as an indication of weakened immunity.
2.1.2. Hypoxia
Echinoderms rely upon diffusion across organs known as papulae located on the aboral surface of the body to expel waste products and to aid the tube feet in gaseous exchange. Since every surface within a marine environment has a very thin coating of water known as the diffusive boundary layer (Jørgensen & Revsbech, 1985), diffusion is made slightly more difficult. While the papulae and tube feet enable adequate exchange of gasses despite the presence of the DBL, both these adaptations were developed for survival in their native habitat under naturally occurring conditions. When confined within an aquarium, DO concentrations may be far lower than the tropical seas L. laevigata is native to which poses a great threat to health. Hypoxic conditions have been shown to be related to Sea Star Wasting Disease (discussed later) by causing an excessive accumulation of anaerobic bacteria on the aboral surface (epidermis) of asteroids (Jørgensen and Revsbech, 1985). Once this occurs, not even returning the DO concentration to adequate levels can enable adequate oxygen exchange after these bacteria have covered the pedicellariae and papulae. Following this, difficulty in gas exchange is greatly exacerbated in the presence of high nutrient concentrations from DOM (dissolved organic matter). This enrichment from DOM coupled with high temperatures further intensifies the issue due to accelerating the rate copiotrophic bacteria reproduce which can essentially lead to slow suffocation.
2.1.3. Effects of High Temperature On MCT and Coeloemocytes
A commonality in each of the 5 classes of echinoderms is the presence of Mutable Collagenous Tissue (MCT), an extraordinary adaptation which stiffens to offer mechanical support and protection (Motokawa, 2011). MCT makes arm autonomy (the shedding or loss of one or more arms) possible in starfish and increased temperatures may trigger this response due the release of proteinaceous compounds which enter the coelomic fluid, indicating internal stress as previously mentioned (Chaet, 1962), (Mladenov et al., 1989), (Oulhen et al., 2022). Though research documenting the effects of high temperatures conducted with L. laevigata serving as test subjects is scarce, higher temperatures generally increase both the rate of metabolic processes and level of activity, leading to increased energy expenditure in many organisms. Therefore, if energy demands cannot be met due to an inadequate supply of food, a decrease in strength required for the healthy functioning of internal processes including the immune system may serve as yet another plausible explanation for high mortality rates. Additionally, coelomocyte clumping has been observed in multiple Asteroid species when subjected to increased temperatures. This phenomenon is similar to hemostasis in vertebrates and occurs in asteroids in order to prevent coelomic fluid loss (Aquino et al., 2021). The occurrence of coelomocyte clumping serves as evidence that temperature does affect immune system response (although the magnitude of significance is likely to be highly species dependent).
2.2. Sea Star Wasting Disease (SSWD)
Sudden, irreversible health declines observed in certain species such as Linckia laevigata are symptomatic of one particularly dreaded disease known as Sea Star Wasting Disease or SSWD. Though not all causes have been identified, credible research suggests that the previously mentioned environmental stressors are highly likely to be integral in development of this disease. As a rapidly progressing illness, SSWD leads to mass mortality of many asteroids globally even within their natural environment (Aquino et al., 2021). Initial symptoms include the development of minor lesions on the epidermis which continue to enlarge and soon-after limbs may begin to twist unnaturally (Bucci et al., 2017). In advanced stages, outer tissues of the body dissolve and begin to fall away, leading to an appearance many describe as dissolving or melting (Eisenlord et al., 2016). Death has been known to occur within days following the first visible signs of illness.
As definitive causes of SSWD have yet to be determined and no widely accepted consensus exists among researchers to explain the recent mass mortalities attributed to this mysterious disease, many plausible explanations have been proposed. Although the densovirus, pathogenic bacteria, and a variety of other organisms were first assumed to be responsible (Hewson et al., 2014), a knowledge gap later led to the discovery that the densovirus is unlikely to be the primary cause of SSWD after compelling research concluded that this particular virus is a prevalent and common component present in the microbiome of many species of healthy starfish (Jackson et al., 2020). The belief that pathogenic bacteria are solely to blame has also been widely abandoned by the scientific community due to the emergence of new theories and recent advancements in both techniques and methods for the examination and analysis of the internal functions of echinoderms. Most modern discoveries support the theory that SSWD is driven by a multitude of factors, with those previously mentioned thought to act synergistically in the development and progression of SSWD.
3. Conclusion
As is evident, asteroid species in general (and particularly those belonging to the Linckia genus) are a class of highly unique and delicate organisms with an anatomy sensitive to many environmental factors. The great variations among asteroid species in regards to susceptibility to malnutrition and starvation, sensitivity to stressors, and biological reactions to stress, do make it difficult to diagnose the causes of mortality as L. laevigata specimens are not frequently used as test organisms. However, in the occurrence of rapid decline leading to death, symptoms observed such as epidermal lesions, reduced activity, and the melting away of tissue would suggest Sea Star Wasting Disease to have likely been induced by these unnatural conditions. If this is in fact the cause, it would appear the only solution may be close monitoring of all chemical and physical parameters through accurate testing and maintaining records of the test results in order to track any shifting trends as treatment attempts have rarely been effective. Mortalities which occur after several months during which a reduction in size or weight is observed would suggest starvation to be the likely cause of death if all parameters are maintained within semi-natural ranges. A final concern worth mentioning is the stress induced by differing collection methods, handling procedures, and transportation times, which could have potentially led to a decline in health before purchase. While stress is a common concern taken into account before purchase of fish or corals, the treatment of invertebrates pre-purchase is a seldomly discussed topic and investigation may reveal this to be an important factor in the survival of individuals of this species given their high sensitivity.
3.1 Additional Speculations In Need of Future Research
While the aforementioned factors are likely attributable to the majority of deaths in captivity, further research is required to determine precisely why L. laevigata appears to be far more prone to the development of SSWD than other asteroid species in captivity within such a remarkably short period of time. Bearing in mind that sensitivity to adverse conditions is highly species-specific, further investigation is necessary for the determination of why this species exhibits particularly low tolerance even when housed within systems well equipped with technology designed to detect shifting parameters and maintain stability. Although pure speculation, future research may reveal specific anatomical characteristics such as a thinner epidermal layer in comparison to other asteroid species or perhaps an unexpected peculiarity regarding the function of the immune system of L. laevigata which deviates from that of other commonly kept species and leads to increased rates of health decline. With consideration to the exorbitant number of individuals collected for the global aquarium trade, the remarkably frequent observations of rapid health decline which are consistent in description warrant further investigation.
4. Moving Forward: A Final Word of Hope
As is evident from published research, asteroid species (and particularly those belonging to certain genera) are a class of highly unique and delicate organisms with an anatomy sensitive to many more environmental factors than the majority of organisms commonly kept by hobbyists. While it may appear a futile effort for the majority of aquarists to maintain species such as L. laevigata, it would be a mistake to consider it an impossibility simply due to the historical survival record alone. In fact, multiple individuals of this species have previously been maintained in captivity under carefully controlled laboratory conditions for a period of 14 months during which significant growth of specimens was recorded throughout the study period (Yamaguchi, 1977). However, this should not be perceived as encouraging any attempt to house L. laevigata in the absence of a comprehensive understanding of the biology and requirements ofthis specific species, particularly due to the fact that the vast majority of commonly touted “keys to success” are anecdotal and lack supporting evidence. Though some may disagree, the path to success is far more likely to be achieved through proper research as opposed to personal anecdotes. Just as maintaining SPS and other stony coral species was considered an impossibility only decades ago, these captivating creatures may someday be a common sight in many home aquariums if research is coupled with the passion, willpower, and indomitable determination required for the development of methods which adequately meet the needs of this fragile species.
Linckia laevigata starfish and many other asteroid species are known to possess an “eye-spot” located at the tip of each arm to avoid wandering into the open ocean, away from their primary source of food. This sometimes leads to an individual mistaking similarly shaped organisms for the biofilm-coated rocks it desires to feed upon.
References
Alcazar, D. S., & Kochzius, M. (2015). Genetic population structure of the Blue Sea Star Linckia Laevigata in the Visayas (Philippines). Journal of the Marine Biological
Association of the United Kingdom, 1–7. https://doi.org/10.1017/s0025315415000971
Aquino, C. A., Besemer, R. M., DeRito, C. M., Kocian, J., Porter, I. R., Raimondi, P. T., Rede, J. E., Schiebelhut, L. M., Sparks, J. P., Wares, J. P., & Hewson, I. (2021). Evidence that microorganisms at the animal-water interface drive sea star wasting disease. Frontiers in Microbiology, 11. https://doi.org/10.3389/fmicb.2020.610009
Bucci, C., Francoeur, M., McGreal, J., Smolowitz, R., Zazueta-Novoa, V., Wessel, G. M., & Gomez-Chiarri, M. (2017). Sea star wasting disease in asterias forbesi along the Atlantic coast of North America. PLOS ONE, 12(12). https://doi.org/10.1371/journal.pone.0188523
Chaet, A. B. (1962). A toxin in the coelomic fluid of scalded starfish (Asterias forbesi). Experimental Biology and Medicine, 109(4), 791–794. https://doi.org/10.3181/00379727- 109-27337
Dupont, S., Ortega-Martínez, O., & Thorndyke, M. (2010). Impact of near-future ocean acidification on Echinoderms. Ecotoxicology, 19(3), 449–462.
https://doi.org/10.1007/s10646-010-0463-6
Eisenlord, M. E., Groner, M. L., Yoshioka, R. M., Elliott, J., Maynard, J., Fradkin, S.,Turner, M., Pyne, K., Rivlin, N., van Hooidonk, R., & Harvell, C. D. (2016). Ochre Star mortality during the 2014 Wasting Disease Epizootic: Role of population size structure and temperature. Philosophical Transactions of the Royal Society B: Biological Sciences, 371(1689), 20150212. https://doi.org/10.1098/rstb.2015.0212
Hewson, I., Button, J. B., Gudenkauf, B. M., Miner, B., Newton, A. L., Gaydos, J. K., Wynne, J., Groves, C. L., Hendler, G., Murray, M., Fradkin, S., Breitbart, M., Fahsbender, E., Lafferty, K. D., Kilpatrick, A. M., Miner, C. M., Raimondi, P., Lahner, L., Friedman, C. S., … Harvell, C. D. (2014). Densovirus associated with sea-star wasting disease and mass mortality. Proceedings of the National Academy of Sciences, 111(48), 17278–17283. https://doi.org/10.1073/pnas.1416625111
Jackson, E. W., Pepe-Ranney, C., Johnson, M. R., Distel, D. L., & Hewson, I. (2020). A highly prevalent and pervasive densovirus discovered among sea stars from the North American Atlantic Coast. Applied and Environmental Microbiology, 86(6). https://doi.org/10.1128/aem.02723-19
Jørgensen, B. B., & Revsbech, N. P. (1985). Diffusive boundary layers and the oxygen uptake of sediments and detritus1. Limnology and Oceanography, 30(1), 111–122. https://doi.org/10.4319/lo.1985.30.1.0111
Mayer, M. P., & Bukau, B. (2005). Hsp70 chaperones: Cellular functions and molecular mechanism. Cellular and Molecular Life Sciences, 62(6), 670–684. https://doi.org/10.1007/s00018-004-4464-6
Mladenov, P. V., Igdoura, S., Asotra, S., & Burke, R. D. (1989). Purification and partial characterization of an autotomy-promoting factor from the Sea Starpycnopodia helianthoides. The Biological Bulletin, 176(2), 169–175. https://doi.org/10.2307/1541585
Motokawa, T. (2011). Mechanical mutability in connective tissue of starfish body wall. The Biological Bulletin, 221(3), 280–289. https://doi.org/10.1086/bblv221n3p280
Oulhen, N., Byrne, M., Duffin, P., Gomez-Chiarri, M., Hewson, I., Hodin, J., Konar, B., Lipp, E. K., Miner, B. G., Newton, A. L., Schiebelhut, L. M., Smolowitz, R., Wahltinez, S. J., Wessel, G. M., Work, T. M., Zaki, H. A., & Wares, J. P. (2022). A review of asteroid biology in the context of sea star wasting: Possible causes and consequences. The Biological Bulletin, 243(1), 50–75. https://doi.org/10.1086/719928
Remple, K. L., Silbiger, N. J., Quinlan, Z. A., Fox, M. D., Kelly, L. W., Donahue, M. J., & Nelson, C. E. (2021). Coral reef biofilm bacterial diversity and successional trajectories are structured by reef benthic organisms and shift under chronic nutrient enrichment. Npj Biofilms and Microbiomes, 7(1). https://doi.org/10.1038/s41522-021- 00252-1
Yamaguchi, M. (1977, January 1). Population structure, spawning, and growth of the coral reef asteroid Linckia Laevigata (linnaeus). ScholarSpace. https://scholarspace.manoa.hawaii.edu/handle/10125/1178
Zhang, W., Ding, W., Li, Y.-X., Tam, C., Bougouffa, S., Wang, R., Pei, B., Chiang, H., Leung, P., Lu, Y., Sun, J., Fu, H., Bajic, V. B., Liu, H., Webster, N. S., & Qian, P.-Y. (2019). Marine biofilms constitute a bank of hidden microbial diversity and functional potential. Nature Communications, 10(1). https://doi.org/10.1038/s41467-019-08463-z
Linckia laevigata: An Analysis of Plausible Causes of Mortality in Captivity
By: Malcolm Levison
By: Malcolm Levison
The indisputable beauty of Linckia laevigata has led to this fragile organism becoming an icon of the global aquarium trade. However, survival rates in captivity are tragically low. Epitomizing beauty with an exotic appearance and arms as blue as the ocean itself, Linckia laevigata, (commonly referred to as the “Blue Linckia”), seduces thousands of marine aquarium hobbyists into an ill-informed purchase year after year despite their dismal survival rate in captivity. Records reveal that L. laevigata accounts for an astounding 3% of all wild- collected invertebrates for the global marine aquarium trade (Alcazar & Kochzius, 2015). Why then, if so many attempts have been made to maintain them in captivity, have there been so few success stories? Is there something drastically different about this asteroid species which has led many hobbyists to consider them impossible to maintain long term? While most hobbyists would agree that many species of starfish (and particularly certain species such as those within the Linckia genus) are prone to nutritional deficiencies when housed in captivity and ultimately succumb to starvation generally within 6-8 months, it should be acknowledged that this widespread belief is predominantly based upon assumptions drawn from personal anecdotes and not necessarily supported by documented research. Furthermore, this fails to offer an explanation as to why many starfish of this particular species tend to simply “disintegrate” sometimes within only a week after introduction into the aquarium. Many explanations have been proposed and widely circulated ranging from the outrageous to highly plausible. However, often even these plausible explanations fail to take into account the unique and complex anatomy of asteroid species which is required to logically unravel the cause(s) of mortality or ill health. Thankfully, recent studies as well as a review of older research into the Asteroidea class of Phylum Echinodermata can aid in shedding light upon possible causes of death. To further advance the husbandry of every hobbyists’ favorite 5-armed friend and to understand why success has remained dismally low, we must first turn to what is known regarding the biology of this particularly irresistible Linckia species in order to identify what may lie behind the shockingly high mortality rate.
1. Nutrition
A common misperception of many asteroids is the assumption that all non-carnivorous species of starfish feed exclusively on microalgae or can survive long-term by consuming the scant amount of uneaten fish food and fish excrement present in the typical aquarium. Although this is a false belief, it continues to be spread repeatedly by store employees who are either motivated by sales figures or are misinformed themselves. While we (as humans) aren’t entirely sure what the specific nutritional requirements of L. laevigata may be, we do know that in their natural habitat they feed on biofilm. Biofilm is a highly complex assemblage of microalgae, bacteria, archaea, protozoans, and countless other microscopic organisms which forms on rocks and other surfaces in marine ecosystems. While some reports have documented that Linckia species consume carrion and certain other foods in the wild and are therefore considered generalist feeders, biofilm continues to be accepted as their primary source of nutrition. This isn’t to say that no other foods will be accepted, only that there is an absence of scientific data documenting alternative food sources which are known to sufficiently meet their nutritional requirements well enough to completely replace their natural diet and support good health. This offers an answer as to why starvation is frequently cited as the likely cause of death when hobbyists report that an individual of this species had been “doing well” for months before it suddenly ceased to actively forage in the aquarium and succumbed to death shortly thereafter. It is important to note that “doing well” is a subjective term generally used to indicate that no obvious signs of ill health have been observed. In the absence of any assessment to accurately measure health quantitatively, one should not blindly assume that the starfish in question is truly healthy or far from a swift decline likely to culminate in death. In fact, it’s common for asteroid species to appear physically healthy just days before a rapid decline in health becomes evident. Unfortunately, once symptoms indicating a decreased state of health become visible or pronounced enough to elicit concern, little is known regarding what can be done to aid in recovery. It may initially seem to be impossible to explain why these mortalities are so commonly observed in this particular species, but an examination of the biological requirements and sensitivity to both biotic and abiotic factors can lend insight.
1.1. Biofilm Formation in Captivity
As mentioned previously, biofilm is extremely complex, dynamic in nature, and astoundingly diverse in its composition. Not only is it exceedingly difficult to accurately identify and/or culture the bacterial strains present even under laboratory conditions, but studies also show that enormous variability exists among biofilms geographically--largely due to the great influence environmental factors have in the determination of which bacterial species will thrive and become most prominent. Researchers are frequently making discoveries which reveal just how little we actually know regarding biofilm formation. One such study, published in 2019, found great variation between biofilm formation on artificial substrates and biofilm formation on natural rocks. The biofilm composition on each substrate deviated to such a great extent that new bacterial species were discovered on the artificial panels (Zhang et al., 2019). This clearly leads to the conclusion that the surface of the substrate plays a role in selection of bacterial species and other microfauna, raising the question of whether natural rock originally sourced from the ocean would form biofilm even remotely close to that of synthetic rock alternatives. Furthermore, biofilm formation occurs successionally, meaning that it passes through distinguishable phases prior to maturing. During these phases,the bacteria, microfauna, and microalgae species dominating the film shifts dramatically (Remple et al., 2021). There may be partial merit to the common claim suggesting that the maturity of a captive system can increase the likelihood of food availability for L. laevigata and similar biofilm-feeding starfish. However, given the unpredictable variability and uncertainty regarding bacterial strains present in any given biofilm, it would not appear likely that the biofilm formed on surfaces of an artificial habitat would remotely mimic those of any natural habitat. Due in part to this variability, it seems particularly likely an aquarium started with artificial rock and dry sand would lack the sheer bacterial diversity which constitutes natural biofilms, and, since the presence of different bacterial species can affect the formation of the biofilm through competition as well as signaling, biofilms which form in aquariums started through this method would likely vary from that of the natural environment to a greater extent than those started with live rock derived from the ocean. This is not to say, however, that simply because an aquarium was built with any amount of live rock (regardless of origin) that these specialist feeders are guaranteed to fare better. We, as humans, simply cannot condense one of the most complex (and least understood) ecosystems on planet Earth into a tiny, artificial environment and expect life to carry on as it would in nature.
L. laevigata spends much of its time slowly crawling through the rocks of its benthic habitat in search of food.
2. Internal Biological Changes
Similar to other wild-harvested aquarium fauna, after being placed into a captive environment differing parameters and environmental conditions are highly likely to cause internal biological changes in L. laevigata. However, the vast majority of research has been limited to the study of acute biological effects induced by captive conditions, leaving a knowledge gap regarding chronic and semi-chronic effects caused by environmental changes. This leaves us with many unknowns regarding the extent to which these internal changes may alter the overall fitness and health of the organism. As the anatomy of echinoderms is strikingly unique due to the adoption of some rather bewildering adaptations to aid in survival--including radial symmetry (referred to as penta-radial symmetry in class Asteroidea) and even an alternative to a traditional circulatory system, diagnosing ill-health or attempting to determine the cause of death can be exceedingly difficult. (Some of the internal changes which may be of particular relevance to the high mortality rate will be discussed following a brief overview of the internal anatomy.)
2.1. The Internal Anatomy and Effects Due to Environmental Stress Factors
Unbeknownst to many, the anatomy of members of the Phylum Echinodermata is as highly complex as those of many invertebrate species. Asteroid species such as Linckia laevigata are no exception, and at least a rudimentary understanding of the internal anatomy is imperative to gain a basic understanding of the physiological changes induced by common stressors in captive environments. While complex vertebrates (including humans) possess a circulatory system through which blood cells carry required nutrients throughout the body, deliver oxygen required for cellular respiration, and remove waste material, asteroids have developed a water vascular system to serve a similar purpose. Coelomic fluid (analogous to blood) flows through the water vascular system, bathing the internal organs and performing many of the same functions as our blood while also supporting other essential life functions such as reproduction (Aquino et al., 2021). Only one of many components of this system, coelomocytes perform the crucial functions of fighting infections and internal parasites as well as preventing fluid loss following injury. These cells are an integral part of the immune system of starfish and are very similar to the leukocytes (white blood cells) of vertebrates. Just as inhibition of leukocytes leads to extensive and even life-threatening conditions such as HIV in humans, any interference with the normal functions of coelomocytes within the water vascular system--such as clumping or a reduction in coelomocyte count--can result in great disruptions in the organism’s ability to fight foreign or harmful pathogens, hinder recovery rates following injury, and prevent the efficient transport of vital nutrients to internal organs.
The importance of a healthy and naturally functioning immune system cannot be overexaggerated in any organism and although this may seem obvious, few understand how the anatomy of sea stars increases their susceptibility to alterations in the coelomocytes, which subsequently affects the immune system. While relatively few scientific studies have detailed the importance and factors which affect the concentration and function of coelomocytes, the subject has recently gained attention, revealing the effects of various environmental stressors which are present in typical hobbyist aquariums. Although many environmental conditions are likely to have detrimental effects on the immune system of asteroid species (very few studies have been conducted using L. laevigata as test organisms and therefore, it is probable different species are not affected equally), a few specific conditions which have been studied extensively have been shown to have significant effects on the immune system. pH levels varying from that of natural habitats, hypoxic conditions, and temperatures exceeding those naturally encountered have each been shown to affect coelomocytes, lead to variations in the natural volume of coelomic fluid, and affect bacterial population density which can have detrimental effects in relation to respiration.
2.1.1. Effects of decreased pH on Skeletal Development and the Immune System
The rapidly increasing levels of atmospheric carbon dioxide which have been well documented to cause ocean acidification is an obvious cause for concern for any marine organisms possessing calcified skeletons such as echinoderms. However, we simply cannot make the generalization that all organisms with calcified skeletons will experience equivalent or even similar detrimental health effects without first thoroughly examining the class or genus in question and even applying results obtained from carefully performed studies cannot predict the effects an equivalent decrease in pH will have on separate species with high accuracy. This conclusion has been drawn from numerous studies and continues to be accepted. Shockingly, thorough analysis of scientific literature reveals asteroid species such as L. laevigata are surprisingly resilient to inhibition of skeletal formation due to ocean acidification. Some echinoderms such as echinoids (urchins) are far more sensitive than other classes within Phylum Echinodermata such as crinoids (feather stars), holothurians (sea cucumbers), asteroids, and ophiuroids (brittle stars) with varying sensitivity existing within each class. The most obvious cause for the observed disparities in sensitivity is due to differences in the amount of calcium carbonate required for skeletal formation and characteristics of the skeleton itself. Due to the fact that asteroids possess somewhat flexible skeletons embedded in connective tissue, they appear to be far less sensitive than other classes (Dupont et al., 2010).
Though research pertaining to the effects of ocean acidification on calcification may alleviate some concerns relating to the long term survival of many asteroid species in their natural habitat, pH is a parameter of importance for hobbyists desiring to maintain starfish since significant swings are likely to occur more rapidly in closed artificial environments. Two separate studies conducted with the aim of providing an analysis of the effects of decreased pH on coelomocyte counts support the hypothesis that pH is a particularly important parameter and clearly affects the internal functioning of the immune system. Both studies tested the effects on Asterias rubens (the common sea star). The first of the two studies lasted for a period of just 14 days during which the starfish were housed in seawater at a pH of 7.7 which resulted in a 37% decrease in coelomocytes (Aquino et al., 2021). Results from the second study which lasted for 6 months, during which each specimen was housed in seawater at the same pH of 7.7 (the projected pH of the ocean by the year 2100), revealed the occurrence of immunosuppressive effects. It was determined coelomocyte count decreased by 50%, coelomic fluid pH was reduced, and heat shock proteins which indicate stress dramatically increased in number (Mayer & Bukau, 2005). Due to the fact that both studies which were performed independent of the other are in agreement, pH appears to be of great importance to health as decreasing coelomocyte counts can serve as an indication of weakened immunity.
2.1.2. Hypoxia
Echinoderms rely upon diffusion across organs known as papulae located on the aboral surface of the body to expel waste products and to aid the tube feet in gaseous exchange. Since every surface within a marine environment has a very thin coating of water known as the diffusive boundary layer (Jørgensen & Revsbech, 1985), diffusion is made slightly more difficult. While the papulae and tube feet enable adequate exchange of gasses despite the presence of the DBL, both these adaptations were developed for survival in their native habitat under naturally occurring conditions. When confined within an aquarium, DO concentrations may be far lower than the tropical seas L. laevigata is native to which poses a great threat to health. Hypoxic conditions have been shown to be related to Sea Star Wasting Disease (discussed later) by causing an excessive accumulation of anaerobic bacteria on the aboral surface (epidermis) of asteroids (Jørgensen and Revsbech, 1985). Once this occurs, not even returning the DO concentration to adequate levels can enable adequate oxygen exchange after these bacteria have covered the pedicellariae and papulae. Following this, difficulty in gas exchange is greatly exacerbated in the presence of high nutrient concentrations from DOM (dissolved organic matter). This enrichment from DOM coupled with high temperatures further intensifies the issue due to accelerating the rate copiotrophic bacteria reproduce which can essentially lead to slow suffocation.
2.1.3. Effects of High Temperature On MCT and Coeloemocytes
A commonality in each of the 5 classes of echinoderms is the presence of Mutable Collagenous Tissue (MCT), an extraordinary adaptation which stiffens to offer mechanical support and protection (Motokawa, 2011). MCT makes arm autonomy (the shedding or loss of one or more arms) possible in starfish and increased temperatures may trigger this response due the release of proteinaceous compounds which enter the coelomic fluid, indicating internal stress as previously mentioned (Chaet, 1962), (Mladenov et al., 1989), (Oulhen et al., 2022). Though research documenting the effects of high temperatures conducted with L. laevigata serving as test subjects is scarce, higher temperatures generally increase both the rate of metabolic processes and level of activity, leading to increased energy expenditure in many organisms. Therefore, if energy demands cannot be met due to an inadequate supply of food, a decrease in strength required for the healthy functioning of internal processes including the immune system may serve as yet another plausible explanation for high mortality rates. Additionally, coelomocyte clumping has been observed in multiple Asteroid species when subjected to increased temperatures. This phenomenon is similar to hemostasis in vertebrates and occurs in asteroids in order to prevent coelomic fluid loss (Aquino et al., 2021). The occurrence of coelomocyte clumping serves as evidence that temperature does affect immune system response (although the magnitude of significance is likely to be highly species dependent).
2.2. Sea Star Wasting Disease (SSWD)
Sudden, irreversible health declines observed in certain species such as Linckia laevigata are symptomatic of one particularly dreaded disease known as Sea Star Wasting Disease or SSWD. Though not all causes have been identified, credible research suggests that the previously mentioned environmental stressors are highly likely to be integral in development of this disease. As a rapidly progressing illness, SSWD leads to mass mortality of many asteroids globally even within their natural environment (Aquino et al., 2021). Initial symptoms include the development of minor lesions on the epidermis which continue to enlarge and soon-after limbs may begin to twist unnaturally (Bucci et al., 2017). In advanced stages, outer tissues of the body dissolve and begin to fall away, leading to an appearance many describe as dissolving or melting (Eisenlord et al., 2016). Death has been known to occur within days following the first visible signs of illness.
As definitive causes of SSWD have yet to be determined and no widely accepted consensus exists among researchers to explain the recent mass mortalities attributed to this mysterious disease, many plausible explanations have been proposed. Although the densovirus, pathogenic bacteria, and a variety of other organisms were first assumed to be responsible (Hewson et al., 2014), a knowledge gap later led to the discovery that the densovirus is unlikely to be the primary cause of SSWD after compelling research concluded that this particular virus is a prevalent and common component present in the microbiome of many species of healthy starfish (Jackson et al., 2020). The belief that pathogenic bacteria are solely to blame has also been widely abandoned by the scientific community due to the emergence of new theories and recent advancements in both techniques and methods for the examination and analysis of the internal functions of echinoderms. Most modern discoveries support the theory that SSWD is driven by a multitude of factors, with those previously mentioned thought to act synergistically in the development and progression of SSWD.
3. Conclusion
As is evident, asteroid species in general (and particularly those belonging to the Linckia genus) are a class of highly unique and delicate organisms with an anatomy sensitive to many environmental factors. The great variations among asteroid species in regards to susceptibility to malnutrition and starvation, sensitivity to stressors, and biological reactions to stress, do make it difficult to diagnose the causes of mortality as L. laevigata specimens are not frequently used as test organisms. However, in the occurrence of rapid decline leading to death, symptoms observed such as epidermal lesions, reduced activity, and the melting away of tissue would suggest Sea Star Wasting Disease to have likely been induced by these unnatural conditions. If this is in fact the cause, it would appear the only solution may be close monitoring of all chemical and physical parameters through accurate testing and maintaining records of the test results in order to track any shifting trends as treatment attempts have rarely been effective. Mortalities which occur after several months during which a reduction in size or weight is observed would suggest starvation to be the likely cause of death if all parameters are maintained within semi-natural ranges. A final concern worth mentioning is the stress induced by differing collection methods, handling procedures, and transportation times, which could have potentially led to a decline in health before purchase. While stress is a common concern taken into account before purchase of fish or corals, the treatment of invertebrates pre-purchase is a seldomly discussed topic and investigation may reveal this to be an important factor in the survival of individuals of this species given their high sensitivity.
3.1 Additional Speculations In Need of Future Research
While the aforementioned factors are likely attributable to the majority of deaths in captivity, further research is required to determine precisely why L. laevigata appears to be far more prone to the development of SSWD than other asteroid species in captivity within such a remarkably short period of time. Bearing in mind that sensitivity to adverse conditions is highly species-specific, further investigation is necessary for the determination of why this species exhibits particularly low tolerance even when housed within systems well equipped with technology designed to detect shifting parameters and maintain stability. Although pure speculation, future research may reveal specific anatomical characteristics such as a thinner epidermal layer in comparison to other asteroid species or perhaps an unexpected peculiarity regarding the function of the immune system of L. laevigata which deviates from that of other commonly kept species and leads to increased rates of health decline. With consideration to the exorbitant number of individuals collected for the global aquarium trade, the remarkably frequent observations of rapid health decline which are consistent in description warrant further investigation.
4. Moving Forward: A Final Word of Hope
As is evident from published research, asteroid species (and particularly those belonging to certain genera) are a class of highly unique and delicate organisms with an anatomy sensitive to many more environmental factors than the majority of organisms commonly kept by hobbyists. While it may appear a futile effort for the majority of aquarists to maintain species such as L. laevigata, it would be a mistake to consider it an impossibility simply due to the historical survival record alone. In fact, multiple individuals of this species have previously been maintained in captivity under carefully controlled laboratory conditions for a period of 14 months during which significant growth of specimens was recorded throughout the study period (Yamaguchi, 1977). However, this should not be perceived as encouraging any attempt to house L. laevigata in the absence of a comprehensive understanding of the biology and requirements ofthis specific species, particularly due to the fact that the vast majority of commonly touted “keys to success” are anecdotal and lack supporting evidence. Though some may disagree, the path to success is far more likely to be achieved through proper research as opposed to personal anecdotes. Just as maintaining SPS and other stony coral species was considered an impossibility only decades ago, these captivating creatures may someday be a common sight in many home aquariums if research is coupled with the passion, willpower, and indomitable determination required for the development of methods which adequately meet the needs of this fragile species.
Linckia laevigata starfish and many other asteroid species are known to possess an “eye-spot” located at the tip of each arm to avoid wandering into the open ocean, away from their primary source of food. This sometimes leads to an individual mistaking similarly shaped organisms for the biofilm-coated rocks it desires to feed upon.
References
Alcazar, D. S., & Kochzius, M. (2015). Genetic population structure of the Blue Sea Star Linckia Laevigata in the Visayas (Philippines). Journal of the Marine Biological
Association of the United Kingdom, 1–7. https://doi.org/10.1017/s0025315415000971
Aquino, C. A., Besemer, R. M., DeRito, C. M., Kocian, J., Porter, I. R., Raimondi, P. T., Rede, J. E., Schiebelhut, L. M., Sparks, J. P., Wares, J. P., & Hewson, I. (2021). Evidence that microorganisms at the animal-water interface drive sea star wasting disease. Frontiers in Microbiology, 11. https://doi.org/10.3389/fmicb.2020.610009
Bucci, C., Francoeur, M., McGreal, J., Smolowitz, R., Zazueta-Novoa, V., Wessel, G. M., & Gomez-Chiarri, M. (2017). Sea star wasting disease in asterias forbesi along the Atlantic coast of North America. PLOS ONE, 12(12). https://doi.org/10.1371/journal.pone.0188523
Chaet, A. B. (1962). A toxin in the coelomic fluid of scalded starfish (Asterias forbesi). Experimental Biology and Medicine, 109(4), 791–794. https://doi.org/10.3181/00379727- 109-27337
Dupont, S., Ortega-Martínez, O., & Thorndyke, M. (2010). Impact of near-future ocean acidification on Echinoderms. Ecotoxicology, 19(3), 449–462.
https://doi.org/10.1007/s10646-010-0463-6
Eisenlord, M. E., Groner, M. L., Yoshioka, R. M., Elliott, J., Maynard, J., Fradkin, S.,Turner, M., Pyne, K., Rivlin, N., van Hooidonk, R., & Harvell, C. D. (2016). Ochre Star mortality during the 2014 Wasting Disease Epizootic: Role of population size structure and temperature. Philosophical Transactions of the Royal Society B: Biological Sciences, 371(1689), 20150212. https://doi.org/10.1098/rstb.2015.0212
Hewson, I., Button, J. B., Gudenkauf, B. M., Miner, B., Newton, A. L., Gaydos, J. K., Wynne, J., Groves, C. L., Hendler, G., Murray, M., Fradkin, S., Breitbart, M., Fahsbender, E., Lafferty, K. D., Kilpatrick, A. M., Miner, C. M., Raimondi, P., Lahner, L., Friedman, C. S., … Harvell, C. D. (2014). Densovirus associated with sea-star wasting disease and mass mortality. Proceedings of the National Academy of Sciences, 111(48), 17278–17283. https://doi.org/10.1073/pnas.1416625111
Jackson, E. W., Pepe-Ranney, C., Johnson, M. R., Distel, D. L., & Hewson, I. (2020). A highly prevalent and pervasive densovirus discovered among sea stars from the North American Atlantic Coast. Applied and Environmental Microbiology, 86(6). https://doi.org/10.1128/aem.02723-19
Jørgensen, B. B., & Revsbech, N. P. (1985). Diffusive boundary layers and the oxygen uptake of sediments and detritus1. Limnology and Oceanography, 30(1), 111–122. https://doi.org/10.4319/lo.1985.30.1.0111
Mayer, M. P., & Bukau, B. (2005). Hsp70 chaperones: Cellular functions and molecular mechanism. Cellular and Molecular Life Sciences, 62(6), 670–684. https://doi.org/10.1007/s00018-004-4464-6
Mladenov, P. V., Igdoura, S., Asotra, S., & Burke, R. D. (1989). Purification and partial characterization of an autotomy-promoting factor from the Sea Starpycnopodia helianthoides. The Biological Bulletin, 176(2), 169–175. https://doi.org/10.2307/1541585
Motokawa, T. (2011). Mechanical mutability in connective tissue of starfish body wall. The Biological Bulletin, 221(3), 280–289. https://doi.org/10.1086/bblv221n3p280
Oulhen, N., Byrne, M., Duffin, P., Gomez-Chiarri, M., Hewson, I., Hodin, J., Konar, B., Lipp, E. K., Miner, B. G., Newton, A. L., Schiebelhut, L. M., Smolowitz, R., Wahltinez, S. J., Wessel, G. M., Work, T. M., Zaki, H. A., & Wares, J. P. (2022). A review of asteroid biology in the context of sea star wasting: Possible causes and consequences. The Biological Bulletin, 243(1), 50–75. https://doi.org/10.1086/719928
Remple, K. L., Silbiger, N. J., Quinlan, Z. A., Fox, M. D., Kelly, L. W., Donahue, M. J., & Nelson, C. E. (2021). Coral reef biofilm bacterial diversity and successional trajectories are structured by reef benthic organisms and shift under chronic nutrient enrichment. Npj Biofilms and Microbiomes, 7(1). https://doi.org/10.1038/s41522-021- 00252-1
Yamaguchi, M. (1977, January 1). Population structure, spawning, and growth of the coral reef asteroid Linckia Laevigata (linnaeus). ScholarSpace. https://scholarspace.manoa.hawaii.edu/handle/10125/1178
Zhang, W., Ding, W., Li, Y.-X., Tam, C., Bougouffa, S., Wang, R., Pei, B., Chiang, H., Leung, P., Lu, Y., Sun, J., Fu, H., Bajic, V. B., Liu, H., Webster, N. S., & Qian, P.-Y. (2019). Marine biofilms constitute a bank of hidden microbial diversity and functional potential. Nature Communications, 10(1). https://doi.org/10.1038/s41467-019-08463-z
