Despite the diversity of sea stars that abound in tropical waters, relatively few species find their way into aquariums with any regularity. One of the most interesting species—morphologically, behaviorally and evolutionarily—is also one of the most seldomly encountered: the “Striking Sea Stars” of the genus Euretaster. Almost nothing has been written on this group in terms of aquarium husbandry, and the scientific literature is similarly empty on basic elements of its biology. So let’s draw back the curtain and summarize what little is known of these enigmatic creatures. [above image credit: Mark Rosenstein]
The position of Euretaster within the sea star tree of life has been contentious. Traditional morphological studies have placed it as a highly derived group amongst the more “advanced” clades. On the other hand, recent molecular studies have shown Euretaster belonging to the basalmost clade, which indicates it belongs to a particularly ancient and undiversified lineage.
Euretaster is classified within the Family Pterasteridae, whose other common representatives are the “Slime” or “Cushion Stars” (Pteraster) commonly found in tidepools throughout the temperate waters of the Northern hemisphere. The remaining genera and species are ecologically limited to extreme depths, as are the members of two small and closely-related families. These three families together constitute the Order Velatida, whose only shallow water tropical species are indeed Euretaster.
As the presumed earliest diverging lineage of living sea stars, the Velatida grant us valuable insight into the origins of sea stars as a whole. The closest living relative appears to be the bizarre, abyssal Xyloplax—a small genus of tiny, armless xylophages (wood eating) so unusual that it was originally described as an entirely new taxonomic class of echinoderm.
From this modern ecological data, it is tempting to infer that the last common ancestor of the sea stars would have evolved in a similar abyssal habitat, but Cretaceous fossils indicate the shallow water species (much like Euretaster and Pteraster) were formerly plentiful. Instead, it appears that these ancient groups have been outcompeted in shallow waters by more recent taxa.
Pterasterids possess a truly unique morphology that is instantly recognizable. Unlike any other known sea star, they have a “canopy-like, secondary dorsal covering” known as the supradorsal membrane, and at the center of this membrane is a small opening called the osculum, which is muscular and capable of contracting like a sphincter [above video]. The space between these two dorsal epidermal layers is known as the nidamental cavity, and it is within this cavity that many unique adaptations can be seen. The muscular nature of this cavity allows for the inner volume to fluctuate when contracted, forcing water in and out of the osculum in much the same way a fish flushes water over its gills by moving its operculum. And like the gills of a fish, the nidamental cavity is filled with structures that allow for respiration to take place.
Nearly all known pterasterids brood their young, as opposed to the standard echinoderm strategy of releasing planktonic gametes. Incredibly, these young are fed by the parent, though in a rather indirect and involuntary manner that has been referred to as “cannibalistic ectoparasitism”. The developing larvae are able to irritate the epidermal tissue, which in turn stimulates the production of a nutrient-rich mucosal secretion that nourishes the young. By the time they leave this quasi-marsupium they have grown into a perfectly miniature sea star over a centimeter across. These are truly the kangaroos of the sea star world!
The common name slime star is fitting for this group, as many species are capable of producing a thick, slimy mucous that deters predators. This mucous contains a variety of chemicals (primarily soaplike saponins) that behavioral studies have shown can inhibit attacks from larger, predatory sea stars nearly without fail. The mucus is excreted out through small openings in the supradorsal membrane known as spiracles. It’s uncertain if Euretaster is similarly capable of producing toxic mucous, as studies have indicated they do not contain saponins (though they do contain other noxious chemicals) and aquarium specimens seemingly fail to produce mucous when agitated.
Euretaster is composed of three described species: E. cribosus of the Indian Ocean, E. insignis of the West Pacific, and E. attenuatus from New Caledonia. But, as is often the case in poorly known creatures like these, things are not quite so simple. E. cribosus and E. insignis are reported to differ in the relative number of spiracles (~40 vs. 45-100) and ambulacral spines (3-4 vs. 5-6). But a thorough examination of specimens from various geographic regions indicated that these numbers are too variable to distinguish between the species reliably, with specimens occurring where the two species’ ranges meet having intermediate morphologies.
This pattern indicates that we are potentially dealing with one widespread and variable species. The unique form described in 1984 from New Caledonia, E. attenuatus, is morphologically similar as well, primarily differing in having thinner (i.e. attenuated) arms. Clearly more specimens are needed to determine if there is a legitimate third species lurking in the South Pacific. Genetic sequencing will go a long way towards unraveling this taxonomic mystery. Until then, the best way for aquarists to identify their Euretaster to species is to find out where the specimen originated (though any species identification is provisional at best).
Aesthetically speaking, Euretaster is one of the more attractive sea stars for the home aquarium. The supradorsal membrane (topside) is varyingly mottled with reds, whites and browns. In addition to the base coloration of the membrane, there is a net-like pattern of thickened ridges, which in large specimens breaks down into a series of disconnected spine-like protuberances. This overlaid pattern also varies in coloration from red to bright white, which, combined with the variable underlying coloration, creates a seemingly endless combination of colors and patterns. It’s little wonder why this species is known commonly as the “Striking Sea Star”. The net-like pattern is alluded to in the scientific name Euretaster, which can be roughly translated from Ancient Greek as: the “true netted star”.
This sea star is rarely available—the only two specimens I’ve personally seen for sale were darkly-colored, with a highly contrasting red “netting” pattern. Max size for Euretaster is stated to be near 30cm, but the specimens I’ve seen were less than half that. The apparent rarity of Euretaster may be due to its chosen habitat in the wild, with most studies indicating a preference for waters of 30-50m on sandy or muddy ocean bottoms away from reefs—areas with limited aquarium collection occurring.
Furthermore, Euretaster is generally sporadic in its distribution, being absent from habitats that it should seemingly want to occupy. Confounding things further is a brief mention that E. insignis was formerly common along rocky shorelines of Labrador Island in Singapore, but that it has since become noticeably rarer—it’s even listed as endangered in Singapore! Given its vast geographical distribution, large depth range and occurrence in varying habitats, this is likely to be a species with wide ecological tolerances.
When discussing sea star husbandry, diet is first and foremost on every aquarist’s mind. Specific dietary preferences are poorly known for most tropical sea stars, and they are often described under the catchall term “benthic omnivore”. There is essentially zero authoritative research published on the dietary preferences of Euretaster, so the bulk of our knowledge has to be inferred from research done on related pterasterids and brief mentions found in some field guides. For instance, Richardson (2002) lists this species as being corallivorous, whilst Coleman (2007) describes it as preferring sponges and detritus. It’s uncertain if either author conducted any serious study on the actual gut contents or if these dietary inferences are based on behavioral associations of specimens found in the wild.
Studies done on related genera indicate a wide range of gut contents, ranging from the normal benthic animalian fare (sponges, hydroids, bryozoans, tunicates) to sediment to bivalves to ophiuroids (brittle stars!). There’s even mention of a Pteraster that has been proposed as a mucous-ciliary feeder, much like many corals are. The aquarium specimens I’ve dealt with have eaten pieces of clam and krill, and it can be presumed that any meaty item will be ingested. As to whether this species is safe to keep with corals is difficult to answer. Perhaps Euretaster is best described as opportunistic in its feeding strategies.
It seems unlikely that corals, either stony or soft, would make up a large part of the natural diet, especially given the presence of Euretaster in muddy habitats far away from coral reefs. But it’s also likely that in the confines of an aquarium largely devoid of food certain corals would present themselves as a tasty snack. Trial and error will determine what Euretaster will and won’t eat, but I’d wager that chemically-defended soft corals (Sarcophyton, Lobophytum, Nephthea, etc.) would probably be safe, while bivalves and tubeworms would almost certainly be on the menu.
For those looking to recreate interesting interspecies relationships in their aquarium, it bears mentioning that in the wild Euretaster is frequently observed with the emperor shrimp (Zenopontonia rex). This is a commensal relationship, whereby the shrimp is ferried about by its sea star companion. There is no clear advantage or disadvantage to the sea star, but the shrimp likely gains some amount of protection camouflaged against its colorful and distasteful host, and it further benefits in encountering a greater variety of feeding opportunities brought about by the sea stars constant wandering. For the intrepid aquarist looking for something a little different, a small aquarium with a sandy bottom and minimal live rock would complete a most unusual biotope for this fascinating sea star and its caridean companion.
References
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