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 genusEuretaster。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 ofEuretaster在海星中，生命是有争议的。传统的形态学研究使其成为更“先进”进化枝的高度衍生群体。另一方面，最近的分子研究表明Euretasterbelonging to the basalmost clade, which indicates it belongs to a particularly ancient and undiversified lineage.
Euretasteris 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 indeedEuretaster。
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, abyssalXyloplax—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 likeEuretaster和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.
与释放浮游配子的标准Echinoderm策略相反，几乎所有已知的翼刺病都会使他们的年轻人繁殖。令人难以置信的是，这些年轻人是由父母喂养的，尽管以一种相当间接和非自愿的方式被称为“食人族的外寄生虫主义”。发育中的幼虫能够刺激表皮组织，这反过来刺激了营养丰富的粘膜分泌的产生，从而滋养了年轻人。当他们离开这个准泥块时，他们已经长成一厘米的绝小海星。这些确实是海星世界的袋鼠！
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 ifEuretasteris 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.
Euretasteris composed of three described species:E. Cribosusof the Indian Ocean,E. insignis西太平洋，以及E. attenuatusfrom New Caledonia. But, as is often the case in poorly known creatures like these, things are not quite so simple.E. Cribosus和E. insignisare 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 theirEuretasterto species is to find out where the specimen originated (though any species identification is provisional at best).
Aesthetically speaking,Euretaster是一个更具吸引力的海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 nameEuretaster, 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 forEuretasteris stated to be near 30cm, but the specimens I’ve seen were less than half that. The apparent rarity ofEuretaster可能是由于其在野外选择的栖息地所致，大多数研究表明，偏爱距离礁石的沙质或泥泞的海底，偏爱30-50m的水域 - 地区的水族馆收集有限。
Furthermore,Euretasteris generally sporadic in its distribution, being absent from habitats that it should seemingly want to occupy. Confounding things further is a brief mention thatE. insigniswas 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 ofEuretaster,所以我们的大部分知识推断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 aPterasterthat 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. PerhapsEuretaster最好在其喂养策略中被描述为机会主义。
It seems unlikely that corals, either stony or soft, would make up a large part of the natural diet, especially given the presence ofEuretasterin 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 whatEuretasterwill 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 wildEuretasteris frequently observed with the emperor shrimp (Zenopontonia rex）。这是一种共同的关系，因此，虾被其海星同伴围绕着。海星没有明显的优势或劣势，但是虾可能会掩盖其彩色和令人讨厌的宿主的一定程度的保护，并且在遇到海星带来的更多喂养机会时，它的进一步好处是不断流浪。对于那些寻找一些不同的东西的勇敢的水族馆，一个带有沙质底部和最小生命岩石的小水族馆将为这位迷人的海星及其caridean伴侣完成最不寻常的生物底座。
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