Scientific name : Asteroidea
If a contact exists betweeen marine biology and science fiction, this must be close to sea stars. And not only for their shape, recalling space trips. Analyzing closely the anathomy of a sea star, we will se how many details exceed the creativity of the best sci-fi writers.
Sea stars or starfish belong to the phylum Echinodermata, class Asteroidea. All Echinoderms are original animals, adopting unique survival strategies (see also sea urchins). Even if far from the main evolutionary line, their success is clear, looking at the number of species and at the abundance in some environments.
Like most Echinodermata, they have a radial simmetry: that is they have no head and tail, but a star-shaped body, mouth in the center of the lower side, no anus. Sea stars' simmetry is usually pentaradial, that is the majority of the organs is repeated along 5 arms.
The body wall is reinforced by hard dermic plates, and it is quite rigid. In the lower part 5 radial grooves start from the mouth (at the center), or anyway one for each arm. Those protect the pedicellaria or tube feet, tubular appendages filled with water, flexible and very mobile, with a sucker at the tip, used to crawl but also to handle some items (for instance, to open a bivalve shell).
The pedicellaria movement uses a unique system in the animal kingdom: an aquiferous system, with vessels similar to the blood vessels, but containing circulating sea water from the external environment. Pumping water the sea star can inflate the pedicellaria and keep their mobility. This circulating water comes from the outside environment, through a porous plate visible on the back of the star, usually away from the center, often with a different colour.
Sea stars are obviously benthic animals, living in all the marine environments from the shallow bay, lagoon, back reef, to the reef front, external reef, deep reef.
The majority of the species are carnivorous, feeding on benthic invertebrates like bivalve molluscs, sea urchins, hard corals. Some species are scavengers, some are herbivorous and feed on algae and detritus.
An interesting aspect: even being predators, starfishes are teethless. Some species swallow the whole prey, rejecting the undigested parts (hard shells), other species can vomit the stomach out of their mouth, pre-digest the prey on site (inside its shell if it is a bivalve, on the colony surface for hard corals), and suck it when liquified.
Fecundation is external, eggs and sperm emission is synchronised and the planktonic larvae, after many developmental stages, will settle on the bottom.
Sea stars have a unique shape, even if one can have doubts with armless species like this Culcita novaeguineae. When we turn it upside down we can see clearly the pentaradial structure.
The more similar group is that of brittle stars (class Ophiuroidea), having 5 thin, long and very flexible arms, around a rounded or pentagonal central disc. Sea stars' arms are more rigid.
Another feature of starfishes is their regeneration power. If an arm is detached, a star can easily regenerate the missing limb. Infact many species can mutilate themselves (autotomy) abandoning an arm to the predator but saving life.
A detached arm in most species can regenerate the missing parts if it has at least a portion of the central disc still attached. In the long-armed starfishes (genus Linckia) the regeneration of a whole new sea star can start from only a piece of an arm
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