Sea stars can be found from shallow tidal pools to the to the deepest ocean trenches. They can sometimes be found in brackish water, but never freshwater. While they are commonly called starfish, they are not related to fish and are an invertebrate, which is why is more correct to refer to them as sea stars. Their closest relatives are basket and brittle stars, which are often referred to sea stars themselves.
Scientific & Common Names
Kingdom - Animalia
Phylum - Echinodermata
Subphylum - Asterozoa
Class - Asteroidea
Orders - Brisingida, Forcipulatida, Notomyotida, Paxillosida, Spinulosida, Valvatida, Velatida
Common Names: Starfish, sea star (considered more proper)
Starfish are most commonly seen with five arms in the traditional star shape, thus their name. There are species, however, that can have many more arms, even up to 40, although they still have five sections. Sea stars are typically only a few inches but the sunflower sea stars can have a radius of over three feet. They can be muted earth tones or display a cartoonish splash of purples, greens, blues, reds, and yellows. Stars have no blood and use filtered sea water as a vascular system. They can famously lose and arm to escape from predators and then slowly regenerate the limb. A few species are able to generate a full body from a limb, but it not as common as is often believed. Of the nearly 2000 identified species, not all the life spans are known, but they can range from 10-30 years.
Starfish can reproduce sexually or asexually, using fission where the body splits and eventually generates full bodies from the fragments. Those that produce sexually use spawning. The adults can be male, female, or hermaphrodites, which have both male and female organs. There are also species that can switch from male to female at a certain point in their maturity. After spawning, the eggs of most species become free floating plankton, but a few types will attack their eggs to rocks or even brood them. Those that are brooded feed from a yolk but the others receive nutrients from the water.
Starfish are unable to swim but they can move along the bottom or on any surface they can reach, sometimes with surprising speed. Stars can be pure predators, scavengers, or a combination. When consuming things like clams, they pry open the shell and then partially expel one of their two stomachs into the gap, enveloping the prey and consuming it in its own shell. When this stomach, called the cardiac stomach, retracts into the body the contents are then transferred to the pyloric stomach.
Due to their physical structure, sea stars do not tend to fossilize well, thus much of their history may have been lost. Fossils have been found as far back as the ordovician period and there is evidence of massive species extinctions in the Jurassic. Modern sea stars are closely related to urchins , as well as the similarly named brittle and basket stars.
Knobbly sea star (Protoreaster nodosus), often collected for the pet trade, are listed as endangered. Sea Star Wasting Syndrome has been noted since the 1970's, but it has recently increased in frequency, area, and virulence. The syndrome has been attributed to both bacterium (vibrio) and a virus. The syndrome is still being studied in an attempt to alleviate the massive die-offs in the American Pacific Northwest. There are species, such as the crown of thorns, that can be considered problematic if their numbers are too high in that they consume coral and can cause considerable damage.
Reef Secrets by Alf Jacob Nilsen and Svein A. Foss?
Guide to marine Life (Caribbean, Bahamas, Florida) by Marty Snyderman and Clay Wiseman
Petersen Field Guides Southeastern and caribbean Seashores by Eugene H. Kaplan
Reef Creature Identification (Florida, Caribbean, Bahamas) by Paul Humann and Ned Deloach
Invertebrates A Quick Reference Guide by Julian Sprung