Starfish is not a true fish, except that it shares the same marine habitat with other fish in the ocean. Possessing a radially symmetrical body pattern (instead of bilaterally symmetrical) and tube feet for locomotion, it shares some similar traits with sea urchins. Thus, both are grouped under the echinoderms, and the correct name for starfish is sea star. Around 2,000 different types of starfish are identified from varied habitats of the oceans, including the tropical coral reefs and deep sea floor.
Starfish Identification and Classification
The features and adaptations exhibited by this echinoderm are amazing. At one glance, you can identify a starfish from its distinctive arms, which are usually five in number. But, it is a misconception that starfish have strictly five arms, as there are species having more than 20 arms. Simply flip over a starfish, you will notice small tube like feet that serve as locomotory systems. Also, its mouth is present in the ventral side (at the near center), and eyes are located at the tips of the arms. The scientific classification of a common starfish is presented below.
Common Types of Starfish
Maximum number of starfish are found in the Pacific Ocean. And various starfish types that differ in their size, shape, color and the number of arms are identified. All of them share some common traits, like they neither have brains nor blood running in their system. They are carnivores, and feed on fish, clams, sponges and oysters. The size of these echinoderms averages 12 - 24 cm, while other species measure as small as 1 cm, or as large as 30 cm. Likewise, average weight of a starfish is 5 kg, but it varies for some species. Different types of starfish and how they differ from each other are presented below.
The true sea stars or starfish belong to the class Asteroidea. Under this taxonomic class, there are again variant types of starfish - horned sea star, bat star, blue sea star, carpet sea star, eleven armed star and so on. An interesting fact about starfish is, they have the ability to regenerate their lost arm and parts (if any). Thus, a whole organism can be generated from smaller starfish fragments. The body is covered by spine like structures, which help in defending themselves from prospective predators.
This group of echinoderms are categorized under Crinoidea class. As the term suggests, they have feathery arms. All types of crinoids or sea lilies have 5 arms, and each arm is bifurcated into two, resulting in 10 arms. These arms are further divided into many branches, giving a feather like appearance. Unlike the true sea stars, some of the feather stars can swim freely. In short, the feathery arms are an adaptation for locomotion purpose. A beautiful species of this group is the passion flower feather star, which looks like a pinkish flower.
Brittle stars, also known as serpent stars, are found abundantly in the shoreline and deep sea floor. Grouped under Ophiuroidea, the number of brittle stars is highest than any other sea star. The individual arms of these sea stars are flexible, and appear very fragile, hence the name. In comparison to true sea star, their central disk is much smaller. While some feed on detritus and organic matter that settle down at the sea bottom, other brittle stars eat suspended matter or hunt mollusks and planktons. Examples include Alexander's brittle Star and spiny brittle star.
Basket stars share the same taxonomic class Ophiuroidea with brittle stars. They are differentiated from their cousin brittle stars by the presence of several branching arms, which look like a basket. This complex branching of arms is a specialty and helps in catching planktons. Interested in their appearance, many aquarists select this sea star for aquarium decorations. It is quite challenging to keep basket stars in captivity, as they require large tanks and special feeding requirements.
There is no doubt that starfish survive for several years in the natural habitat, with their lifespan recorded to be 35 years. Despite the long lifespan, the population of these amazing echinoderms is decreasing. Main threats to starfish are marine pollution and mass catching for keeping as pets. Though they are not threatened or endangered, there is a strong need for conservatory programs to save these unique marine organisms and increase their number.