Starfish are marine invertebrates common throughout the world’s oceans. There are nearly 2,000 known species and no doubt, as with practically all forms of marine life ( in particular the smaller ones), there are almost certainly many others as yet undocumented. They range from exceedingly small (less than half an inch) to an impressive few feet and come in just about every colour you can imagine – including blue, yellow, purple, red, and brown.
Starfish belong to the phylum Echinodermata, a group that includes sea urchins, sea cucumbers. brittle stars and sand dollars. While all animals are more or less symmetrical others have bilateral symmetry. Echinoderms have radial symmetry. In the case of starfish this, unsurprisingly, takes the form of a symmetrical star, usually with 5 arms but sometimes many more. Another unique characteristic is their method of moving, walking by means of hundreds of tube feet.
Although starfish all have the same basic plan there are many variations on the outer appearance . In combination with the often vivid colours many develop dramatic looking and sometimes poisonous spikes, spines and other protrusions. Although on first sight this looks like an exoskeleton (an external skeleton, such as possessed by insects and crustaceans) starfish skeletons are actually internal and covered by skin – the spines and spikes grow from this and poke through the skin.
Starfish show remarkable regeneration abilities. Losing an arm, or even a large part of their body is often not a death sentence for a starfish. The arm grows back. This ability improves their chances of survival after a predator taking a chunk out of them. They can also utilise it to reproduce asexually – one arm provided there is a portion of the starfish centre there can grow into a new individual.
Generally though they reproduce sexually. Most are either male or female with a few species being sequential hermaphrodites {they change from one gender to another during their life). They spawn by getting together and releasing sperm and eggs into the open water where a sufficient number will meet up and develop into embryos. Here an indication of their evolutionary history can be seen in the bilateral larvae they have. While in many ways they are fairly simple animals it looks like their radial symmetry is not a primitive state and in fact evolved from bilateral ancestors.
The diet varies from starfish species to starfish species – many are active predators eating other smaller and/or slower marine creatures including mussels, snails and other starfish. Other species ‘graze’ on immobile animals. One such species is the fearsome looking and aptly named Crown of Thorns starfish, which has been known to wreak havoc on coral populations in the tropics. Yet others are scavengers feeding on decomposing organic material and a few feed on plankton suspended in the water.
Starfish have a relatively complex nervous system as compared to, for example, sponges. While they do not have a central brain as such they are sensitive to light, orientation, smell and touch. Experiments have shown that some starfish can detect whether other starfish are hungry or well fed by smell and behave accordingly in their search for the next meal. It is easy to underestimate invertebrates and their behaviour frequently turns out to be a lot more complex than it at first appears. There however is still a lot to be learnt about how starfish ‘brains’ function.
