These stars of the sea twinkle from all depths of the ocean, being found at the intertidal margins through to the abyssal depths and are abundant in both tropical and polar oceans. Together with their close relatives, sea urchins, cucumbers, lilies and brittle stars they have a rich and ancient fossil record. Their ancestors were first seen near the start of the Cambrian period around 540 million years ago, proving their resilience in a world of change over the aeons. But what are they exactly and what do they do? What is life like for a starfish? Here we take a look at some of the peculiar habits and traits of starfish and some of their glorious kin; behold the Echinoderms.
Starfish belong to a group of invertebrate animals termed Echinoderms which means spiny skinned in Greek. These entirely marine creatures were first described by Aristotle in the 4th century BC and include sea urchins, sea cucumbers, sea lilies and feather stars, starfish and brittle stars. Although, rather confusingly not all have spiny skin, they do all possess an external skeleton of calcified plates held together by a flexible mesh of collagen fibres. This is very much apparent in sea urchins for example, whose plates are fused in to a rigid ‘test’. In starfish it forms armour plating that is reinforced at the vulnerable arm tips and corners for extra protection from predators. They also share a body plan which has a radial or five-fold symmetry and a unique water vascular system that powers hundreds of sucker-like tube feet and transports nutrients and gas around the body.
Starfish appear to lead a sedentary life as they are mostly seen motionless on the seabed. A casual observer may wonder that they move at all in fact, until you realise that life carries on at a much slower rate for starfish than humans. A thorough and laborious study in the seventies involving hours of scuba diving and filming revealed their behaviour to be extremely complex. Bouts of wrestling between individual starfish were observed but not as dynamic displays of physical prowess. Instead these bouts involved discrete arm movements, a sustained push here or an arm held aloft there. These protracted confrontations are usually for dominance at feeding sites where competition can be fierce. Most starfish are carnivorous and feed by everting the stomach, engulfing the prey and secreting digestive enzymes. When feeding on shellfish such as mussels, the tube feet on opposing arms may also be used to pull apart shells to access the soft flesh. Others scavenge on corpses and use the same technique to dissolve the flesh. Carried out at leisurely starfish pace this is a sustained yet ponderous affair by our measure and therefore only works on prey that cannot amble away.
The slow-motion, sedentary life style of starfish means they need some effective strategies for defending themselves, hence the heavily calcified armour and spines in some starfish. Others harbour a chemical surprise in their body walls in the form of saponin, which tastes extremely unpleasant to predators. Some species, like the tropical starfish Astropecten polyacanthus have gone a step further. Their tissues are infused with the deadly tetrodotoxin. This potent nerve-blocker is 100 times more poisonous than cyanide and has the ability to subdue an attacker far larger than the diminutive starfish. A careless nibble on this echinoderm can lead to respiratory failure and death, a powerful deterrent indeed. Such an encounter is much less traumatic for the starfish as they also possess remarkable powers of regeneration. Severed limbs can be regrown several times over, so long as a portion of the central disc remains. Stories of fishermen chopping starfish in to pieces to protect mussel beds from predation, only resulting in more starfish are true. In typical starfish style though regrowth is a lengthy process and it can take up to a year for an arm to reach former proportions.
Relatives of starfish, the sea cucumbers have a rather less deadly but gorier defence tactic. When ambushed they eject their guts and other organs which are sticky and unpalatable to the attacker. The cucumber then regrows the digestive system and must feed by absorbing nutrients through its body wall in the meantime. Extreme measures are required for a creature that lies otherwise unprotected on the seabed.
Aristotle’s writings were the first recorded observations of starfish and marine life in general and are the reason he is often referred to as the father of marine biology. When he and fellow Greek philosophers of the 4th century BC looked to the ocean they saw stars. But they also gazed at the other great mysterious expanse that surrounded the earth, the night sky and pondered the scintillating celestial bodies above. Their insight remains valid to this day and set the stage for modern-day astronomy. Recent discoveries have shown that all elements found in the universe were made in the heart of stars during super novae explosions billions of years ago. Over the course of those years these elements have been recycled in to many different entities. It is humbling to think that some of the atoms that make up our bodies may in the last 400 million years at least, have been part of a primitive echinoderm. And that some tiny part of each of us may well know what life is like for a slow-motion starfish…