You may be familiar with sea stars or starfish, sea urchins, and sand dollars if you are beachcombers and tide pool explorers, but what many may not know is that all are related. Additionally, sea cucumbers, brittle stars, and sea lilies, less known animals of the ocean, round out the family, Echinodermata, a word with Greek origins meaning ‘hedgehog skin.’ These curious little creatures can be found right here in Portugal, usually in rocky pools or even on wide sandy shallows if you are lucky. They technically aren’t fish at all, but belong to the class Asteroidea, and are often referred to as brittle stars, basket stars, sea stars or just asteroids, and they do not have gills, scales, or fins like fish do, and because of that, they are not classified as fish.
Around 1,900 species occur on the seabed around the world, from warm tropical zones to frigid polar regions, and in depths as far down as 6,000m deep below the surface.
Any number of arms
Starfish are marine invertebrates, and typically have a central disc and five ‘arms’, but some species have more, with the ‘sun star’ having as many as 40 arms. Their upper surface may be smooth, grainy or spiny and covered with overlapping plates, and can be brightly coloured reds, oranges and blues, or dull greys and browns, and can live up to an amazing 35 years.
With tube feet operated by a hydraulic system and a mouth at the centre of the oral or lower surface, they are opportunistic feeders and are mostly predators of invertebrates that live on the seafloor. The species have specialised feeding behaviours including eversion of their stomachs - a pretty weird habit - which basically means that when they capture prey, they have tiny suction cups to grab their food, then their stomach exits the mouth to digest the food, and re-enters the body when they’re done eating. They have complex life cycles and can reproduce both sexually and asexually.
Another fact is they have no brain nor even a brain-like organ in their body. But despite this, they do have a nervous system, albeit a simple one. Surrounding its mouth is a nerve ring that's connected to each of its arms via a radial nerve. Neurons stimulate the muscles on each of their tube feet, which are located on the underside of their body. They have no blood either and use filtered seawater to pump nutrients through their nervous system, and cannot survive in freshwater.
Move along, please
Movement is another thing they can achieve, despite seeming motionless, looking like a rock just sitting on the ocean floor. In reality, the hundreds of tube feet on their underside stretch and contract to create movement, and to hold prey.
Any one tube foot can act on its own in response to stimuli, but coupled together, they can synchronise to produce a bouncing motion—their version of running. For years, researchers have wondered exactly how they accomplish this, given it has no brain and a completely decentralised nervous system. The answer, according to researchers at the USC Viterbi School of Engineering, was published in the Journal of the Royal Society Interface - they couple a global directional command from a ‘dominant arm’ with individual, localised responses to stimuli to achieve coordinated movement. In other words, once they provide an instruction on which way to move, the individual feet figure out how to achieve this on their own, without further communication.
And they have a handy trick of regenerating missing limbs – but it can take up to a year to grow one back.
If a starfish is cut into pieces, each of the pieces can grow into a complete animal due to this ability to regenerate. Regeneration is a natural process of replacing or restoring missing cells, tissues, or organs. They can even grow their entire body parts to full function using this ability.
I don’t know why, but images of starfish always seem happy, and make me think of a little figure jumping for joy!