As with most marine organisms, life as a baby sea star begins with a planktonic larval stage. Their bodies resemble a spearhead and require high magnification to be seen. Later, they metamorphose into pentamerous juveniles, at which stage they begin to develop their well-recognised star shapes. It is at this point that their body parts begin to become more recognisable; mouths for eating, eyespots for detecting light brightness, and feet for walking. All this extra mass allows them to settle on the sea bed and life in its adult shape can begin.
Locally, a prominent species is the spiny sea star Marthasterias glacialis. This is primarily an eastern Atlantic species whose range extends from the UK all the way down to South Africa. It has been found on rocky or sandy environments, either subtidal or up to a depth of around 200 metres, a testimony to its adaptability. It is a very hungry sea star which consumes many types of bi-valve molluscs and other invertebrates. Often, they are found consuming a human sized portion of mussels, except, they like theirs alive. Without defined mouth parts, sea stars push their stomachs out of the bodies and digestive juices break down the prey externally. They then suck in the soup into the stomach within their bodies and the food is passed to digestive glands located in the arms. It is this strange way of eating which allows sea stars the ability to consume prey items much larger than themselves if required.
People often say that sea stars have five legs but this is not true at all. One of the features of Echinoderms is that they travel using a water vascular system. In essence it is similar to a water balloon; push on one end and the opposite end swells up. Sea stars have thousands of legs with sucker cups on the end in a system which resembles the octopus suckers. These stick to the surface of rocks, allowing the arm to hold on whilst at the same time releasing another arm and moving it along in the direction of travel. With five arms to choose from, this a quite a complicated walk but as any climber will attest to, a very stable one. They can achieve speeds of up to 15km per hour. Uncharacteristically for most animals, the eyespots are located at the end of the arms and this helps determine which limbs it moves. In some species like the brittle stars, these eyespots are speckled over the upper side of their body.
Sea stars have their own predators to fear and as part of that defence system our local spiny sea star can shed an arm as an escape response. This is similar to a lizard losing a tail and the process does not harm the animal in any significant way. The arm can be re-grown in a matter of months. On rare occasions it is possible that the single arm can be replaced by two which produces a six-armed sea star. This is rarer that the four-leaf clover so don’t spend too long looking for one!
Sea stars are commonly considered keystone species due to its wide-ranging diet. They help suppress the populations of many invertebrates and ecologically are very important indeed for maintaining ecosystem balance and biodiversity. Studies on the effect of sea star removal found that mussel beds were able to out compete all other organisms. It is a good sign that our local marine coastline has many examples of these sea stars which in turn, keep our mussel populations in check.
Evolutionary analysis through embryology has determined that of all the invertebrates the Echinoderms are our closest living relatives, a fact that can be hard to believe. The evidence lies in that they, like us, are deuterostomes, meaning “two mouths”. Our mouth is connected by a long gut to our bottoms, the two holes being the “mouth”. This system is exactly the same with all deuterostomes and is completely distinct to feeding mechanisms found in Arthropods or Molluscs. Therefore, this implies a common ancestry, albeit millions of years ago. This is further evidenced through a plethora of studies investigating genetic commonalities, which have found similar organ structural development even though their respective functions might have diverged. On the tree of life our branch then splits off to form the Chordates and it is from here that the Vertebrates arise.
So next time you walk down a beach and happen upon a sea star, take a good long look and remember that we share common ancestry, just 450 million years ago!
Images © Melanie Soiza-Stagnetto.