Stars of the sea
Biscuit star Pentagonaster pulchellus
Words and photos by Dr Malcolm Francis
When is a fish not a fish? Answer: When it’s a starfish, a jellyfish, a crayfish, a shellfish or a blackfish! We seem to like calling marine animals ‘fish’ regardless of whether they are or not. Hundreds of years ago, marine mammals were thought to be fish, so calling a pilot whale a blackfish was understandable. But the others are clearly not fish, though there are fisheries for many of them (including jellyfish in some places!). These days, we are gradually replacing inappropriate names with better ones. Crayfish are now called rock lobsters, and starfish are usually called sea stars.
Sea stars are a highly visible and important component of the rocky reef ecosystem of the Wellington south coast. The cushion star (Patiriella regularis) is the most abundant sea star in the Taputeranga Marine Reserve. Although it is small (up to 7 cm across), it is often seen scattered across rock flats, cobbles and sediment covered reef in shallow water. Cushion stars are often blue-green to greyish, but they can be pale orange-yellow and occasionally bright red, plus all colours in between. Like most sea stars, a cushion star feeds by everting its stomach through its mouth, and pouring enzymes on to its food, before slurping up the pre-digested soup. Clusters of cushion stars often aggregate on small patches of seaweed, rock or sediment covered rocks, and if they are gently turned over, the everted stomachs can be seen underneath. It’s not clear what they are actually feeding on – perhaps it’s the small crustaceans, molluscs and worms that inhabit the weed and sediment, but they may also gain nourishment from the organic detritus and algal films present in these habitats.
Seven-armed sea star
Some sea stars are large and predatory. The seven-armed sea star (Astrostole scabra) grows to over 40 cm across. It is easily recognised by its colour – the upper surface is greyish-purple, and its numerous tube feet are bright orange. Seven-armed sea stars, and their cousins the 11-armed sea stars (Coscinasterias muricata), are voracious, highly mobile predators that specialise in catching or forcing open molluscs such as paua and mussels. Paua don’t often move very fast, but I recently saw a yellow-foot paua (Haliotis australis) being chased (in a slow motion kind of way) out of a crevice and across a rock by a roving seven-arm star.
The biscuit sea star (Pentagonaster pulchellus) is often seen on rock walls covered with colourful encrusting invertebrates. It is usually brown and about 8 cm across, and is recognisable by its knobbly arm tips and the plates on the margins between the arms. Cushion and biscuit stars usually have five arms, but occasionally animals with four, six or seven arms can be seen. Related to the cushion star, but larger (up to 10 cm across) and rarer, the inflated cushion star (Stegnaster inflatus) inhabits rock crevices and walls. It arches its body so that a tunnel or ‘cave’ is created beneath itself, and then flattens itself downwards to trap any small prey such as shrimps and crabs that walk underneath. Inflated cushion stars come in a gorgeous array of colours – they are often grey-green or bright orange, but may also be purple or red.
Inflated cushion star
Sea stars move, capture their prey, and force open shells using their tube feet, which are driven by an amazing hydraulic system. Water is drawn into the body through a filter plate (madreporite) which is visible as a different-looking, often pale, plate on the upper surface. The water is pumped through a system of canals running around the mouth and along the arms. The tube feet, which are housed in grooves on the underside of each arm (yes, their feet are inside their arms!), extend as the water pressure increases. When the tube feet touch a shell or rock surface, the sea star pumps water out of the canals creating a slight vacuum, which turns the tips of the feet into minute suction cups. Large sea stars can exert high pressures with their tube feet over several hours, enabling them to slowly overpower the muscles of their prey, and prise them open or off the rocks. No wonder that yellow-foot paua was motoring away!