Countryside Ranger column: What can be found in rockpools?

Despite this, there are a number of organisms which have evolved to survive these conditions.

There’s a little bit of everything in there – there are plants, grazing herbivores, filter feeders, scavengers and predators; a whole miniature ecosystem, in fact.

When it comes to rockpool predators, you might think of shore birds or carnivorous fish, or perhaps various species of crab. The latter, while equipped with fearsome claws, are actually more used to life as scavengers rather than hunters. In fact, some of the most voracious of predators are a little less obvious.

Dog whelk (right) with periwinkle

Dog whelks are one of many species of mollusc found on the shore. They’re often found alongside common periwinkles and other sea snails, but can usually be distinguished by their longer, pointier and lighter-coloured shell – periwinkles, by comparison, are more rounded and darker.

Dog whelks don’t look particularly scary but they are fearsome hunters of barnacles and mussels (admittedly not the most agile of prey).

They are equipped with a radula – rather like a tongue fitted with hard teeth – with which they can bore a small hole through the shell of their prey. The dog whelk can then squirt the prey with paralysing chemicals and digestive enzymes, which turn the poor beastie into a soupy mess that is then sucked up.

One predator that most will recognise is the common starfish, with its familiar five arms and warty, orange-brown upper side.

Common starfish

On the underside, things get very interesting. Along each of the arms are many small tube-feet which allow the starfish to attach itself to rocks and other surfaces. These help the animal to move around, but are also used in feeding.

Sometimes, the tube feet will simply pick up scavenged bits of food and pass it into the mouth, located in the middle of the central disc. However, the tube-feet can also be used to pick up larger prey and even to pry apart the two halves of mussel shells; and this is where things get really macabre. Once the shell is open, the starfish pushes part of its stomach out through its mouth and starts to digest the mussel. Once the prey is suitably liquified, the stomach will be pulled back in, bringing the remains of the mussel along.

It all sounds pretty grim, especially for mussels, which seem to be a target for all sorts of predators. However, they’re not entirely defenceless. They produce strong, sticky threads which they use to stick themselves to rocks. They can also use these threads to tether approaching dog whelks, effectively immobilising them. Unable to escape, the dog whelk will then starve to death whilst surrounded by its favourite food. Isn’t nature wonderful?