Day 6 of Life Beneath the Keel

On Sunday 13 September, at 1800 BST, The Island Trust is live streamingLife Beneath the Keel’ from Plymouth Yacht Haven as part of the Ocean Institute’s virtual Maritime Festival. This is the sixth in a series of blogs about the marine life we hope to discover then.

Day 6:  5 September 2020 | Crustaceans

Crustaceans are related to insects, are mostly marine, and there are lots of different species. But most of them are quite small and tend to be disregarded, which is a shame as we can find plenty of smaller crustaceans on our marina pontoons which repay closer attention. There is more to crustaceans than crabs and lobsters!

We’ll start with one group, which many would be surprised to find are crustaceans. Like many crustaceans, barnacles start their lives in the plankton before gluing their heads to something like a rock or ship’s bottom. They build a castle-like structure of bony plates around themselves, through the doors of which they wave their legs to filter stuff like plankton from the water and kick it to their mouths. If you have particularly good eyesight you may be able to see this happening in the wild, but it is really well worth a look under the microscope (image 1). In this YouTube video at the foot of the page you will also see the barnacle stop feeding when the light is dimmed. This ‘shadow reflex’ is a defence against predation by fish like shannies which find barnacle feeding appendages a tasty morsel.

Other less obvious and well known crustaceans you might find crawling around the pontoon jungles include Isopods (flattened top to bottom, like woodlice image 2) and amphipods (flattened side to side image 3).

You will find more characteristic crustaceans there, too, like these little crabs in images 4, 5 and 6.

Bigger crabs live on the sea bed, however you are not usually allowed to swim, snorkel or dive in marinas without permission (which is not easy to get). But you can catch them and there’s lots of advice on how to do that, and to report what you find in the Crab Watch Project. Or you can drop a baited camera to the seabed and see what they do in their natural habitat, as we will do on our ‘Life Beneath the Keel’ event. The Squabbling crabs in image 7 shows the usual situation. On the right, 3 Carcinus maenas, common shore crabs, squabble over the bait, while keeping at bay (foreground, left) a Necora puber, Velvet swimming crab, normally considered one of our most feisty crabs. At home in our waters, the common shore crab is known elsewhere as one of the ‘world’s worst alien invasive species’. Just visible in the background is a Maja brachydactyla, spiny spider crab.

Occasionally a European lobster might muscle in on the fray, but generally not for long as their claws don’t seem up to the fiddly task of extracting the bait from the net bag (image 8).

John Hepburn, Ocean Discoverability Project Manager
5 September 2020