Day 7 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 seventh in a series of blogs about the marine life we hope to discover then.

Day 7:  6 September 2020 | Molluscs

Molluscs are slugs and snails and things like that. Surprisingly, that includes squid, cuttlefish and octopus.

  1. The slipper limpet, a sort of sea snail or gastropod is an invasive non-native species, from N America. It out-competes native species, and by growing on scallops makes it difficult for them to escape predators.
  2. Limpets are also gastropods. Not often found on pontoons, the blue rayed limpet is my favourite marine species. When young, its gorgeous blue stripes make everyone want to see one and they go “ooh!, it’s so cute” but when it’s old, looks like a toenail clipping and lives hidden away in the holdfast of some kelp, no one loves it. Bit like life, really.
  3. Chitons look like woodlice, but are more like limpets. Overlapping plates (or valves) help them bend round sharp corners.
  4. The Pacific oyster has two shells making it a bivalve. We already had a native species of oyster, but then introduced this one for oyster farming not expecting it to reproduce. Wrong. So it is now widely spread as an invasive non-native species, and can often be seen on harbour walls.
  5. The mussel is another bivalve, very common on marina pontoons. They create a biogenic reef, a rough carpet of shells held together by byssus threads – strong, silky strands that each mussel produces to anchor itself to the seabed and to other mussels. Barnacles and worms fix themselves to the shells and small crabs, brittlestars and sea snails hide in the gaps. Fish, crabs, gulls, whelks and even otters feast on them. They store carbon in their shells and the sediment below that would otherwise be released into the atmosphere. As filter feeders they suck water in, take what they need out of the water – oxygen, plankton, dead stuff and poo – and squirt it out again. Just one square metre of mussel bed can filter 150,000 litres of water per day. The resulting increase in water clarity lets light penetrate more deeply encouraging greater primary productivity, and thus greater secondary productivity. And the same might apply to mussel farms. On the down side the reef structure slows the water flowing over the mussels and increases turbulence resulting in a three-fold rise in ingested plastic.
  6. If you know any divers, you’ll know they get excited about sea slugs and nudibranchs, but you can find them on the pontoons. Polycera quadrilineata are quite common in UK marinas. These are laying eggs. Finding eggstrings is often an indicator that they are about. They eat sea mat – coming up tomorrow in blog 8.
  7. Squid, cuttlefish and octopus are also molluscs, and very occasionally (only once, in my case) one will appear on CrabCam.

John Hepburn, Ocean Discoverability Project Manager
6 September 2020