By Dr Deborah Robertson-Andersson
Last week I suggested you to go down to the seashore to see what has been washing up on our beaches. I am lucky in that I work at the Two Oceans Aquarium and have access to a microscope which means that I can look at some things a little closer. The animals that we are seeing are very interesting as we seldom see them, as they are members of a community that lives close to the water surface and has been called neuston or pleuston.
Pleuston are organisms whose bodies project at least partly into the air (e.g. Blue bottles)
Neuston are organisms that live underneath the water surface film (e.g. Purple bubble raft snail)
Even though I spent a good while looking at the animals washing up on the beach I was surprised to find something very unique. It was a blue worm which I saw inside one of the buoy barnacles we had collected. When I asked my colleges, they had never seen or heard of it either. What followed over the next two weeks was the marine biologist’s version of CSI. And here are the results…
Hipponoa gaudichaudi agulhana Audouin & Milne Edwards, 1833
This worm is a slow moving carnivore. There is a single record of this worm in the world and it is at the South African museum and it was found by Prof. John Day off Port Elizabeth in 1967.
It is an amphinomid polychaete and belongs to the same class of worms as fire worms. Most fire worms are found on corals, rocks and other hard substrata covered with attached organisms. They swim well and are brilliantly coloured. These worms when irritated will raise their bristles which are hollow and contain poison, and they break off on contact with a predator. Ask anyone who has had a close brush with these worms it HURTS when they get you!!! The genus Hipponoa is entirely parasitic on goose barnacles and they have hooked bristles which are very efficient clinging organs (perfect for staying on a floating buoy barnacle. If you do see one, it’s best to pick them up with forceps. Unfortunately the only pictures I have are taken with my cell phone. But good luck to you in finding one.
Buoy goose barnacle Dosima fascicularis :
Are “the most specialised of the goose barnacle” species. They hang downwards from the water surface, held up by a float of their own construction, and are carried along by ocean currents. They are found world wide between the latitudes 71° North off Siberia to 57° South near Cape Horn and are at home in the warmer waters of the Atlantic and Pacific Oceans and are sometimes stranded in large numbers following onshore winds. It is easy to distinguish them from other species of goose barnacles as their shell (exoskeleton – they are related to crabs) is exceptionally thin and brittle and they have a float attached to them. The young (cyprid larvae) are planktonic, and must attach to a float for metamorphosis (change) into the adult form. They usually settle on small floating particles in the water, and as they grow they produce a spongy secretion from modified cement glands, comparable in texture to polystyrene, which keeps them near the surface of the water and is increased in volume as the barnacle grows. Other individual barnacles attach to the float and in this way a large colony is constructed.
Why are they washing up? This is a more scientific answer. Buoy barnacles and other neuston or pleuston collect at the South Atlantic sub tropical convergence zone. This is the area in the ocean where a major boundary between the warmer, nutrient poor Subtropical Surface Water in the north meets the colder, nutrient rich Subantarctic Surface Water of the Southern Ocean. Because mixing is very slow in water, organisms and marine plastic pollution tend to collect and create a visible boundary line. There are accounts of these lines extending for over 200 nautical miles. This line moves slightly with the seasons and it is normally around 40 °S. The South Atlantic current runs west to east along the front and passes west of Cape Town. This current would carry neuston or pleuston from the subtropical convergence zone and brings it past Cape Town. During the last month we have been receiving a lot of westerly winds. These winds would bring neuston or pleuston in this current to our seashores. This is why we have been getting all of these wash ups of interesting animals.