Sea swallow
Photo Emil H Niksch
The sea swallow is a sea slug - nudibranch, but unlike most nudibranchs it is not a reef-dweller, but instead floats (upside down!) at the surface, which it achieves by ingesting air bubbles. Its resemblance to a portuguese-man-of-war bluebottle is not co-incidental - those form part of its prey, so it makes sense to look like one, in order to avoid becoming prey yourself - not that anything is bound to eat it, as it has the ability to pass the stinging cells of the blue bottle through its system without "firing" them, and these cells then become parts of its defence!
As with blue bottles, it spends its life out at sea, and only when there's a combination of the right wind (or wrong wind, from its point of view), and current, is it thrown inshore, as with blue bottles. It is not recommended picking it up, as it is most likely going to sting as much as bluebottles will too.
The species itself is rarely seen, but wide-spread throughout the world.
For more details on this species, see "Nudibranchs of the Cape Peninsula and False Bay" or "A field guide to the Marine Animals of the Cape Peninsula", available directly from SURG or better dive and book stores in Cape Town.
Nudibranchs of the Cape Peninsula and False Bay, G Zsilavecz, 2007
A field guide to the Marine Animals of the Cape Peninsula, G Jones, 2008
Nudibranchs of Southern Africa - a guide to the opisthobranch molluscs of southern Africa, T. Gosliner, 1987
Nudibranchs of the world, H. Debelius, R. Kuiter, 2007
Two Oceans, A guide to the marine life of Southern Africa, GM Branch, CL Griffiths, ML Branch and LE Beckley, 1994, Revised Edition 2010