Two views of the glass squid. In the top image one can see a diver's tank in the back.
Both photos Kate Jonker
Kate spotted this little (8 cm long) squid at Rooi Els, and had this to say: "Where we spotted the glass squid was about 10m from the edge of the reef in a wide but sheltered gully with a sandy bottom. The depth of the water was 6m and the little chap was halfway up the water column, so I would say it was at about 3m. Quite a playful little fella zooming around in a space of about 50cm square when we approached him. He was about 8cm long from one end to the other and about 4cm in diameter. I seem to recall he had a "sort of light" - similar to a comb jelly.
There had been a strong SE wind blowing for about a week before this (date of photos 19 December 2009) so he must have been swept in from outside the bay. We didn't see any more at the time. Water temperature was about 18 degrees, visibility 10m."
Members of the "glass squids" (Family Cranchiidae) are open-water species, spending much of their time near the surface where their translucency renders them much less visible. The opaque bit inside the squid is a digestive gland, which is usually held vertically to reduce its visibility from above and below, where most predators would be. It may possess a light organ at the bottom as well. Having a light emiting organ at the lower surface is something other marine animals (including some fish) use to reduce their silhouette against a pale sky - I am assuming that's the function for the light organ in the squid as well, given its the only real opaque bit apart from the eyes.
The family consists of 13 genera with about 60 species, many of which are undescribed.
Thanks to Mark Norman for confirming that this is a glass squid.
Young, Richard E. and Katharina M. Mangold (1922-2003). 2008. Cranchiidae Prosch, 1847. Bathyscaphoid squids. Version 22 April 2008. http://tolweb.org/Cranchiidae/19411/2008.04.22 in The Tree of Life Web Project, http://tolweb.org/
Cephalopods, a world guide, M Norman, 2000