Part of the new residential development on Thesen's island, Knysna.
14 shaggy sea hares are highlighted in this photo, in shallow water.
and 18 shaggy sea hares in this one!
A few together
An adult photographed at Long Beach, Simon's Town
A younger individual also photographed at Long Beach, Simon's Town
Detail of the young individual, showing the pattern and dark-edged blue spots.
All images Guido Zsilavecz.
Shaggy sea hares are known to occasionally occur in large numbers - to quote Gosliner, 1987: "... population numbers fluctuate sporadically. Thousands of individuals can be present in a small area of a lagoon, in very dense aggregations, but may be entirely absent a few weeks later." The calm waters formed by the new channels dug into Thesen's island, as part of the marina for the new residential development, seems to be perfectually adequate for these sea hares, and they are now abundant, as the images show.
A similar population explosion was experenced in Cape Town in mid 2007, where large numbers were found at Long Beach, which in many ways offers a similarly sheltered environment (with less current, if somewhat more wave action.) They were absent a few months later.
Shaggy sea hares feed by ingesting sand, filtering out anything nutricious, and discarding the rest. They reach a length of at least 15 cm, and the smaller ones are worth looking at in detail, as they exhibit a beautiful pattern between the shaggy bits, including some dark-edged blue spots, as the last photo shows.
For more details see "Nudibranchs of the Cape Peninsula and False Bay", now available from SURG and better Cape Town dive shops.
Nudibranchs of Southern Africa - a guide to the opisthobranch molluscs of southern Africa, T Gosliner, 1987.
Nudibranchs of the Cape Penisula and False Bay. G Zsilavecz, 2007.