On 5 November 1958 our attention was drawn, by Mr A. K. Nagabhushanam, to the presence in Port Erin bay of large numbers of swimming gastropods. These gastropods proved on examination to be Pleurobranchus membranaceus (Montagu), a species generally believed to be exclusively benthic in habit. This paper is a report on this occurrence and on some aspects of the biology of P. membranaceus.

For 4 days these animals were abundant in the bay and hundreds could have been captured with a hand-net from the steps of the Raglan Pier. Only approximately forty specimens were taken, this number being ample for our purpose. Between 5 and 9 November, numbers were stranded on the beach by the receding tide. Although many local fishermen consented to look out for these easily recognizable animals, and the Port Erin research vessels were out each day, no record of their swimming outside the bay was obtained, although specimens were occasionally dredged. On 9 November they disappeared from the bay without trace, and otter trawl and D-net hauls in the bay yielded only two further specimens; none were found on frequent shore collecting expeditions. However, on 2 December five more specimens were captured swimming near the Raglan Pier and occasional individuals were seen in the same place on 9 December. During the periods of their abundance it was ascertained that they swam during the hours of darkness as well as in daytime.