From Smiths' Sea Fishes
Sequence Peter Southwood
As mentioned in the News Section of April 8, Peter Southwood reported observing a biscuit skate "walking" along the bottom, with two odd fins appearing now and then from underneath the trailing edge of the pectoral disc. On a recent dive Peter once again saw a skate and decided to film, rather than photograph, this behaviour. The fin in question is the one shaded red in the diagram, which has the edge of the disc turned over to show the fin. In the video it is very clear that the skate pushes itself along the bottom, using both fins at the same time.
While it is using these small fins the pectoral fins are not used, except to generate a small amount of lift to allow for easier forward movement. Now and then a rippling along the trailing edge of the disc allowed the skate to speed up, but it only took up the more commonly known "flying" motion when it had enough of Peter's persistent following.
There is a group of skates called "leg skates", consisting of a number of genera, who have similar fins which are thought to be used for propulsion. The biscuit skate does not fall within that group, but clearly does use this method of slowly moving forward.
Unfortunately space restrictions does not allow us to publish the whole video, but a short clip is available, as an mpeg, here. It is recommended to loop your viewer, as it shows the motion better.
See "Coastal fishes of the Cape Peninsula and False Bay", available from SURG and better Cape Town dive shops, for more details about the biscuit skate.
Coastal fishes of the Cape Peninsula and False Bay, a divers' identification guide, G Zsilavecz, 2005.
Smiths' Sea Fishes, MM Smith and PC Heemstra, 1986.
Guide to the Sharks and Rays of Southern Africa, LJV Compagno, DA Ebert, MJ Smale, 1989.
www.fishbase.org