Predation of Sagartiogeton Laceratus Upon Aurelia Aurita in Shallow Water

Extract

Sagartiogeton laceratus Carlgren. was observed in 1979 in large numbers (11–20 m) in shallow water (5 m below chart datum Sullom Voe) at Whaal Firth, Yell, Shetland, at a site named Stany Pund. It was recorded at a nearby site, Poita, at a lower density (1–2 m) in deeper water (6–8 m below CD.), and was also recorded as ‘common’ at the shallow head of the firth.

Very large numbers of Aurelia aurita (L.) were observed within the sheltered, shallow arm of the firth, within the upper few metres of water.

Each year since 1979 the same occurrence has been noted, and in the summer of 1980 the predation of Sagartiogeton upon Aurelia was observed and photographed on several occasions. These observations were repeated in 1982.

Sagartiogeton laceratus is found attached to shell fragments in silt, the column inclined towards the current. During ingestion the tentacles shorten, and the mouth, disc, and column, distend many times their normal dimensions. (Estimated to be 400% for the column, disc.)

It seems likely that the occurrence of both species in sufficiently large numbers to make predation noteworthy is due to the position and depth of Whaal Firth. It forms a north facing ‘trap’ on the eastern side of Yell sound through which ebb and flood tides move rapidly. The total length of Whaal Firth is approximately 7 km of which 4 km are relatively exposed, and 3 km sheltered.