Colonies of Stentor polymorphus associated with Ramshorn snails

This article and the accompanying video illustrate the consistent association of Stentor polymorphus colonies with Ramshorn snails in a Cambridge pond.

Avid pondwatcher, Michelle Fleming, had posted images on Facebook of her Ramshorn snails sporting green somethings in the hollows of the centre shell whorls. Her pond is situated in a residential garden, just off Newmarket Rd in Cambridge.

The images were tantalising enough for me to invite Michelle and partner Mike to visit with some specimens for a closer view under the stereo microscope.

Viewed between 6x and 20 times magnification, green trumpet shapes became visible, that retracted on being touched. At less than a millimetre in length, they were identified as a colony of Stentor polymorphus.

Stentor species are large single celled organisms with cilia at the end of their bell shaped aperture, which waft in food to be eaten. S. polymorphus absorbs chlorella algae, which live symbiotically within its body, giving the Stentor species its distinctive green colour.

The unusual feature of the specimens from Michelle's pond is that they appear to be consistenly associated with the Ramshorn snails (Planorbarius corneus). Furthermore. They preferentially form in the centre of the snail shells whorl, which forms a bowl shaped environment. Whether this is due to selective settlement by the Stentor or because of removal by abrasion during the snails motion is not known, thought the former appeared more likely.

The specimens studied were unaffected by the snails motion or occasionally being agitated in the water during sharp turns of the petri dish in which they were observed and it took physical contact with a leaf or brush to induce contraction.

Future readers could check their ponds to see if they observe a similar association and whether it is limited to snails with a concave centre to their whorls.