When feeding, the large, whitish, translucent proboscis is everted like a sock turned inside out by muscles increasing the hydrostatic pressure of the haemolymph within it. Fretter & Graham (1994) stated that Trivia species feed on compound ascidians by biting through the indigestible test to reach the zooids within. The proboscis is rarely seen as it is usually hidden within the ascidian when everted. This image of a conveniently transparent species, Corella parallelogramma, shows that unitary ascidians are also eaten, and that in this case the test is avoided by forcing the proboscis through the ascidian’s oral siphon to feed on its internal organs. Apparent feeding by Trivia on other unitary ascidians, including Ciona intestinalis, has been observed by D. Kipling (pers. comm. 11/11/2018).

1: yellow rim of oral siphon of a transparent, unitary ascidian (sea squirt).
2: transparent, almost invisible, test of ascidian.
3: everted feeding proboscis of T. arctica inserted deep into ascidian.
4: faint, dull orange oesophagus within translucent proboscis. Colour may be ingested ascidian material.
5: opaque, dull orange tip within proboscis where buccal mass with jaw and radula are located. Colour may be of buccal mass and/or ingested ascidian.
6: inhalent respiratory siphon of T. arctica. Many on-line images of gastropods mistake the frequently extended respiratory siphon for the more rarely seen proboscis.
7: foot of T. arctica.

Oban, Scotland. October 2018. © D. Watson.