Daily tracking of the locomotion of the nudibranch Tritonia tetraquetra (Pallas 1788 = Tritonia diomedea Bergh 1894) in nature and the influence of water flow on orientation, crawling, and drag

We observed orientation and locomotion of the nudibranch Tritonia tetraquetra in its natural habitat using SCUBA over many sequential days, in three different months. The slugs oriented significantly headfirst to tidal currents. Nevertheless, the direction of locomotion of the slugs over hours was not usually correlated with tidal flow direction (i.e. not indicative of consistent rheotaxis). We did not find evidence of consistent body axis orientation to the geomagnetic field, but the direction of locomotion of some groups of slugs over hours was significantly correlated with geomagnetic direction. Independent of direction, each slug changed position by an average of ∼2 m during a single tidal phase (∼6 h), and changed position by an average of ∼4 m over a full tidal cycle (∼25 h). Orientation to flow reduced drag, and reduced the probability that a slug will be dislodged from the soft bottom, in laboratory experiments. Slugs deprived of olfactory and flow cues exhibit a search-like pattern of multiple an...