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

Abstract

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 and frequent turns.

†The species described here is Tritonia tetraquetra (Pallas 1788). In the recent modern literature, it has been referred to as Tritonia diomedea (Bergh 1894), but this name now has junior synonym status (Martynov 2006).

Keywords:

• Mollusca
• Gastropoda
• nudibranch
• Tritonia tetraquetra
• Tritonia diomedea
• rheotaxis
• geomagnetic
• tidal flow
• orientation
• counter-turning

Acknowledgments

The authors thank all those divers who helped in collecting data for this study – Glen Brown, Kenny Broad, Jerry Mahoney, Nat Scholz, and Michelle Herko. Figure 7 was created by Elizabeth Murray. The authors acknowledge Mike Huynh for video analysis, and Andreas Beckmann for deckhand, culinary, technical, and moral support. The authors are particularly grateful to Russell Wyeth for helpful discussions and construction of the Lab 11 flume. The assistance of the Friday Harbor Labs staff is gratefully acknowledged for slug collection and dive equipment. The authors thank three anonymous reviewers for comments that helped improve the manuscript. The California State University East Bay provided research support, including a Faculty Support Grant and a College of Science Sieber Interdisciplinary grant. This study was also supported by a grant from NIH (RO1 NS22974) to A.O.D. Willows, by research funding to A.P. Jones from HHMI-MILES Program (UMT), and by an NIH training grant (PHS 5T32GM07108-19) and Grass Foundation Fellowship to J.A. Murray.

Notes

†The species described here is Tritonia tetraquetra (Pallas 1788). In the recent modern literature, it has been referred to as Tritonia diomedea (Bergh 1894), but this name now has junior synonym status (Martynov 2006).