The largest side-gilled slug here, often with a commensal shrimp riding on it.
A large side-gilled slug to about 25 cm, the back covered in flat-topped polygonal tubercles set close together like paving and separated by paler grooves; the colour is brown, red-brown or cream and varies a good deal. A broad oral veil sits at the front and a single feathery gill lies along the right side between mantle and foot. The large Pleurobranchus of the region are not cleanly separated in the field — P. grandis shares the mosaic of tubercles — and the characters used in the literature are the shape of the tubercles and the colour of the grooves between them, which photographs frequently cannot resolve. Size helps: this is the one that reaches the length of a forearm.
Nocturnal, and feeds on colonial ascidians. Several species in the genus secrete mucus of very low pH, with sulphuric acid measured in some, which is why almost nothing troubles them; a disturbed animal may also swim clear of the bottom by flapping the mantle margin. Large individuals often carry the commensal shrimp Zenopontonia rex, long listed as Periclimenes imperator, sitting between the tubercles on the back.
Recorded from about 5 to 30 m on rubble and silt, and almost always after dark.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.
