Diguet’s Warty Sea Slug, Pleurobranchus digueti

Diguet’s Warty Sea Slug, Pleurobranchus digueti, Underwater photographs taken in Zihuatanejo Bay, Guerrero, March 2019. Photographs courtesy of Ron Woheau, Zihuatanejo.

Phylogeny: Diguet’s Warty Sea Slug, Pleurobranchus digueti (Rochebrune, 1895), is a member of the Pleurobranchidae Family of Side-gill Slugs. The Genus Pleurobranchus is one of seven genera in that Family, and there are twenty-one species within the genus. Diguet’s Warty Sea Slug is also know as  the Pacific Side-gill Slug and Warty Side-gill Slug. In Mexico it is called  babosa de mar verrugosa de Diguet. Historically, Diguet’s Warty Sea Slug was identified as the Side-gill Slug, Pleurobranchus areolatus, which is found in the Atlantic. They were separated in into separate species in 2005.

Morphology: Diguet’s  Warty Sea Slug is uniquely structured with a flat mantel that is covered with numerous bubble-like tubercles of varying sizes.  The rhinophores have horizontal striations that run from the base to the tips; some individuals have white spots on the rhinophores. The foot, which can project from the mantel in some individuals, is semitransparent and tan to orange in color with gray-brown irregular spots. In juveniles the border between the tubercles is white while in more mature animals it is a light tan to peach color. Their inner shell is calcareous and purplish-white in color. Diguet’s Warty Sea Slug one of the largest species of side-gilled slugs or Pleurobranch found along the Pacific coast of North and Central America. The inner shell is small, 4 mm to 5 mm, and oval in shape with a uniform width for its entire length. Diguet’s Warty Sea Slugs reach a maximum length of 19.0 cm (7.4 inches).

Habitat and Distribution:  Diguet’s Warty Sea Slugs are found on, and under, rocks. They reside intertidally and to depths up to 30 m (98 feet).  They range from Southern California to Ecuador. In Mexican waters, Diguet’s Warty Sea Slug is found in all waters of the Pacific including the entire Gulf of California.

Diet:  Diguet’s Warty Sea Slugs are predators that feed primarily on ascidians (sea squirts).

Predators:  Little is known about predation of Diguet’s Warty Sea Slugs, other than that they are known to be preyed upon by pycnogonids (sea spiders). Similar species of sea slugs are known to be eaten by sea turtles. Sea slugs in this genus secrete an acidic fluid through their skin as a defense mechanism which is believed to limit predation to specially adapted predators.

Reproduction: Diguet’s Warty Sea Slugs are hermaphrodites. They reproduce sexually, with internal fertilization and reciprocal copulation. The eggs are laid in a wavy ribbon of egg capsules. Once the eggs hatch, they become planktonic veliger larvae, which feed on other plankton.

Ecosystem Interactions:  Diguet’s Warty Sea Slugs are thought to host ectoparasitic copepods. Otherwise, little is know about their relationship with other species.

Human Interactions:  Diguet’s Warty Sea Slugs have no significant impact on human activities. From a conservation perspective they have not been formally evaluated however they are relatively common,  with a fairly wide distribution and should be consider to be of Least Concern.

Synonym:   Pleurobranchus (pleurobranchus) digueti