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 Nudibranchs. The Pleurobranchus Genus is one of seven genera in the Pleurobranchidae Family, and there are twenty-one species within the Pleurobranchus Genus. They are also know as the Pacific Side-gill Slug and Warty Side-gill Slug and in Mexico as  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 into separate species in 2005. The genus name Pleurobranchus comes from the Greek words meaning “side-gill”. This species was named in honor of the French naturalist Léon Diguet, who made extensive studies and species collections on the Baja Peninsula.

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. They have a feathery gill between the mantle and the foot on their right side. 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 of 19.0 cm (7.4 inches) in length.

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). Diguet’s Warty Sea Slug is a subtropical to tropical Eastern Pacific species that is found in all Mexican waters of the Pacific Ocean.

Diet:  Diguet’s Warty Sea Slugs are predators that prey 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.