Sorting out Sea Slugs of the Genus Pleurobranchus

My colleagues Ángel Valdés, Yolanda Camacho-García, Vinicius Padula, Michael Schrödl, Juan Lucas Cervera, Terrence Gosliner, and I have published a new paper in the Zoological Journal of the Linnean Society. 

Figure 1. Pleurobranchus varians from the Hawaiian Islands (left) and Vanuatu (right)

More than 200 years ago, the great French Zoologist Georges Cuvier established the genus Pleurobranchus for a group of sea slugs with rudimentary internal shells. Over the years, roughly 70 species names have been proposed for species within the genus, many based on color pattern or features of the internal shell. Nevertheless, fewer species than this are recognized as valid and work on other sea slugs have shown that some species names corresponding to different color morphs are actually a single species with broad color variation, whereas in other cases more than one cryptic species have been united within a single species name. Given that some species within this genus have been shown to possess compounds of medicinal interest, knowing species boundaries and evolutionary relationships within the genus, our main aim of this paper, has broad importance.

Figure 2) Pleuobranchus albiguttatus from Papua New Guinea

Our new paper is based on genetic sequences obtained from 72 representatives of Pleurobranchus sea slugs from around the world. We used these data, along with observations of morphological characters to address the systematics, biogeography and evolution of these sea slugs. Among our samples, we found strong support for fourteen (14) species, for which we provide updated descriptions. Among several notable findings in our paper, we show that there have been multiple colonizations of species from the Indo-Pacific region to the Atlantic. We also resurrect the name Pleurobranchus varians (Figure 1), a species originally described from the Hawaiian Islands. Members of this species have been routinely identified as P. albiguttatus (Figure 2), which is one of the species identified as having a compound that is toxic to melanomas and which appears to be solely found in the Indo-Pacific.

A third interesting story remains unresolved and in need of further research. Pleurobranchus garciagomezi and P. reticulatus (Figure 3) are both native to the eastern Atlantic. P. reticulatus was described in 1832, likely from Principe Island, São Tomé, and P. garciagomezi in 1996 from the Cape Verde Islands. The data we present is consistent with a hypothesis that the two species names refer to just a single species. However, this result hinges on the identification of a specimen from the Canary Islands, which we could not confirm. As a result, we refrained from definitively synonymizing the two as P. reticulatus until more information is available. 

Figure 3) Color variation in Pleurobranchus reticulatus (A, B, C) and P. garciagomezi (D, E)

In addition to a revision and review of the systematics and taxonomy of Pleurobranchus, our paper also looks at the biogeography and times of divergence within the genus.

by Jessica Goodheart [Edited a bit by Allen Collins]

 

Further reading:

Goodheart, Jessica, Camacho-Garcia, Yolanda, Padula, Vinicius, Schrödl, Cervera, Juan L., Gosliner, Terrence M. & Valdés, Ángel. 2015. Systematics and biogeography of Pleurobranchus Cuvier, 1804 sea slugs (Heterobranchia: Nudipleura: Pleurobranchidae).  Zoological Journal of the Linnean Society. doi: 10.1111/zoj.12237.