By Sofía Paz Sedano – Published on 22nd September 2022
Within the phylum Mollusca, nudibranchs (or what may sound more familiar: sea slugs or naked mollusks) are commonly known for their bright and striking colors and shapes. The name nudibranch comes from presenting naked gills, due to one of their great characteristics, the loss of the shell. Nudibranchs are probably the most diverse mollusk group with a wide morphological disparity and ecological adaptations (Figure 1).
Nudibranchs currently include around 2,530 described species, inhabiting in most coastal areas of the world, from the poles to the tropics, from the intertidal to the abyssal zone, being more abundant and diverse in the warm tropical and subtropical waters of the Indo-Pacific region.
The fascinating and striking shapes and colors that make these animals known among nature enthusiasts are usually a consequence of the defense adaptations that the species developed to compensate for the loss of the shell. Some nudibranchs present chemical substances that can be toxic, acidic or repellent for their predators. They can also present stinging cells known as nematocysts. Thanks to the presence of these chemical substances, some species are aposematic, presenting a striking appearance with bright colors that warns of their potential toxicity. In other cases, a species may imitate the striking color of another toxic species, and different species share the same color pattern. However, not all nudibranchs are brightly colored. Some species also have cryptic colors and shapes, similar to their substrate, allowing them to camouflage themselves to avoid being eaten.
Nudibranchs have also developed different structures used as sensory organs. The most notable are the rhinophores, present on the dorsal part of its head. They also have oral tentacles on both sides of their mouths. The nudibranchs are carnivorous, their diet includes a wide diversity of animals such as sponges, anemones, gorgonians, or bryozoans. Nudibranchs are hermaphroditic and their life cycle begins with a trochophore and veliger larvae, which through metamorphosis changes to a juvenile state.
Body structures vary between different species of nudibranchs, morphology being important for species delimitation in taxonomic studies. The classification of nudibranchs has suffered various changes over time. Two suborders are currently recognized: Cladobranchia and Doridina. Both suborders have been the subject of several integrated taxonomic studies to explore species richness, ecology, diversification, and adaptation. These studies resulted in an exponential increase in species discovery, and the systematic rearrangement of them.
Currently, the suborder Cladobranchia stands out for presenting some biological characteristics such as the ability to sequester nematocysts or symbiotic zooxanthellae, or the evolution of a rhythmic motor behaviour. The monophyly of Cladobranchia is supported by both morphological and molecular data. On the other hand, species within the suborder Doridina are mainly defined by morphological variation in gills, anus and food structures. To date, phylogenies including Doridina families highlight the lack of resolution at internal nodes due to incomplete genetic representation.
Despite the high number of studies carried out in the last decades, there are still several groups within both suborders with important gaps in their systematic knowledge. The family Goniodorididae H. Adams & A. Adams, 1854, object of study of my PhD thesis, is not an exception. The family currently includes eight accepted genera (Okenia Menke, 1830, Goniodoris Forbes & Goodsir, 1839, Ancula Lovén, 1846, Lophodoris G. O. Sars, 1878, Spahria Risbec, 1928, Trapania Pruvot-Fol, 1931, Goniodoridella Pruvot-Fol, 1933, and Murphydoris Sigurdsson, 1991).
When I began to study this group of nudibranchs, one of the things that caught my attention was that there were species-rich genera, with dozens of species, and species-poor genera with just one or two. In addition, there were many undescribed species —as would be shown later on. Phylogenetic analyses based on morphological data had been performed only on two out of the eight genera, and the number of available sequences was very small. As I suppose it happens to many scientists when they start to specialize in a subject, the more I tried to know about this group of nudibranchs, the more gaps I found in their knowledge, and it led me to want to continue with studies on the Goniodorididae during my Bachelor’s thesis, Master’s thesis, and now PhD thesis.
From studying a single genus in my Bachelor and Master’s theses, I went on to studying all eight genera for my PhD. I started looking at the morphological characteristics of each genus, and I soon realised that some of the species reported as undescribed Goniodorididae in different papers, books and online posts could be specimens misidentified at the genus level. Therefore, to be able to continue with the description of the species and determination of the evolutionary relationships between them, the first thing to do was to clarify the morphological and anatomical characteristics of each of the currently recognised genera.
After an exhaustive search for the type species of each genus (the first species described with that name and which defines the characteristics of the genera), we found six of the eight type species. These species were studied by micro-computed tomography, with these analyses partially supported by the LinnéSys: Systematics Research Fund 2020 call. This technology allows to scan the anatomy of the animal and to 3D-reconstruct it by using specialized software, which leads to the study of structures maintaining their original position. As an example, it was possible to observe the spicular pattern of some of the species (calcareous structures within the body of the animals), which cannot be observed when dissecting the specimens due to breakage. In addition, we dissected some specimens to study the radula, cuticle and penis using a scanning electron microscope (Figure 2).
Afterwards, thanks to the clarification of the external morphology and internal anatomy of the type species and the differences between genera, integrative taxonomic studies that included morphological and molecular analyses led us to describing four new species. These new species belong to genus Murphydoris (Figure 3). Specimens belonging to these species had been widely cited in field-guides, papers and websites as Goniodoridella spp. However, our study showed that their characteristics do not match Goniodoridella but Murphydoris.
Our study of the Goniodorididae family shows, once again, the importance of integrating different methods in biodiversity studies. There is still much work to do within the family, but our results to date show that the most detailed is a species description (including morphological and molecular data), the smaller the margin of error for mis-identifications and interpretations.
Paz-Sedano, S., M. Candás, T.M. Gosliner & M. Pola (2021). Undressing Lophodoris danielsseni (Friele & Hansen, 1878) (Nudibranchia: Goniodorididae). Organisms Diversity & Evolution, 21(1), 107-117. https://doi.org/10.1007/s13127-020-00470-z.
Paz-Sedano, S., G. Díaz-Agras, M.T. Gosliner & M. Pola (2022). Revealing morphological characteristics of Goniodorididae genera (Mollusca: Nudibranchia). Organisms Diversity & Evolution, 22(1), 93-116. https://doi.org/10.1007/s13127-021-00508-w.
Paz-Sedano, S., D. Smirnoff, M. Candás, T.M. Gosliner &M. Pola (2022). Rediscovering the overlooked genus Murphydoris (Nudibranchia: Goniodorididae): the first phylogeny and addition of four new species. Zoological Journal of the Linnean Society. https://doi.org/10.1093/zoolinnean/zlac002.
Sofía Paz Sedano is a PhD student at the Universidad Autónoma de Madrid, Department of Biology. Sofía and colleagues have recently published two scientific articles on Nudibranchia with the support of LinnéSys, a research fund scheme organised by The Linnean Society of London and the Systematics Association.