Exploring the Evolutionary History of the Spanish Dancer

Kingdom: Animalia

Phylum: Mollusca 

Class: Gastropoda

Order: Nudibranchia 

Family: Hexabranchidae 

Genus: Hexabranchus 

Species: Hexabranchus Sanguineus  

Welcome back! As we have explored Hexabranchus sanguineus throughout this journey, we have discovered that these dorid nudibranchs are a species from a subclass of gastropods called Opisthobranchia. In this post, we will be taking a deeper dive into the taxonomy of the Spanish Dancer by analyzing a few phylogenetic trees to investigate where this species fits in the larger evolutionary context of its relatives!

First, let’s look at figures 1, 2 and 3, all from a study done by Angel Valdés in 2002, which show the evolutionary relationships of nudibranchs within the larger group of gastropod mollusks. The study analyzed the phylogeny of the cryptobranch dorids, a group of nudibranchs that includes the Hexabranchidae family. Figure 1 shows a phylogenetic tree of the order Nudibranchia based on morphological characters, and it suggests that Hexabranchus sanguineus is part of a clade called Doridoidea, which includes several other families of dorid nudibranchs. Within Doridoidea, Hexabranchidae is placed as the sister group to a clade consisting of the families Discodorididae and Chromodorididae. This tree suggests that Hexabranchidae is a relatively basal group within the Doridoidea, meaning that it is one of the earliest diverging lineages within this clade. Figure 2 shows a molecular-based phylogenetic tree of the order Nudibranchia based on a mitochondrial gene. This tree confirms the placement of Hexabranchus sanguineus within the clade Doridoidea and suggests that Hexabranchidae is more closely related to Discodorididae than to Chromodorididae. Figure 3 shows another molecular-based phylogenetic tree of the order Nudibranchia based on a different mitochondrial gene and is also consistent with the placement of Hexabranchidae as a basal group within the Doridoidea. Across all three phylogenetic trees, nudibranchs are a diverse group with many different characteristics. However, they all share certain features, such as the absence of a shell, the presence of rhinophores for sensing chemical cues in the environment, and a radula for feeding. In terms of their evolutionary relationships, the trees suggest that Hexabranchus sanguineus is a member of the family Hexabranchidae, which is a relatively basal group within the Doridoidea clade of nudibranchs. They also suggest that Hexabranchus sanguineus is more closely related to other dorid nudibranchs in the families Discodorididae and Chromodorididae than to other lineages within the Doridoidea.

 

 

The phylogeny from Figure 4 uses data based off Valdés study to represents various spicule arrangements in cryptobranchidae dorid nudibranchs. As we can see, H. Sanguineus is most closely related to Bathydoris aioca. Both species share common characteristics such as lacking spicules. Bathydoris is the sister taxon to the Doridina which was discovered through both morphological and molecular analyses. Hexabranchus is the sister taxon to the Cryptobranchia or is in a more basal position within the Doridina. The tree also suggests that H. sanguineus more closely related to Chromodorididae rather than Discodorididae.

One feature that unifies members of the genus Hexabranchus is the presence of six gills arranged in a circular pattern on the dorsal side of the body. This is where the genus name “Hexabranchus” comes from, which means six-gilled. In contrast, other members of the Doridina clade typically have three or four gills. Another striking feature of Hexabranchus sanguineus that sets it apart from its closest relatives is their striking bright red or reddish-orange coloration, which is where it gets the other part of its name “sanguineus” or “blood-colored.”

 

 

References 

MolluscaBase eds. (2023). MolluscaBase. Hexabranchus sanguineus (Rüppell & Leuckart, 1830). Accessed through: World Register of Marine Species at: https://www.marinespecies.org/aphia.php?p=taxdetails&id=220381 on 2023-04-01

Penney, Brian K. “Phylogenetic Comparison of Spicule Networks in Cryptobranchiate Dorid Nudibranchs (Gastropoda, Euthyneura, Nudibranchia, Doridina).” Acta Zoologica, vol. 89, no. 4, 2008, pp. 311–329., https://doi.org/10.1111/j.1463-6395.2008.00320.x.

VALDÉS, ÁNGEL. “A Phylogenetic Analysis and Systematic Revision of the Cryptobranch Dorids (Mollusca, Nudibranchia, Anthobranchia).” Zoological Journal of the Linnean Society, vol. 136, no. 4, 2002, pp. 535–636., https://doi.org/10.1046/j.1096-3642.2002.00039.x.