Month: April 2023

The Spineless Swimmer

In wrapping up this blog about the extraordinary species, Hexabranchus sanguineus, I have decided to explore how the invertebrate sea slug is such a remarkable swimmer through the following research paper:

Title: Swimming Without a Spine: Computational Modeling and Analysis of the Swimming Hydrodynamics of the Spanish Dancer

Authors: Zhihua Zhou and Rajat Mittal

Journal: Bioinspiration & Biomimetics, 13(1), 015001

Hyperlink: https://iopscience.iop.org/article/10.1088/1748-3190/aa9392

Keywords:

Computational fluid dynamics (CFD): a method used to solve fluid flow problems numerically using computer simulations.

Hydrodynamics: The study of the motion of fluids and the forces acting on them.

Vortices: regions within a fluid where the flow is spinning or rotating around an axis

Drag reduction: the reduction of resistance to movement through a fluid

Lift generation: the creation of an upward force that opposes the force of gravity

Parapodia: fleshy protrusions along the sides of the Spanish Dancer’s body that are used for swimming

The study aims to analyze the unique swimming behavior of the Spanish Dancer, a marine gastropod that uses parapodia to swim, and investigate the hydrodynamics of its movement using computational modeling and analysis. The authors hypothesized that the vortices generated by the Spanish Dancer’s movement play a crucial role in reducing drag and increasing lift, which facilitates its efficient swimming behavior.

Their methods included using CFD simulations to model the hydrodynamics of the Spanish Dancer. They created a 3D model of the animal based on anatomical data, and simulated its swimming motions in water. The simulations allowed them to analyze the flow of water around the animal, including the formation of vortices and the resulting forces of lift and drag. To validate their simulations, the authors compared their results to experimental data from previous studies on the swimming of related animals. They also conducted sensitivity analyses to test the effects of variations in parameters such as swimming speed and body posture on the hydrodynamic performance of the Spanish Dancer. They found that the flapping motion of the parapodia generates complex vortical structures that provide lift and propulsion, while the body deformation helps to maintain stability and control. So the vortices generated by the animal’s movement were crucial in reducing drag and increasing lift. Additionally, the simulations demonstrated that the velocity and direction of water flow along the body of the animal differed depending on the phase of the swimming cycle, indicating that Hexabranchus sanguineus is able to adjust its hydrodynamics to optimize swimming performance. These hydrodynamic mechanisms are unique to the Spanish Dancer and enable it to swim effectively without a spine.The authors also suggest that the research could have implications for the development of new underwater propulsion systems inspired by the Spanish Dancer’s swimming behavior. The study sheds light on the unique and unusual swimming mechanics of the Spanish Dancer, which may inspire new approaches to underwater propulsion in robotics and other engineering applications. Further experiments could explore the specific biomechanics of the slug’s undulations, as well as the ecological factors that have driven the evolution of this unusual swimming behavior.

 

 

Citations

Zhou, Z., & Mittal, R. (2017). Swimming without a spine: computational modeling and analysis of the swimming hydrodynamics of the Spanish Dancer. Bioinspiration & biomimetics, 13(1), 015001. https://doi.org/10.1088/1748-3190/aa9392

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.