The relationship between arm shape and lifestyle of brittle stars

The evolutionary relationship between arm vertebrae shape and ecological lifestyle in brittle stars (Echinodermata: Ophiuroidea)

Mona Goharimanesh, Fereshteh Ghassemzadeh, Barbara De Kegel, Luc Van Hoorebeke, Sabine Stöhr, Omid Mirshamsi, and Dominique Adriaens

Summarized by Emma Nawrot, who is a geology major at The University of South Florida and is currently a senior. Once she graduates, she plans on pursuing a career in Volcanology and Igneous Petrology. In her free time, she enjoys playing video games, hiking, and going to the beach!

What data were used? This study examined species of Ophiuroidea (brittle stars) that were specifically chosen to cover a large range in both species type and lifestyles that corresponded to distinct functions of their arms. Samples were gathered from fossils of the Persian Gulf and Oman between December 2017 to March 2018 and were classified based on their lifestyle and joint type. 

Methods: This study used a 3D analysis of a range of brittle star species to determine the structural relationships of their arm vertebrae (here, meaning the distinct pieces of their arms). Species were carefully chosen to cover a range of operational lifestyles associated to different usage of their arms. This included prehensile (grasping) and non-prehensile (non-grasping) species. At minimum, one specimen per species was utilized, across to seven families throughout the broader groupins within the ophiuroids. To obtain a better resolution of the structures, portions were removed from the middle and outer part of one arm per sample of brittle star for CT scanning. Every sample was CT-scanned using a HECTOR scanner. The vertebrae were digitally segmented from the CT scans produced and 3D models were created using a software called Amira. The acquired 3D model of each arm skeletal piece was then transformed into data that the computer could use to determine the differences in shapes between each of them. 

On the left is a diagram of a phylogenetic tree that displays the twelve different species used in the study and their relationships to one another as they have evolved through time. To the right of this tree are the 3D shapes of their arm skeletal pieces and joint types shown from two different perspectives: distal is shown on the left and proximal is shown on the right. The names of the joints are listed in grey and yellow boxes to the right of the shapes, with the grey representing the distal viewpoint and the yellow representing the proximal. The different species on the phylogenetic tree are highlighted in pink, yellow, and green depending on their lifestyle. Epizoic or living on another animal, are highlighted in pink. Endozoic or living within another animal are highlighted in yellow and epiphytic or growing on the surface of a plant are in green.
Phylogenetic tree of the 12 species used in this study and corresponding arm shape and life habits.

Results: The study showed that there was a significant amount of variability found in the arm vertebrae of different species of brittle stars. The results reflect how these structural differences represent specific adaptations, such as having the ability to hold onto other objects and creatures. Furthermore, unique shapes of arm vertebrae in brittle stars were found to be directly correlated to their functional and environmental lifestyles. It was observed that some species that were not strongly related, still converged to a comparable design in arm shape. Perhaps the most remarkable results that came from the study were the patterns of how the shape of brittle star vertebrae is directly associated to unique evolutionary adaptations. For example, the species Ophiura sarsii has longer distal arm skeletal pieces than Ophiocamax vitrea, which is a non-prehensile organism, meaning it cannot grasp onto objects. Ophiura sarsii is known to take part in significantly more hunting behaviors than Ophiocamax vitrea, and these longer arm skeletal pieces create a greater yielded force. This is an indispensable factor for its hunting activities as an active predator. 

Why is this study important? This study sheds light on the intricate nature of shape deviations in brittle stars and how these changes relate to their distinctive adaptations. It is the first study to connect morphological attributes of brittle stars to their modes of life using 3D modeling of their vertebrae. Through this modeling, insight was gained on the unique functional and ecological lifestyles of different species of brittle stars. Without understanding this relationship, we can’t begin to understand how these organisms changed over time and the evolutionary patterns they show. 

The big picture: Ultimately, this study will be extremely helpful in the future for inferring information that we can apply to the fossil record. For example, paleontologists often find disarticulated bits of ophiuroids where it’s difficult to ascertain their origins and morphological traits, so this could be helpful for researchers in pinpointing these patterns when there’s not much other data to go on.

Citation: Goharimanesh, M., Ghassemzadeh, F., De Kegel, B., Van Hoorebeke, L., Stöhr, S., Mirshamsi, O., & Adriaens, D. (2021). The evolutionary relationship between arm vertebrae shape and ecological lifestyle in brittle stars (Echinodermata: Ophiuroidea). Journal of anatomy.