How the ability to swim affects crinoid arm regrowth rates

Ability to Swim (Not Morphology or Environment) Explains Interspecific Differences in Crinoid Arm Regrowth

Biography: Delaney Young. She is an undergraduate student at the University of South Florida. She is currently working on her geology B.S. and will graduate in the summer of 2023. She then plans to obtain her geology M.S. starting in the Spring of 2024. 

Point of the Paper: The main point of the paper was to determine how arm regeneration rates of feather stars (occurring after injuries), a kind of crinoid, vary. Scientists examined the swimming ability of crinoid species, available food supply, severity of the injury, water temperature, number of regenerated arms, and the total number of arms in order to understand what drives differences in regeneration rates. The authors of this study found that the swimming crinoids regenerated arms up to three times faster than non-swimming crinoids. 

What data were used? 123 adult feather starts from eight different species were collected at depth in the ocean during two sessions (December 2016 – April 2017 and June – October 2018) in Malatapay, Negros Oriental, Philippines. The respective maximum arm length, the maximum number of arms, and the arm regeneration were compared. 

Methods. To study the rates of arm regeneration amongst swimming and non-swimming crinoids, the animals were collected at depths ranging from 5 to 35 meters. In the 2016 expedition, the individuals were captured and had a few arms removed by researchers. The researchers would pinch a feather star’s arm until it was voluntarily released as a means for amputation. The mechanism of voluntary release is used as protection for the crinoids. Researchers caused the crinoids to amputate an average of 3–5 arms, but some amputated up to ten arms. The animals were brought back to their original habitat after they amputated their arms and scientists measured their regrowth rates. In the 2018 expedition, the animals were caught and put in bamboo cages with mesh material on every side. The mesh allowed food particles to enter the cage, and the cage dimensions allowed the feather stars with the longest arms to extend them to the fullest. To mark a starting point for every animal, the measurements of maximum arm length and maximum arm number were taken for each feather star. The swimming or non-swimming ability of eight species from Malatapay, Negros Oriental, Philippines, was recorded and compared to the respective maximum arm length, the maximum number of arms, and the arm regeneration rate. 

Results. Of the eight tropical feather stars collected in Malatapay, Philippines, the rate of arm regeneration ranged from 0.29–1.01 mm/day (Figure 1). The species included two swimming and six non-swimming feather stars. The swimming feather stars experienced regeneration rates of 0.89–1.01 mm/day. The lower of the two rates (0.89 mm/day) was higher than the highest non-swimming arm regeneration rate. The impacts of total arm number and total regenerating arm number on rates of regeneration were larger in non-swimmers than in swimmers. There was no notable relationship between the number of removed arms and the rate of regrowth.

Image showing a graph of arm regeneration rates by color-coded species of feather star, with regenerating arm length on the y-axis and time on the x-axis. The image shows the arm length (millimeters) over time (days) and the mean regeneration rate of eight tropical feather stars. The six non-swimming feather stars of Family Comatulidae (Anneissia bennetti, Capillaster multiradiatus, Clarkcomanthus mirabilis, Comaster nobilis, Comatella nigra, Phanogenia gracilis) are shown in shades of blue. The one swimming family Mariametridae (Oxymetra cf. erinacea, Stephanometra indica) is shown in shades of red, orange, and yellow. The 95% confidence interval of each curve is shown in gray. Oxymetra cf. erinacea and Stephanometra indica show higher rates of growth.
Figure 1: Modified from Stevenson et al. (2022). This graph depicts the arm regeneration rates per species used in this study. Swimming crinoids showed higher rates of regeneration than non-swimming crinoids.

Why is this study important? The researchers found that swimming ability alone best explains the differences in arm regeneration rates amongst the swimming and non-swimming feather stars. Swimming ability in feather stars is thought to be an adaptation from the need to escape predators that live on the seafloor. Feather stars that lost limbs while escaping predators would need to regrow limbs quickly, as having missing limbs would negatively affect the animal’s ability to escape predators in the future. The rate of regeneration, while controlled primarily by swimming ability, is still affected by temperature, but to a lesser degree. In cold water, biological processes slow down, so crinoids in cooler waters with other forms of protection would have had better chances at survival. Scientists could relate this to the way that fossilized crinoids look to help understand the environment they lived in.

Broader implications. This paper can be related to paleontology because knowing if a crinoid was swimming or non-swimming can inform scientists of the likely regeneration rate of arms of organisms in the fossil record. Knowing these pieces of information can potentially give researchers more clues about the predation pressures that a fossil crinoid may have faced. We could hypothesize, for example, that crinoids in cooler waters may have had other forms of protection or retrieval, as a survival mechanism for the slowed biological processes caused by cooler water. The results of this paper could be compared to fossilized crinoids, so researchers can understand the ancient marine environments and ecology of crinoids. 

Citation: Stevenson, A., Corcora, T. C. Ó., Harley, C. D. G., & Baumiller, T. K. (2022). Ability to Swim (Not Morphology or Environment) Explains Interspecific Differences in Crinoid Arm Regrowth. Frontiers in Marine Science, 8. 

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