Arms of larval seastars of Pisaster ochraceus provide versatility in muscular and ciliary swimming.

Sophie B George,Richard R Strathmann,Chung-Ping Lin

doi:10.1371/journal.pone.0213803

Sophie B George, Richard R Strathmann + Show 1 more

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https://doi.org/10.1371/journal.pone.0213803

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Journal: PloS one	Publication Date: Mar 14, 2019
Citations: 4	License type: CC BY 4.0

Affiliation: Georgia Southern University, Luxel (United States)

Abstract

Larval swimming with cilia, unaided by muscles, is the presumed ancestral condition for echinoderms, but use of muscles in swimming has evolved several times. Ciliation and musculature of the arms of brachiolaria-stage larvae in the family Asteriidae provide unusual versatility in the use of muscles in swimming. The muscles affect swimming in two different ways. (1) Contraction of muscles moves the arms, propelling the larva. (2) Contraction of muscles changes orientation of the arms, thereby changing direction of ciliary currents and direction of swimming. New observations of the brachiolaria of the asteriid seastar Pisaster ochraceus demonstrate more versatility in both of these uses of muscles than had been previously described: the posterolateral arms stroke in more ways to propel the larva forward and to change the direction of swimming, and more pairs of the arms point ciliary currents in more directions for changes in direction of swimming. Morphology of brachiolariae suggests that these uses of muscles in swimming evolved before divergence of the families Stichasteridae and Asteriidae within forcipulate asteroids. This versatile use of muscles for swimming, both alone and in combination with ciliary currents, further distinguishes the swimming of these brachiolariae from swimming by larvae of other echinoderms and larvae of acorn worms in the sister phylum Hemichordata.

Highlights

Ciliary swimming without the aid of muscles is the general and presumably ancestral case for the feeding larvae of echinoderms [1]
Both uses of muscles occur in swimming by the brachiolaria larvae in the family Asteriidae and perhaps in some other families in the order Forcipulatida
For a larva viewed from its left side, the turn was by a counterclockwise rotation toward the ventral side (Fig 1) or clockwise rotation toward the dorsal side (Fig 2). In these frames from a video recording, the left posterolateral arm is in the plane of focus and the right posterolateral arm is more distant from the plane of focus

Summary

Introduction

Ciliary swimming without the aid of muscles is the general and presumably ancestral case for the feeding larvae of echinoderms [1]. Muscles of echinoderm larvae supplement swimming in two ways. One way is for muscles to create a current by moving part of the larval body. Another way is for muscles to redirect ciliary currents by changing the form of the larval body. Both uses of muscles occur in swimming by the brachiolaria larvae in the family Asteriidae and perhaps in some other families in the order Forcipulatida. Brachiolaria larvae develop 5 pairs of larval arms

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