George SB and Strathmann RR (2019), Arms of larval seastars of Pisaster ochraceus p...

ECB-ART-47049

PLoS One 2019 Jan 01;143:e0213803. doi: 10.1371/journal.pone.0213803.

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

George SB , Strathmann RR .

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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.

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	Fig 1. Turning ventrally by strokes of a posterolateral arm.The right posterolateral arm (pl) is directed ventrally and strokes. The left posterolateral arm is directed dorsally. The larva turns ventrally (a counterclockwise rotation as viewed toward its left side). Numbers at the upper left of the frames are time in seconds. Arrows indicate the direction of motion of the right posterolateral arm.
	Fig 2. Turning dorsally by strokes of the posterolateral arms.The posterolateral arms (pl) are directed dorsally. The larva turns dorsally (a clockwise rotation as viewed toward its left side). Numbers at the upper right of the frames are time in seconds. Arrows indicate direction of motion of the posterolateral arms. From 0 through 10 seconds both posterolateral arms stroke. From 11 to 13 seconds the left posterolateral arm moves little while the right posterolateral arm continues strokes.
	Fig 3. Swimming forward with side-to-side strokes of the posterolateral arms.The posterolateral arms are directed posteriorly and move from side to side like a fish’s tail. Numbers at the lower right of the frames are time in seconds.
	Fig 4. Lateral strokes of the posterolateral arms.The posterolateral arms move apart laterally and then together medially. Numbers at the upper right of the frames are time in seconds.
	Fig 5. Pointing ciliary currents by pointing arms.The posterodorsal arms (pd) point anteriorly and the postoral arms (po) point posteriorly. Ciliary currents that run from base to tip of arm turned the larva dorsally (a counterclockwise rotation as viewed toward the larva’s right side). The posterolateral arms (pl) pointed posteriorly and did not stroke. Numbers at the upper right of the frames are times in seconds.
	Fig 6. Brachiolariae swimming in a column of water.The larvae had been introduced at the bottom of a column 45 cm tall. The line on the column is 15 cm from the top, and each dot is 1 cm away from the line. The dots and line mark the position of a halocline with 30‰ below and 20‰ above. Most but not all larvae are swimming upward, with anterior ends toward the top of the figure. Posterolateral arms of most individuals are pointed posteriorly. Posterolateral arms of two individuals (at lower right of the figure) are directed anteriorly.

References [+] :

Allen, Applying Movement Ecology to Marine Animals with Complex Life Cycles. 2018, Pubmed