Thursday, June 9, 2011

Brachiolar arms in late-stage asteroid larvae

This is the brachiolaria larva of a starfish, Pisaster ochraceous,that I raised during the Embryology class. Larval anterior end is up, and you are looking at the ventral side. The brachiolaria is characterized by the presence of brachiolar arms and an adhesive disk, and the bottom image shows a close up of the pre-oral protuberance of the frontal lobe that bears these structures. The brachiolaria larva follows the bipinnaria larval stage in the forcipulates, a group of starfish.

The brachiolaria is the last larval stage in these asteroids. It is characterized by three brachiolar arms (the three stubby arm buds near the top of the larva in the first image and zoomed in on in the second image) which surround a central adhesive disk (the brown spot between the lower two brachiolar arms in both images). These appendages have sticky cells and are used to make contact with the substratum when the larva is competent to settle. Some other asteroids, that have lecithotrophic, or non-feeding development, skip the bipinnaria stage, and directly produce large yolky brachiolaria with three brachiolar arms and an adhesive disk also used for settlement and attachment.

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