Tuesday, May 14, 2013

The epitokes of Autolytus: swarming to mate

On April 24th, the spring 2013 Embryology class collected invertebrates in the large boat basin in Charleston. We scooped up rapidly swimming polychaete worms from the water's surface. These members of the Autolytus genus from the Syllidae family are benthic dwellers (benthos is Greek for “depths of the sea”). To reproduce, they undergo an incredible reproductive transformation known as epitoky (epitokos, Greek for “fruitful”). Depending on the species, either the entire worm or its posterior end develops chaetae for swimming and large eyes, and the gut is degenerated as well. This epitoke then swims up to swarm and mate. In cases where only a portion of the worm is transformed, the rest of the worm resumes its normal benthic life, but the epitokes usually die after spawning. For the subfamily Autolynae, the female epitokes are called sacconereis. We most often see bright orange epitokes, though the picture at left shows a brilliant blue-green sacconereis about 30 mm long.

Swarming in epitokes may be triggered by such factors as rising water temperatures and the lunar phase. The male circles the female, wrapping her in a mucous trail of sperm. The eggs are fertilized as they emerge, and either undergo planktonic development or, as is the case in the species shown here, collect in several brood sacs on the ventral side of the sacconereis. Even after larvae hatch from the brood sack, they hang on to the female. Can you see their eyes staring out?

This larva from one of the brood sacks of the above individual is a 500 µm long nectochaete, with chaetae bristling from three segments like legs on a cartoon stick figure. The apical tuft at its anterior end contains the sensory cells that may be involved in detecting the chemical cues during larval settlement. Once out of the brood sack, the larva may propel itself using several transverse bands of cilia. Three of the ciliary bands are visible here - one on its head, one just anterior to the first pair of chaetae, and one at the posterior end. The chaetae may also aid in locomotion. Four large ocelli (eye spots) are arrayed anterior to its large muscular pharynx, the clear oval shape anterior to the gut. The gut itself is filled with greenish yolk droplets packed into the egg by the mother. There will be no need for this nectochaete to feed during its brief life in the plankton. It will live on yolk reserves until settling out to begin its benthic existence.

Dorresteijn A & Westheide W. (2010). Reproductive strategies and developmental patterns in annelids. Dordrecht London: Springer.

Rouse G & Pleijel F. (2001). Polychaetes. Oxford New York: Oxford University Press.

Rouse G & Pleijel F. (2006). Reproductive biology and phylogeny of Annelida. Enfield, NH: Science Publishers.

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