Wednesday, June 2, 2010

Nudibranch veliger larva

Here are a few pictures of the veliger larva of the frosted nudibranch (a type of sea slug), Dirona albolineataIn the top picture, you can see two larvae inside an egg capsule. Many other capsules in the same egg mass held 6-8 larvae in each. The shell of the veliger protects the internal organs, e.g. the nervous system, digestive tract, and the retractor muscles, which allow the veliger to pull the velum and foot into the shell. A pair of statocysts (little capsules that work as balance organs) are visible as well, each with a statolith (a tiny granule) inside. One can also see the velum, which has two rounded lobes with long cilia used for locomotion in the water column once the veligers hatch from the egg capsules. 

The bottom picture shows a hatched veliger from the side, with a developed foot. On the back of the foot is the thin, visible operculum, which acts as a trap door, closing the shell opening when the velum and the foot are pulled in. When the velum is pulled in, the larva can’t swim and sinks to the bottom. Once the larval stage is complete, settlement cues in the environment induce metamorphosis  (transformation into juvenile stage). This includes permanent loss of the velum and shell (and other larval organs, like the muscle retractor), as the foot becomes the main locomotory organ of a sea slug.

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