Friday, May 17, 2013

Development of the nudibranch Janolus fuscus

Nudibranchs are arguably some of the most flamboyant animals of the Pacific coast. Janolus fuscus (shown here), a sub-tidal species that is commonly found on the floating docks in Oregon, is no exception. Surprisingly, it’s brilliant and delicate morphology is a beautiful display of camouflage. Janolus fuscus is remarkably difficult to spot when it is on its primary food source, the bryzoan Bugula pacifica. Its distinctly lined cerata break up the nudibranch's outline, making the body less conspicuous. Its translucent flesh allows the nudibranch to blend in. The two characteristics seem gaudy until the nudibranch is viewed in it’s native habitat, where they act as a veil. Often, the egg masses of J. fuscus are more visible than the adults.

Nudibranchs are simultaneous hermaphrodites, so a mature adult can mate with any other mature adult of the species. Nudibranchs deposit eggs in characteristic gelatinous masses. An egg mass of J. fuscus depicted here is a cylindrical, jelly-filled cord with egg capsules
arranged in a single row, resembling white beads on a string.

As you can see, each capsule contains numerous embryos. The embryos undergo spiral cleavage and develop into trochophore larvae at around the forth day after egg deposition.

Two days later, the trochophores develop into veligers, with a shell and velum. Veligers hatch after about two weeks of development, and spend about six weeks in the plankton feeding on unicellular algae. Then the larvae settle on a bryzoan (preferably Bugula pacifica) and undergo meta-morphosis to become a juvenile (e.g. loose the shell and become elongated). Unlike the larvae, the juveniles feed on bryzoan lophophores. The juveniles grow into mature adults, all the while camouflaged by the bryzoan upon which they depend.

M. Wolf. 2012. The reproductive ecology of a northeastern Pacific nudibranch, Janolus fuscus, with an examination of its endoparasitic copepod, Ismaila belciki, Biol. Bull. 222: 137–149.

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