Thecacera darwini Pruvot-Fol, 1950

In 1845, Charles Darwin was off the Chilean coast of South America. As ships naturalist on the Beagle he collected a variety of species which were then farmed out to specialists or housed in museums. As far as we know, only one nudibranch species was collected by him, and its fate is now unknown. However, it was mentioned twice in the literature, first by Alder & Hancock (1855), who identified it as a species of Thecacera collected by Darwin in the Chonos Archipelago, southern Chile and then by Bergh (1883) who listed the undescribed species.

It took one hundred and five years before the Frenchwoman, Alice Pruvot-Fol, found on the shelves of the Natural History Museum in Paris several jars of Thecacera from Isla Hoste, Chile, and described the species which she named in honor of Charles Darwin. The color of these preserved species was unknown and although Ernst Marcus (1959) redescribed the species, he could only add that it had spots and yellow edgings. Now, thanks to Michael Schroedl, who recently published a photograph and contributed this one to slug site viewers, we see this common Chilean species in its true splendor.

Thecacera, is a small genus in the family Polyceratidae with only six known species . They are visibly distinguished by having no frontal veil or oral tentacles and by having contractile rhinophores with incomplete sheaths and unbranched appendages on each side of the non-retractable gills. The species are found mainly in tropical waters, although the cold temperate Thecacera darwini is an exception. Reaching a size of 45 mm, Thecacera darwini advertises its presence with a white body and contrasting black spots. Golden yellow pigment tips the perfoliate rhinophores, papillate edges to the rhinophore sheathes and the five tripinnate gills.The extra branchial appendages, which are believed to be defensive, have white bases, a strong band of golden yellow and opaque white tips. Golden yellow lines are on either side of the tail and its tip. The Y shaped protrusion behind the rhinophore bears the genital openings, the male opening , which has an eversible spiny penis is anterior to the female.

This animal is extremely visible in the shallow subtidal and on the sides of salmon farm floats, where it feeds on the bryozoan Beania magellanica. Its body contains the chemical triophamine, which probably deters predators.

The known range of this species is from Juan Lopez, near Antofagasta, northern Chile (Schroedl, 1996) to Isla Hoste, off the Chilean side of Tierra del Fuego (Pruvot-Fol, 1950).

© The Slug Site, Michael D. Miller 1995, 1996, 1997,1998, 1999. All Rights Reserved.

---