Re: white Dendronotus frondosus

Dear Jim,
Sometimes papers slip past me. I found the following summary of Mikael's project on the Malacological Society of London's website. It certainly does seem relevant, and further illustrates that we still have a lot to learn about these animals.

"Dendronotus frondosus (Ascanius, 1774) is a boreal nudibranch which appears in several colour variations. Some of these forms seem to be more distinct and the question is whether these are better regarded as separate species. In particular, I found some specimens on the Swedish West Coast that had an opaque white colour with dark purple-blackish blotches; the common Dendronotus here is brownish. As I had specimens of both colorations from the same locality, the biological species concept could be applied and tested using allozyme electrophoresis. Allozyme electrophoresis makes it possible to score the genotype as well as the alleles carried by each specimen. Under the null hypothesis that only one population is present in the sample, the genotype frequencies are expected to follow Hardy-Weinberg frequencies. That is, if we have two alleles at a particular locus with frequencies p and q (as counted from the electrophoretic analysis), the two homozygotes should be present in frequencies p2 and q2 respectively, and we should have a frequency of 2pq heterozygotes.
In the Dendronotus study, I found four loci where one allele was present in 100% of the brown specimens (i.e. all homozygotes) and another allele in 100% of the white specimens (all homozygotes for the other allele). Thus p = q = 0.5, and we expect that half of the specimens should be heterozygotes, yet none was found. As this pattern was repeated for four independent loci (Mpi, Mdh, Idh and Acp), this is strong evidence against the specimens coming from one population. Since the specimens came from dredge hauls at the same locality, they can be considered to be different species according to the biological species concept.
As this was further supported by morphological differences, I reinstated the name Dendronotus lacteus (Thompson, 1840) for the white specimens. What we should call D. frondosus remains another topic ..."
.. Tholleson, M., 1999(August) Nudibranch systematics and molecular data. [In] Bulletin of the Malacological Society of London, 33. http://socrates.edsc.ulst.ac.uk/bull/Bull33/b33.html.

Thanks,
Bill Rudman