Date of Award
Program or Major
Doctor of Philosophy
Larry G Harris
The Pycnogonida is a class of arthropods with interesting life histories. Pycnogonids prey on hydroids and some invade hydranths while larvae. Males brood the eggs and larvae hatch as protonymphons. Questions relating to the evolution of life history characteristics were addressed. Evolutionary relationships were poorly understood. It was necessary to determine the relationships within the Pycnogonida and compared to other arthropods.
Twenty-four morphological characters were coded for twenty-three pycnogonid genera and one fossil ancestor, Palaeoisopus problematicus. A branch and bound analysis resulted in fifteen most parsimonious trees. The Nymphonidae were found to be basal. The Ammotheidae were paraphyletic and led to two clades. The first contained the Callipallenidae, and Phoxichilidiidae. The second contained the remaining pycnogonids.
A phylogeny was also compiled using sequences of the D3 expansion segments of 28S rDNA. This resolved relationships of sampled families as follows (Ammotheidae + ((Nymphonidae + Colossendeidae) + (Endeididae + (Pycnogonidae + Phoxichilidiidae)))). The Ammotheidae was found to be paraphyletic and basal. The results from the D3 region yielded perplexing relationships when compared with morphology.
Phoxichilidium tubulariae Lebour 1947 is a valid species. It appeared to be specialists on the hydroid Tubularia larynx. Annual population dynamics of P. tubulariae were seasonal. Density of adult animals was highest in mid to late summer with reproduction being greatest in July and August. The abundance of pycnogonids peaked as the hydroid population declined. Some populations were shown to have two generations. Adult migration may play a larger role in the distribution of this species than larval dispersal.
Phoxichilidium tubulariae had an atypical protonymphon type developmental mode that reduced the typical number of molts, and developed rapidly in the gastrovascular cavities of the host. It decreased developmental time from 35--40 days to 15--20 days. This was adapted to exploit the seasonal abundance of Tubularia larynx. The male looped the egg mass over his oviger. The larvae hatched, infected the hydroid, and developed inside the gastrovascular cavity of T. larynx. The larvae developed for several molts and then hatched, destroying the hydranth. The ancestral pycnogonid stock were external parasites. The internalization of the larval stages appeared to have happened at least twice.
Lovely, Eric Carl, "Evolution of pycnogonid life history traits" (1999). Doctoral Dissertations. 1975.