Observations on Life History of a Gregarine of the Striped Shore Crab,Pachygrapsus crassipes. ∗

Abstract

The common striped shore crab, Pachygrapsus crassipes, of the Pacific Coast is frequently infected by a Cephaline gregarine which inhabits the mid- and hind-gut, the infection rate sometimes running from 70% to 100%. No correlation is apparent between the extent of infection and the locality or ecological habitat of the host, nor does there seem to be seasonal distribution of the infection.

Apparently, the entire life history of this gregarine takes place in the lumen of the mid-gut and hind-gut of the host, no intra-cellular stages having been seen. Trophozoites, forms “in copula”, encysting stages, and cysts may all be present at the same time in the digestive tract.

The classification of this gregarine depends largely upon the method by which the infection is transmitted from one host to another. In the family Porosporidae, the encysted gregarines are transferred in the form of a multinucleated cyst to a mollusk; this cyst invades the gills and produces infective spores for the crab. On the other hand, there are certain genera, such as Uradiophora and Cephaloidophora for which no intermediate hosts are necessary.

It is possible to obtain parasite-free crabs in the laboratory by starving and isolating them for 2 weeks. “Clean” crabs so obtained were kept alive in separate aquaria for several months by feeding them on a diet of chopped liver. The intestinal contents of infected animals were then fed to these clean crabs, and the latter were examined at intervals for gregarines. It was possible to transmit the infection from crab to crab; control animals captured at the same time but not fed infected material showed no parasites in their gut. This artificially transmitted infection proved in some cases to be exceptionally heavy. All of the various stages of this parasite could pass through the anterior portion of the digestive tract of the crab unharmed. Infection thus may be transmitted without the intervention of any intermediate host.