ASPECTS OF THE LIFE HISTORY OFCARCINONEMERTES ERRANS(NEMERTEA: CARCINONEMERTIDAE), AN EGG PREDATOR OF THE CRABCANCER MAGISTER

Abstract

The nemertean genus Carcinonemertes has been known for over a century as an epibiont of brachyuran crabs. Even though symbiotic life styles are highly unusual for nemerteans, only a limited amount of research has been devoted to the study of Carcinoneniertes. Worms of this genus infest crabs after a planktonic larval stage. They next remain dormant, encysted on the host's exoskeleton. Worms on female hosts migrate into the host egg clutch when it is oviposited for brooding under the abdomen. There they grow, mature, and lay their own eggs. While the basic outline of the life history of Carcinonemertes has long been known, it was only recently confirmed as a predator of its hosts' eggs (Wickham, 1978) . Earlier workers had suspected that Carcinoneniertes was an egg predator, but never actually observed feeding ( Humes, 1942 ; Kuris, 1971 ) . The worms have been considered generalists in terms of host specificity. But little is known of the extent to which Carcinoneniertes is capable of affecting host population dynamics. Carcinoneniertes errans Wickhani is the first of these worms found to be specific to a single host species : the commercially important Dungeness crab, Cancer niagister Dana. This worm recently was found to exist at epidemic levels on the Central California Dungeness crab population (Wickham, 1979a) . The egg mortality it has caused in this region is high enough (averaging over 50% of the eggs produced annually) to implicate this worm in the collapse of the Central California Dungeness crab fishery. Carcinonemertes occurs on a large number of ecologically and economically important brachyuran crabs, and is now known to be capable of having a significant effect on its hosts' reproductive output. The following study on the life history of C. errans was initiated for this reason. Aspects of the life history of C. errans were studied in both the laboratory and the field. Worm abundance in the field was investigated using crabs caught in trawls and traps at Bodega Bay and Eureka, California. These crabs were dissected and all worms observable were counted and their locations on the exoskeleton noted. Samples also were obtained from the egg clutches of ovigerous female crabs by fishermen in the field. Sampling was done by inserting forceps into the egg clutch approximately at the middle of the exposed portion of the clutch. Care was taken to insert the forceps deeply enough to remove complete egg-bearing setae. The egg samples obtained contained an average of approximately 100 egg-bearing setae which each held about 200 eggs. Dungeness crabs carry up to 2,500,000 eggs in a clutch, so these samples were approximately 1% of the clutch. The egg samples were placed in vials with a 5 % formalin sea water solution and examined later with a dissecting microscope.