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The pea crab Pinnixa chaetopterana (Brachy- ura: Pinnotheridae) lives in the tubes of the host poly- chaetes Chaetopterus variopedatus and Amphitrite ornata. While attraction to host odor cues is common in sym- biotes, P. chaetopterana resident in Chaetopterus tubes apparently show no attraction to water-borne cues from either host. Because crabs resident in Amphitrite tubes are significantly reduced in their maximum body size and thus clutch size, switching between hosts should occur. We therefore tested the hypothesis that the crabs use odor cues from conspecifics or from the competing crab Po- lyonyx gibbesi (Anomura: Porcellanidae) when locating a new host. Crabs collected from Amphitrite are signifi- cantly attracted to Chaetopterus hosts which contain a mixed sex pair of conspecifics. Females show a strong tendency to avoid hosts containing Polyonyx, but males do not. Neither sex is attracted to unoccupied Chaetop- terus or Amphitrite hosts, but both are significantly at- tracted to isolated conspecifics. This attraction could serve as a reliable means of locating suitable new hosts when switching hosts in the field. Attraction to conspecific odors would also increase the chances of finding a suitable mate already present in a new host.

1. The pea crab Pinnotheres maculatus is a host-generalist symbiotic brachyuran. When subadult posthard and adult crabs are removed from the bivalve Mytilus edulis, Mytilus host odor increases their nonspecific movement as well as their location of the odor source. Odors from other previously recorded host species, however, generally do not affect their searching behavior.2. Such response specificity may be due to olfactory induction to their host. Adult crabs removed from the blue mussel Mytilus edulis can be induced to respond to odor from the bay scallop Argopecten irradians concentricus. Such induction supports the plastic "chemical search image" concept.3. Antennules are the principal sites of chemoreceptors used in host location.

ABsTRACr Pea crabs living within the mantle cavities ofa variety ofbivalve hosts have several adverse effects. In blue mussels (Mytilus edulis L.), the crab Pinnotheres maculatus (Say) steals food strands and causes gil lesions. We studied the long-term stress of P. maculatus on its host by measuring shell accretion in the field, and by numerically characterizing shell shape. Shell form in M. edulis is presumed to reflect environmental and physiological history. We computed growth increments in infested populations of mussels over a three-month period at two sites with high and low nutrient regimes. When growth was measured by change in shell length, significant differences between mussels with and without large pea crabs occurred at the low nutrient site, but not where mussels enjoyed a high nutrient regime. To integrate very long-term disparity in growth rates associated with infestation, we used mussels from a robust, naturally occurring population. We recorded and analyzed mussel sithouette shapes with a video digitizer. Elliptic Fourier approximation completely characterized the two-dimensional outlines of shells in such a way that the allometric dependence of shape variables on shell size could be easily removed. At this evidently favorable site, infested mussels displayed significant shell shape distortion characteristic of reduced growth rates. Thus, even in apparently benign environments, pea crab infestation appears to be a chronic stress to M. edulis.

1. It is shown that Pinnotheres ostreum invades its host, Crassostrea virginica in the first crab stage and not in the hard-shelled stage as hitherto believed.2. The finding of the first crab stage both in plankton samples and inside oysters marks the first find in nature of this stage for any pinnotherid crab.3. A preliminary description is given of the following pre-hard stages, which were the last unknown stages in the developmental cycle of P. ostreum. Both sexes of these stages were found only in the oyster and are never free-living. A full life-history of a pinnotherid crab is now known for the first time.4. Invasion of the oyster in Delaware Bay takes place in late summer at a time when most of the oyster spat have grown sufficiently large to harbor one or more crabs.5. More crabs invade spat than yearlings and older oysters, but the survival rate is higher for crabs invading the latter groups of oysters.6. The growth rate of the crab from the invasive to the mature stage is shown to be positively ...

1. The early stages of Pinnotheres ostreum have been reared in the laboratory from the egg to the first crab.2. There are four zoeal stages and one megalops stage. The four zoeal instars are distinguished by having 4, 6, 8, and 10 swimming hairs on the exopodites of both maxillipeds.3. The time required for development from the early egg (from orange-colored egg mass) to the first crab instar was 38 days. From hatching to the first crab required 25 days.4. The first crab being only 0.6 mm. wide, at least one more instar must intervene before P. ostreum reaches the "first stage" described by Stauber as the youngest crabs which invade the oyster.5. Oyster crabs are widely distributed and abundant in Virginia waters. They tend to keep oyster meats in relatively poor condition, but have never been observed to cause mortality of oysters in Virginia.

1. All the parasitic stages of Pinnotheres ostreum are described, many for the first time. Besides the invasive first stage male and female crabs, one other male stage and four other female stages are reported.2. The first crab stage (male or female) is hard-shelled, relatively flat and hairy with a distinctive brown color and white markings.3. Subsequent stages are quite different with membranaceous, yielding carapace, slender legs, and more rounded body form.4. The later stages of the females are distinguished from one another by size, color, width and shape of abdomen with relation to the rest of the body and size and differentiation of the pleopods. Maturity is usually reached in the fifth stage female and ovigerous crabs have been seen.5. All stages of crabs have been found in oysters either singly or in various combinations except that no more than one specimen of a Stage III, IV, or V female has ever been observed within a single oyster.6. Multiple infestations have been observed which involve chie...