Abundance of Invasive and Native Crab Larvae in the Mouth of Delaware Bay:Hemigrapsus sanguineusandUca Pugnax

Abstract

We compare larval distributions of the invasive Asian shore crab Hemigrapsus sanguineus and the native fiddler crab Ucapugnax. Our study was conducted in the coastal ocean adjacent to Delaware Bay on the east coast of the United States. Subtidal circulation in the study area is dominated by the Delaware Coastal Current, which is formed by the estuarine plume that extends southward out of the bay and along the inner continental shelf. The offshore edge of the current is marked by a strong density front and by convergent circulation along the frontal boundary. Surface plankton tows were taken at fine-scale intervals along saw-tooth transects traversing the front on 3 consecutive days in early August. Coincident physical measurements delivered hydrographic data throughout the investigation. Results provided a synoptic view of larval distribution near the bay mouth and showed patchy occurrence of the 2 species within the sampling area. Approximately 8.7 × 105 U. pugnax larvae were collected during the study, compared with 2.5 × 104 H. sanguineus. Analysis of spatial distribution with respect to the coastal current showed a marked difference between early- and late-stage larvae for each species. Zoea stage I larvae were most common in the main stream of the coastal current. In contrast, the distribution of advanced stages was skewed toward the frontal region. Larvae entrained in the frontal circulation are likely retained near Delaware Bay, whereas those in the main stream of the coastal current may undergo long-distance transport along the continental shelf.