Molecular identification, coastal distribution and recruitment of Cucmaria frondosa larvae in the western Gulf of Maine

Understanding of the physical transport of larvae requires information on adult source populations, larval distributions, larval behavior and physical oceanography. Identification of three species (Cucumaria frondosa, Psolus fabricii and Chiridota laevis) of holothurian larvae in the western Gulf of Maine was done with a simple detection protocol, using oligonucleotide probes, for the 16S rRNA portion of the mitochondrial DNA genome. More than 2000 larvae and new recruits were analyzed. This technique revealed that of the three species present in the plankton, Cucumaria larvae dominated the samples comprising $\u3e$90% of the larvae and 95% of new recruits during 1993. Cucumaria larvae were most abundant during 1994 and 1995, as well. The spatial distribution of Cucumaria larvae was documented over the spawning season during 1993, 1994 and 1995 in the western Gulf of Maine. Active versus passive vertical movements were examined by comparing the specific gravity of the larvae with the ambient density of sea water. Typically, the specific gravity of the larvae was less than that of sea water, which implies that in order to settle, the larvae must be able to overcome positive buoyancy by swimming. I examined whether Cucumaria track a particular water mass by comparing the vertical density of larvae to local temperature, salinity and density distributions. There was no association between the larval distribution and hydrography. The influence of wind on the horizontal distribution of larvae was examined by determining the offshore extent of the highest abundances of larvae with upwelling and downwelling conditions. In 10/15 of the non-neutral cases, Cucumaria larvae were more abundant closer to shore after downwelling and were more abundant further offshore after upwelling, but neutral cases gave similar results. The mixed layer depth was shallower than the depth of the highest abundances of larvae, which suggests that Cucumaria larvae may avoid the surface layer by swimming down through the pycnocline. Recruitment and juvenile abundances of Cucumaria were examined in the field during 1993, 1994 and 1995. During the spring of 1993, recruitment at mussel beds was high. In the fall of 1994, no recruits were present on the benthos; juveniles were more abundant in coralline algae than in mussel beds or kelp holdfasts. In 1995 recruitment was significantly higher in mussel beds than in coralline algae or kelp holdfasts. A two-day intensive field study indicated that recruits were significantly more abundant in mussel beds than in kelp holdfasts or coralline algae, while juveniles were significantly more abundant in the mussel beds than in coralline algae. Mussel beds may enhance early survival by providing a refuge from predation