Patterns of larval release by the Asian shore crab Hemigrapsus sanguineus : periodicity at diel and tidal frequencies

Journal of Shellfisheries Research, August, 2005 by Susan Park, Charles E. Epifanio, Raymond B. Iglay

Our study had two components. The first component investigated patterns of larval release as related to diel and tidal cycles. This study consisted of laboratory-based observations of larval release by recently collected H. sanguineus females. The second component consisted of plankton sampling in waters adjacent to adult habitat to confirm the timing of larval release in the field. In the laboratory experiment, we used ovigerous females collected from the natural environment during peak spawning season (June and July) in 2001. In an earlier study, we developed an index relating the color of an egg mass to the developmental state of embryos within the eggs (Epifanio et al. 1998). We used this index to select hatch-imminent females for use in the experiment. Females were collected during the diurnal low tide immediately preceding the start of the trials. The majority of crabs used in the study released larvae within 30 h of collection, and none were held in the laboratory for more than 78 h.

Laboratory Investigation of Diel and Tidal Periodicity

This investigation consisted of 3 experiments. The first experiment was designed to determine the diel pattern of larval release. In this study, 35 females were maintained individually in 23-cm diameter glass fingerbowls under constant conditions (25[degrees]C, 30[per thousand]). To simplify the experimental design, we held the crabs in complete darkness. This assured that any observed rhythm was either endogenous or already entrained. The crabs were observed using a far-red light (incandescent light passed through a Kodak red filter with less than 1% transmission of wavelengths <600 nm), which cannot he detected by the crabs (Shukoliukov et al. 1980). Crabs were placed in experimental containers <1 h after collection, and initial observations were made immediately before sunset. Observations then continued on the following sunrises/sunsets until all females had released their larvae (<3 d). Larval release was confirmed by the presence of zoeae in the bowls. By making observations twice a day, we could determine whether release occurred diurnally (between sunrise and sunset) or nocturnally (between sunset and sunrise).

A second experiment was designed to determine the tidal pattern of larval release. This study also used 35 ovigerous females collected within 4 h of initiation of the experiment and held under conditions similar to those described earlier. Crabs were observed every 15 min under far-red light starting approximately 2 h before sunset (18:00 EDT) and continuing until 06:00 the following morning. The initial occurrence of intense flapping of the abdomen, the initial occurrence of zoeae in the water, and the occurrence of full larval release were noted for each female. Observations continued for 3 consecutive nights. A third experiment, again designed to determine tidal periodicity and this time using 28 females, was conducted approximately 2 weeks later.

Pooled data from the 2 tidal-periodicity experiments were analyzed using circular statistics (see later). To conduct this analysis, the observed times of larval release were converted to degrees relative to the occurrence of high tide in the field (i.e., the time of high tide = 0[degrees]; a complete semidiurnal tidal cycle = 360[degrees]; and 1 h = 29[degrees]). Thus, larval release occurring 1 h before high tide in the field would convert to 331[degrees] (360[degrees]-29[degrees] = 331[degrees]). All tidal information was obtained from verified water-level data collected at the NOAA/NOS water level observation station at Lewes, Delaware.