Estimating blue crab (Callinectes sapidus) larval release sites in the Gulf of Mexico using an oceanographic particle-tracking model

Small, widely-dispersing, pelagic larvae are difficult to monitor through direct observation, but biophysical models can provide greater understanding of dispersal patterns. Here, we used a technique of reversing time in a biophysical model to estimate larval release sites of the blue crab, Callinectes sapidus, in the northern Gulf of Mexico by backtracking from known locations of postlarval settlement. We modeled surface ocean movement in reverse time from four settlement sites (Grand Isle, Louisiana; Ocean Springs, Mississippi; Dauphin Island, Alabama; and Pensacola, Florida). In the Gulf of Mexico, blue crab fisheries are managed at the state level. We found on average 73% of the larvae were traced back to larval release sites in the same state as settlement. Barrier island release sites accounted for 11% of settlement at Grand Isle and 17% at Dauphin Island. Less than 0.1% of larvae that settled in Grand Isle were tracked back to a known blue crab spawning area located to the south west, Ship Shoal. The distance traveled by larvae was highest in Pensacola and Grand Isle. The coefficient of variation of distance traveled was highest in Dauphin Island. The average distance traveled to each settlement location ranged from 31 to 124 km. Distance traveled explained 24% of empirical settlement numbers (F1,89 = 28.82, P < 0.001;R2 = 0.24). Settlement success was higher at release sites predicted by this study than at randomly generated sites suggesting female selection of larval release sites.

Dispersal

Dispersal of benthic crustaceans primarily occurs by larvae, which can be transported far from parents. However, larval dispersal is reduced by depth regulation in a sheared water column, where surface and bottom currents flow at different rates or directions, and navigation by postlarvae recruiting to adult habitats. Larvae undertake migrations between adult and larval habitats that range from retention near adult habitats to cross-shelf migrations. The extent of these migrations is regulated by depth preferences and vertical migrations that are timed exogenously or endogenously by diel and tidal cycles over planktonic development. Depth regulation is cued primarily by gravity, hydrostatic pressure, and light, and secondarily by temperature, salinity, and turbulence. Settlement stages navigate to suitable settlement sites using hierarchies of acoustic, chemical, visual, and celestial cues that are effective at different distances. The extent of larval migrations between adult and larval habitats as well as diel vertical migrations may be set by the vulnerability of larvae to abundant planktivorous fish in estuaries and nearshore waters. The timing of larval release and vertical swimming by larvae changes across tidal regimes to conserve migrations between adult and larval habitats across species ranges while minimizing predation.

Annual and lunar breeding rhythmicity in females of the terrestrial hermit crab Coenobita rugosus H. Milne Edwards, 1837 (Decapoda: Anomura: Coenobitidae) in Okinawa, Japan

We investigated the reproductive characteristics of a terrestrial hermit crab (Coenobita rugosus H. Milne Edwards, 1837) inhabiting Okinawa-Jima. Monthly changes in the gonadosomatic index and ovarian histology revealed that this species had fully developed oocytes from May to October. Data showing that females with (ovigerous females) and without (non-ovigerous females) embryos on their pleopods appeared simultaneously on the shore from June to September 2014 demonstrates that this species undergoes larval release several times for four months. When non-ovigerous females were reared under a combination of two day lengths and two temperatures, oocyte development was induced in the high-temperature group, suggesting that temperature is a proximate factor in ovarian development. The weekly collection of females during the spawning season revealed that the proportion of fully developed oocytes in an ovary increased toward the time of the new moon and decreased around the first-quarter moon, suggesting that this species is a lunar-spawner with a new-moon preference. Some females releasing larvae around the new or full moon were recaptured at approximately 30 d intervals, suggesting at least two major groups with lunar-synchronized larval release. The ovarian development and larval release of C. rugosus are likely entrained to external factors, which change periodically in their habitats.

Larval Behavior and Protective Implications for Sea Scallops and Surf Clams within the Middle Atlantic Bight: Part Two, Variability of Physical Environment and Larval Behaviors

Larval growth and settlement rates are important larval behaviors for larval protections. The variability of larval growthsettlement rates and physical conditions for 2006-2012 and in the future with potential climate changes was studied using the coupling ROMS-IMBs, and new temperature and current indexes. Forty-four experimental cases were conducted for larval growth patterns and release mechanisms, showing the spatial, seasonal, annual, and climatic variations of larval growthsettlement rates and physical conditions, demonstrating that the slight different larval temperature-adaption and larval release strategies made difference in larval growth-settlement rates, and displaying that larval growth and settlement rates highly depended upon physical conditions and were vulnerable to climate changes.