Changes in the abundance and distribution of sav along Florida’s springs coast: a comparison based on aerial photography acquired in 1992 and 1999

Intra-year class cannibalism in early juvenile blue crabs

Deborah A. Kilbane* and Ken Heck. University of South Alabama and Dauphin Island Sea Lab, Dauphin Island, AL 36528, USA.

Intra-year class cannibalism among juveniles significantly influences the density of several crab species. This study tests the hypothesis that intra-specific predation on recently settled blue crabs (Callinectes sapidus) is a primary controller of the density of juvenile blue crabs. Preliminary work to determine which sizes of intra-year class juveniles are able to consume new settlers and whether cannibalism is density dependent. I have found that cannibalistic mortality rates increase with increasing conspecific predator carapace width and increasing settler densities. This work will continue both in the field and in the lab to further determine the role that cannibalism plays in regulating the abundance of juvenile blue crabs.

Fundulus majalis as a potential predator of the invasive asian shore crab Hemigrapsus sanguineus

A. K. Kim and N. J. O’Connor.* Department of Biology and School for Marine Science and Technology, University of Massachusetts Dartmouth, No. Dartmouth, MA 02747-2300, USA.

In the rocky intertidal zone of the east coast of the United States, Hemigrapsus sanguineus is a successful recent invader whose ecological impacts are little known. For example, predators of H. sanguineus have not been well identified. One potential predator of H. sanguineus is the striped killifish, Fundulus majalis, an opportunistic bottom feeder on sandy and pebbly substrates whose diet includes crustaceans. Laboratory experiments were performed with and without a natural substrate (sand and rocks) to determine the ability of F. majalis to consume megalopae (postlarvae) and juvenile H. sanguineus. Results demonstrate that F. majalis can consume H. sanguineus megalopae and first stage crabs, but crabs ≥ 3 mm in carapace width were rarely consumed. Although fewer first-stage crabs were consumed when substrate was present, megalopae were eaten in large numbers regardless of substrate. There was little difference in consumption rates by male and female F. majalis from 60 to > 100 mm total length. F. majalis might prey on settling megalopae and small juvenile H. sanguineus in the field, but crabs likely escape predation by killifish at relatively small sizes.

Biomass of invertebrates in mussel beds at hydrothermal vents and cold seeps

K. Knick,* C. Jenkins, M. Doerries, S. Rapoport, E. Raulfs, J. Salerno, M. Turnipseed, M. Ward, and C. L. Van Dover. Department of Biology, College of William and Mary, Williamsburg, VA 23187, USA; e-mail [e-mail].

Hydrothermal vents and cold seeps are both chemosynthetic ecosystems, but they vary in resource availability, fluid chemistry, temperature, age, and other environmental features. These characteristics seem likely to influence ecological measures such as biomass, but patterns of distribution of biomass at vents and seeps are poorly known. In this study, we compared the biovolume of macrofaunal invertebrates associated with mussels beds at three sites along the Northern East Pacific Rise, three sites along the Southern East Pacific Rise, and at the Florida Escarpment, Gulf of Mexico cold seep. Mussel beds occur globally at vents and seeps and have shallow-water analogs, so they are useful habitats for comparative studies of biomass. There was no significant difference in biovolume among active sites. Biovolume of mussel beds was relatively lower at a dying site on the Southern East Pacific Rise. There was greater variation of biovolume within sites than between sites, indicating a local heterogeneity. Distribution of biovolume among major taxa varied among sites, and 1 or 2 species typically accounted for greater than 60 % of the total biovolume of invertebrates within mussel beds. These species also tended to be the numerical dominants. We conclude that even though vents and seeps are different ecological systems, they produce comparable amounts of invertebrate biomass within mussel beds.

Effects of seagrass shoot dynamics on distribution and recruitment of epifaunal encrusting bryozoa

N. Kouchi,¹* M. Nakaoka,² and H. Mukai.^{1 1}Akkeshi Marine Station, Hokkaido University, Aikappu, Akkeshi-cho, Akkeshi-gun, Hokkaido 088-1113, Japan; ²Graduate School of Science and Technology, Chiba University, Yayoi-cho 1-33, Inage, Chiba 263-8522, Japan.

Sessile invertebrates are one of the important components of seagrass community. Seagrass leaves are ephemeral habitats, and their dynamics may affect the distribution of epifauna living on them. Zostera caulescens is remarkable in a point that it forms a high canopy (5–7 m from the sea bottom) composed of flowering shoots. It, thus provided the habitat for epifauna with the wide range of height in the water column. We studied distribution and recruitment pattern of encrusting bryozoa Microporella trigonellata, most abundant epifauna on Z. caulescens in Funakoshi Bay, northeastern Japan, by monthly field sampling. We also constructed field experiments to determine the effects of seagrass shoot type (flowering and vegetative shoots) and the position in the water column (high and low) on the bryozoan recruitment.

Bryozoan density on the flowering shoots varied seasonally, reaching maximum in summer and low in winter. In contrast, it was constant and low on the vegetative shoots through a study period. The variation in the bryozoan density well coincided with that of early recruited colonies, and thus the recruitment process was important to determine their distribution. Field experiments showed that the recruitment rate of M. trigonellata was significantly higher at the higher position in summer season, however, this site selection was not observed in autumn when the most of the flowering shoots fell down. The seasonal change in habitat selection by bryozoan was considered to be adaptive to maintain their population on the seagrass leaves that show seasonal dynamics.

Population age structure as a record of recruitment history: How large a grain of salt do we need?

Jacob P. Kritzer* and Peter F. Sale. Department of Biological Sciences, University of Windsor, Windsor, Ontario N9B 3P4 Canada.

Information on spatial and temporal patterns in recruitment of coral reef fishes is central to addressing a range of ecological questions. Unfortunately, unlike commercially important species in northern temperate waters, extensive time series of recruitment are lacking for tropical reef fishes. Several researchers have suggested that a snapshot sample of a population age structure will illustrate the relative strength of different year classes and, therefore, can serve as a proxy for recruitment history in the absence of explicit recruitment data. However, this approach has been criticized on the basis that post-settlement demography subject to temporal variability, density dependence or a combination of both can obscure the link between initial recruitment and cohort strength at a later time. This criticism suggests that recruitment information stored within an age structure must at best be taken with a grain of salt. We use computer simulations to evaluate how large this grain of salt must be. Each simulation spanned 20 years of population dynamics of a hypothetical reef fish species. A new cohort of variable size recruited to the population at the start of each year, and was then modified in each subsequent year first by juvenile mortality rates and then by adult mortality rates. Distinct simulations incorporated different combinations of underlying ecological assumptions about variable recruitment (normally versus log-normally distributed) and post-settlement demography (density-dependent versus density-independent). The age structure at the end of each simulation was compared with the known history of recruitment to assess their similarity. Our study suggests the relative importance of the factors that can negate the link between recruitment and subsequent age structure. Our study also highlights the importance of continued empirical research on the modification of recruitment strength through time in order to best exploit the information potential of population age structures.