Patterns of microzooplankton growth in dilution experiments across a trophic gradient: Implications for herbivory studies

To investigate the growth and grazing patterns of microzooplankton (MZP) in environments of differing productivity, dilution experiments measuring phytoplankton growth (μ) and grazing mortality (m) rates were performed using samples from contrasting locations along the Texas coast. Samples were collected from estuaries, coastal lagoons and offshore Gulf of Mexico locations in the spring and summer of 2001. MZP growth rates were determined in each dilution treatment. Although MZP biomass changed over time in most dilution treatments, adjusting μ and m for the actual grazer gradient (represented by geometric mean MZP biomass) did not cause a significant deviation from the nominal dilution gradient. Likewise, these adjustments did not yield significant regressions where none existed before adjustment. The dynamics of MZP taxonomic groups (ciliates, dinoflagellates) and size categories differed suggesting that in some cases internal predation may lead to trophic cascades. MZP biomass was higher in productive coastal waters and included a larger proportion of dinoflagellates than in the oligotrophic, ciliate-dominated waters of the Gulf of Mexico. The MZP biomass-to-chlorophyll a ratio was lowest in the hypereutrophic Nueces River, where MZP biomass significantly increased in all dilution treatments (net growth rates up to 2 day⁻¹) suggesting a strong top–down control. In the brown-tide dominated Upper Laguna Madre and the oligotrophic seagrass-dominated Lower Laguna Madre MZP growth was decoupled from that of phytoplankton. At these sites, MZP were likely fueled by bacterial carbon and mixotrophy, respectively. Observing the growth response of MZP in dilution experiments can provide insight into trophic structure and efficiency of the microbial food web.     

Latitudinal variation in the life history features of the black turban snail Tegula funebralis (Prosobranchia: Trochidae)

At higher latitudes along the west coast of the USA the herbivorous intertidal snail Tegula funebralis (Adams, 1854) lives longer, grows more slowly, but attains a larger size than further south. To the north, age distributions are less predictable and often far from stationary. The differences, which have some generality, can be explained either by a primary temperature effect joined with increased hazards to planktonic larvae in the north, or else may result from a combination of interactions between intensity of predation, population density and food supply. Growth rates of transplanted individuals suggest a genetic basis for the latitudinal differences.     

Optimal divorce and re-mating strategies for monogamous female birds: a simulation model

Although the extensive variation in divorce rates among monogamous bird species has stimulated several theoretical accounts, the mechanisms underlying divorce strategies remain poorly understood. Here, we use an individual-based simulation model to investigate the adaptiveness of mechanisms of mate choice in the context of remating. Our model compares the fitness of females that choose a mate during each breeding season using one of two different decision rules; best-of-n females sample n potential partners and then select the male with the highest quality, whereas better option females choose a mate whose quality is maximal among the non-mated individuals they sampled the season before. It is assumed in the model that best-of-n females have no a priori information about the quality of potential partners and systematically decide to divorce at the beginning of each breeding season before searching for a new mate. Conversely, better option females use the information they gained the season before, and may retain their previous partner if they have no opportunity to mate with an individual of better quality. Results from simulations indicate that the best-of-n decision rule should be favoured when there is a large variation in male quality and low costs of mate sampling. On the other hand, the probability that the better option rule may invade the population is predicted to increase with male survival rate. However, changes in male mortality had no marked influence on the expected proportion of divorcing pairs, contrary to previous theoretical expectations.Communicated by H. Kokko