John P. Ebersole

A key phase in the recruitment dynamics of coral reef fishes: post-settlement transition

SynopsisRecent studies of recruitment dynamics in demersal fishes have placed major emphasis on presettlement mortality, and little on events bridging late larval and early juvenile periods. Observations on 68 taxa of Caribbean coral reef fishes before and during settlement revealed the existence of a distinct post-settlement life phase called the transition juvenile, associated with the act of recruitment. Transition juveniles were found as solitary individuals, in conspecific groups, or in heterospecific groups. The groups were either uniform or heterogenous in appearance. The complexity of the transition phase and its apparently widespread occurrence in coral reef fishes suggests that important aspects of population structure may be determined between settlement and first appearance as a full-fledged juvenile.

Rapid growth and out-crossing promote female development in zebrafish (Danio rerio)

Sex determination in fishes is often enigmatic, a situation that is often made even more complex by the fact that the process of sexual differentiation in many species may be influenced by environmental conditions. This situation is typified in zebrafish, a popular model organism. Despite the vast array of information available for the species, the genetic controls of sex are unknown. Further, environmental parameters, such as rearing densities, seem to exert an influence on the sex ratios of captive stocks. In an effort to dissect the genetic and environmental controls underlying the expression of sex in this species, we manipulated growth of pure-bred and out-crossed zebrafish by varying their food supply during development. Faster-growing zebrafish were more likely to be female than siblings that were fed less, and out-crossed broods had higher proportions of females than broods from pure-bred crosses. The dependence of sex ratio on feeding rate is readily understood in terms of adaptive sex allocation: zebrafish life history seems to confer the greater pay-off for large size on females. A similar male/female difference in the pay-off for hybrid vigor could similarly account for the female bias of out-crossed broods—and it could be a manifestation of Haldane’s rule.

Microtopography and the organization of two assemblages of coral reef fishes in the west indies

Abstract Two walls of Salt River Canyon, St. Croix, U.S. Virgin Islands are at the same depth in the fore reef environment and are separated by only 100 m, but they differ in microtopography. Replicate visual censuses of the fish assemblages on these two walls, by four saturation divers, gave consistent estimates of the species assemblages on the two walls. Significant faunal differences between the two walls are best understood in terms of combined features of coloration, predator avoidance, and feeding habit displayed regularly by individuals occupying a given microhabitat. It appears that characteristic sets of environmental circumstances occur within a habitat with sufficient frequency to give a consistent direction to natural selection, producing the patterns we have observed. Thus, form and function is predictable in relation to microhabitat, although species occurrence is not. The “order” and “chaos” concepts of community structure in coral reef fish, with their narrow focus on the predictability of species occurrence, underemphasize the possibility for a simple and direct role for natural selection in shaping the functional characteristics of an assemblage on a small scale

Optimal foraging: The responses of Peromyscus leucopus to experimental changes in processing time and hunger

SummaryFood density, degree of hunger prior to foraging, and food processing difficulty were varied in order to determine their effects on the diet diversity of captive Peromyscus leucopus foraging for buried seeds. No relationship was found between diet diversity and food density. However, diet diversity exhibited a significantly positive relationship with hunger, and a significantly negative relationship with seed processing time. These results strongly support optimal foraging theory.

Mechanisms Affecting Recruitment Patterns of Fish and Decapods in Tropical Coastal Ecosystems

The early benthic life history of fishes and decapods in tropical coastal ecosystem can be partitioned into three main stages—settlement, post-settlement transition, post-settlement stage—which culminate in recruitment. Although most species go through these early life history stages, not all species follow the same strategy. Life history strategies occur in three general categories: habitat specialists, habitat generalists, and ontogenetic shifters. Despite this variation in life history strategy, common processes affect the early life history stages of tropical marine fishes and decapods. The life history transition from planktonic larva to benthic post-larva connects oceanic and coastal habitats. However, benthic features and benthic processes affect early life history stages so that settlement and post-settlement distributions are not perfect reflections of larval supply patterns. The abundances and distributions of settlement and post-settlement life history stages result from complex interactions of larval supply, larval behavior, and the interactions of early settlers with the benthic environment. Since much of the very high mortality that occurs during settlement and early post-settlement appears to be due to predation, the direct effects of predators may be the most important factors acting on these early life history stages. Habitat selection, priority effects, predator avoidance, inter- and intra-specific competition, and aggression during and after settlement are also important influences on abundances and distributions of settlement and post-settlement fishes and decapods. The connection between nursery habitat availability and adult population abundances has been demonstrated, so it is likely that these other interactions of early life history stages with the benthic environment have demographic implications that are not yet understood.

Natural history, distribution, and management of Lepidium latifolium (Brassicaceae) in New England

Abstract Lepidium latifolium (perennial pepperweed) is an herbaceous perennial, native to Europe and western Asia, that is now well established at many locations in Massachusetts and Connecticut. This species is considered a major invasive species in the western United States and appears likely to become invasive in the east if left unchecked. We showed that the distribution is greater than previously documented in New England and that populations appear to be expanding. Individuals of the species can produce thousands of seeds and we found that these seeds are tolerant to many days of inundation by salt water so that dispersal by tidal and river currents is likely. In addition, populations can expand at least 2 m/yr from growth by rhizomes, and densities exceeded 50 shoots/m2 in some areas. We identified 17 species of plants that are likely impacted by the expanding populations and 23 families of arthropods associated with L. latifolium in some of these areas. In wetland habitats, repeated pulling of shoots to remove much of the rhizome was effective at stabilizing or eradicating whole, well-defined populations. This treatment was most effective if continued for two or more growing seasons.

Metabolic Rate Variation over Adult Lifetime in the Butterfly Vanessa cardui (Nymphalidae: Nymphalinae): Aging, Feeding, and Repeatability

Questions about the adaptive importance of metabolic rate can be approached only when measurements of differences between individuals are repeatable. We made daily measurements of CO2 production, body mass, and food uptake over the adult life span of unmated Vanessa cardui kept under constant environmental conditions in both fed and unfed treatments. Mass and CO2 production generally declined with age in both treatments, though with much day‐to‐day variability in the fed treatment. For the full samples, metabolic rate was repeatable for the unfed treatment (repeatability r = 0.60) but not for the fed treatment (r = 0.03). Differences between fed and unfed individuals of the same age range were repeatable for the unfed treatment (r = 0.39) but not for the fed treatment (r = −0.20). Removing age effects on CO2 production yielded still higher repeatability in the unfed treatment (r = 0.83), though not in fed butterflies of the same age range (r = −0.02). However, repeatability of CO2 production of fed butterflies increased sharply with age, rising to 0.82 for butterflies age 8–10 d. Although food uptake mass was repeatable (r = 0.52), feeding history explained little variation in CO2 production. We conclude that for V. cardui and possibly for other insects of similar feeding habit, variation in metabolic rate between individuals is best represented by measurements of unfed individuals of the same age or of older fed individuals.