Fish ears are sensitive to sex change

Many reef fishes change sex during their life. The testing of life-history theory and effective fisheries management therefore relies on our ability to detect when this fundamental transition occurs. This study experimentally illustrates the potential to glean such information from the otolithic bodies of the inner-ear apparatus in the sex-changing fish Parapercis cylindrica. It will now be possible to reconstruct the complete, often complex life history of hermaphroditic individuals from hatching through to terminal reproductive status. The validation of sex-change associated otolith growth also illustrates the potential for sex-specific sensory displacement. It is possible that sex-changing fishes alter otolith composition, and thus sensory-range specificity, to optimize life history in accordance with their new reproductive mode.

Parental effects on offspring life histories: when are they important?

Both the parental legacy and current environmental conditions can affect offspring life histories; however, their relative importance and the potential relationship between these two influences have rarely been investigated. We tested for the interacting effects of parental and juvenile environments on the early life history of the marine fish Acanthochromis polyacanthus . Juveniles from parents in good condition were longer and heavier at hatching than juveniles from parents in poor condition. Parental effects on juvenile size were evident up to 29 days post-hatching, but disappeared by 50 days. Offspring from good condition parents had higher early survival than offspring from poor-condition parents when reared in a low-food environment. By contrast, parental condition did not affect juvenile survival in the high-food environment. These results suggest that parental effects on offspring performance are most important when poor environmental conditions are encountered by juveniles. Furthermore, parental effects observed at hatching may often be moderated by compensatory mechanisms when environmental conditions are good.

Influence of prey body characteristics and performance on predator selection

At the time of settlement to the reef environment, coral reef fishes differ in a number of characteristics that may influence their survival during a predatory encounter. This study investigated the selective nature of predation by both a multi-species predator pool, and a single common predator (Pseudochromis fuscus), on the reef fish, Pomacentrus amboinensis. The study focused on the early post-settlement period of P. amboinensis, when mortality, and hence selection, is known to be highest. Correlations between nine different measures of body condition/performance were examined at the time of settlement, in order to elucidate the relationships between different traits. Single-predator (P. fuscus) choice trials were conducted in 57.4-l aquaria with respect to three different prey characteristics [standard length (SL), body weight and burst swimming speed], whilst multi-species trials were conducted on open patch reefs, manipulating prey body weight only. Relationships between the nine measures of condition/performance were generally poor, with the strongest correlations occurring between the morphological measures and within the performance measures. During aquaria trials, P. fuscus was found to be selective with respect to prey SL only, with larger individuals being selected significantly more often. Multi-species predator communities, however, were selective with respect to prey body weight, with heavier individuals being selected significantly more often than their lighter counterparts. Our results suggest that under controlled conditions, body length may be the most important prey characteristic influencing prey survival during predatory encounters with P. fuscus. In such cases, larger prey size may actually be a distinct disadvantage to survival. However, these relationships appear to be more complex under natural conditions, where the expression of prey characteristics, the selectivity fields of a number of different predators, their relative abundance, and the action of external environmental characteristics, may all influence which individuals survive.

Replenishment success linked to fluctuating asymmetry in larval fish

Fluctuating asymmetry (FA), defined as random deviations from perfect symmetry, has become a popular tool with which to examine the effects of stress during the development of bilaterally symmetrical organisms. Recent studies have suggested that FA in otoliths may serve as an indicator of stress in fish larvae. We examined the relationship between otolith asymmetry and temporal patterns in the occurrence of late-stage larvae to a tropical reef (i.e. replenishment) for the Caribbean lizardfish, Saurida suspicio (family Synodontidae). Late-stage larvae were collected in light traps over a period of 18 consecutive lunar months in the San Blas Archipelago, Panama. Asymmetry within otolith pairs was calculated from 24 variables: area, perimeter, longest and shortest axis of the otolith and 20 shape descriptors (Fourier harmonics). Otolith asymmetry was correlated strongly with fluctuations in lunar light trap catches. Two measured variables, otolith area and one of the 20 shape descriptors, accounted for 60% of the variability in lunar replenishment of S. suspicio. Individuals from small replenishment pulses exhibited higher levels of asymmetry compared to larvae from large pulses. When dry and wet seasons were analysed separately, otolith asymmetry explained a surprising 70 and 97% of the variation, respectively. Although the generality of these results remain to be tested among other populations and species, otolith asymmetry may be an important indicator, and potentially a predictor, of larval quality and replenishment success.

Development of nine microsatellite markers for Pomacentrus amboinensis

The relatively long pelagic larval duration of Pomacentrus amboinensis, a tropical fish, suggests the potential for long-distance dispersal; however, several nongenetic studies have found substantial self-recruitment at one location. To analyse patterns of connectivity of this species, primers for nine independent microsatellite loci were developed for P. amboinensis using a magnetic bead enrichment protocol. Twenty individuals from one location were analysed and observed heterozygosities ranged from 0.7 to 0.95. Eight of nine loci were in Hardy-Weinberg equilibrium and no evidence of linkage or null alleles were found.

Survival against the odds: ontogenetic changes in selective pressure mediate growth-mortality trade-offs in a marine fish

For organisms with complex life cycles, variation among individuals in traits associated with survival in one life-history stage can strongly affect the performance in subsequent stages with important repercussions on population dynamics. To identify which individual attributes are the most influential in determining patterns of survival in a cohort of reef fish, we compared the characteristics of Pomacentrus amboinensis surviving early juvenile stages on the reef with those of the cohort from which they originated. Individuals were collected at hatching, the end of the planktonic phase, and two, three, four, six and eight weeks post-settlement. Information stored in the otoliths of individual fish revealed strong carry-over effects of larval condition at hatching on juvenile survival, weeks after settlement (i.e. smaller-is-better). Among the traits examined, planktonic growth history was, by far, the most influential and long-lasting trait associated with juvenile persistence in reef habitats. However, otolith increments suggested that larval growth rate may not be maintained during early juvenile life, when selective mortality swiftly reverses its direction. These changes in selective pressure may mediate growth-mortality trade-offs between predation and starvation risks during early juvenile life. Ontogenetic changes in the shape of selectivity may be a mechanism maintaining phenotypic variation in growth rate and size within a population.

Regulation of protogynous sex change by competition between corticosteroids and androgens: an experimental test using sandperch, Parapercis cylindrica

Cortisol, the dominant corticosteroid in fish, and 11-ketotestosterone (11KT), the most potent androgen in fish, are both synthesized and (or) deactivated by the same two enzymes, 11beta-hydroxylase and 11beta-hydroxysteroid dehydrogenase. Cortisol is synthesized in response to stress (such as that caused by interaction with a dominant conspecific), whereas 11KT is synthesized during protogynous sex change. It has been hypothesized that corticosteroids (such as cortisol) inhibit 11KT synthesis via substrate competition, thereby providing a mechanism for the regulation of socially mediated, protogynous sex change. We tested this hypothesis by administering cortisol (50 microg g(-1) body weight) to female sandperch (Parapercis cylindrica) under social conditions that were permissive to sex change (i.e. in the absence of suppressive male dominance). Twenty-one days later, mean physiological cortisol concentration in cortisol-treated fish was 4.2-fold greater than that in 'socially stressed' female fish maintained in a semi-natural system. Although the dosage of cortisol was therefore considered to be favorable for engendering competitive inhibition of 11KT synthesis, all cortisol-treated fish changed sex, as did all sham-treated and control fish (n=7 fish per treatment). In addition, there was no effect of cortisol treatment on the rate of sex change or on the pattern of steroidogenesis. Thus, our results refute the hypothesis that protogynous sex change is regulated by substrate competition between corticosteroids and androgens.

Social facilitation of selective mortality

Territorial defense by breeders influences access to resources near defended nest sites by intruder species and may have indirect effects on other species within the territory, leading to local patchiness in distribution patterns. The present study demonstrates that adult males of a damselfish, Pomacentrus amboinensis, indirectly facilitate the increased survival of conspecific juveniles through the territorial defense of their nesting site from potential egg predators. Moreover, male territoriality results in a shift in the selectivity of predation on newly settled juveniles. We monitored the fate of pairs of predator-naive, newly settled P. amboinensis placed inside and outside nesting territories. Individuals within a pair differed in size by approximately 1 mm and were tagged for individual identification. Away from male territories larger juveniles had greater survival, while within territories, larger juveniles suffered higher mortality. Behavioral observations indicated that the moonwrasse Thalassoma lunare, a predator of benthic eggs and small fishes, had reduced access to juveniles within male territories, while another predator on small fishes, the dottyback Pseudochromis fuscus, had unobstructed access to male territories. Experimental removal of P. fuscus indicated that the shift in the direction of phenotypic selection on newly settled juveniles was the indirect effect of aggression by nest-guarding male damselfish, which resulted in differential access to male territories by these two predators of small fishes. Evidence suggests that behavioral interactions between the resident community and intruders will influence patchiness in selective pressures imposed on benthic prey by influencing both the composition of predator types that can access the prey resource and their relative abundance. How this spatial and temporal patchiness in predator pressure interacts with spatial patchiness of recruiting prey will have a major influence on the resulting distribution of juveniles and their phenotypic traits.

Habitat choice, recruitment and the response of coral reef fishes to coral degradation

The global degradation of coral reefs is having profound effects on the structure and species richness of associated reef fish assemblages. Historically, variation in the composition of fish communities has largely been attributed to factors affecting settlement of reef fish larvae. However, the mechanisms that determine how fish settlers respond to different stages of coral stress and the extent of coral loss on fish settlement are poorly understood. Here, we examined the effects of habitat degradation on fish settlement using a two-stage experimental approach. First, we employed laboratory choice experiments to test how settlers responded to early and terminal stages of coral degradation. We then quantified the settlement response of the whole reef fish assemblage in a field perturbation experiment. The laboratory choice experiments tested how juveniles from nine common Indo-Pacific fishes chose among live colonies, partially degraded colonies, and dead colonies with recent algal growth. Many species did not distinguish between live and partially degraded colonies, suggesting settlement patterns are resilient to the early stages of declining coral health. Several species preferred live or degraded corals, and none preferred to associate with dead, algal-covered colonies. In the field experiment, fish recruitment to coral colonies was monitored before and after the introduction of a coral predator (the crown-of-thorns starfish) and compared with undisturbed control colonies. Starfish reduced live coral cover by 95-100%, causing persistent negative effects on the recruitment of coral-associated fishes. Rapid reductions in new recruit abundance, greater numbers of unoccupied colonies and a shift in the recruit community structure from one dominated by coral-associated fishes before degradation to one predominantly composed of algal-associated fish species were observed. Our results suggest that while resistant to coral stress, coral death alters the process of replenishment of coral reef fish communities.

Predators target rare prey in coral reef fish assemblages

Predation can result in differing patterns of local prey diversity depending on whether predators are selective and, if so, how they select prey. A recent study comparing the diversity of juvenile fish assemblages among coral reefs with and without predators concluded that decreased prey diversity in the presence of predators was most likely caused by predators actively selecting rare prey species. We used several related laboratory experiments to explore this hypothesis by testing: (1) whether predators prefer particular prey species, (2) whether individual predators consistently select the same prey species, (3) whether predators target rare prey, and (4) whether rare prey are more vulnerable to predation because they differ in appearance/colouration from common prey. Rare prey suffered greater predation than expected and were not more vulnerable to predators because their appearance/colouration differed from common prey. Individual predators did not consistently select the same prey species through time, suggesting that prey selection behaviour was flexible and context dependent rather than fixed. Thus, selection of rare prey was unlikely to be explained by simple preferences for particular prey species. We hypothesize that when faced with multiple prey species predators may initially focus on rare, conspicuous species to overcome the sensory confusion experienced when attacking aggregated prey, thereby minimizing the time required to capture prey. This hypothesis represents a community-level manifestation of two well-documented and related phenomena, the "confusion effect" and the "oddity effect", and may be an important, and often overlooked, mechanism by which predators influence local species diversity.

Maternal condition influences phenotypic selection on offspring

1. Environmentally induced maternal effects are known to affect offspring phenotype, and as a result, the dynamics and evolution of populations across a wide range of taxa. 2. In a field experiment, we manipulated maternal condition by altering food availability, a key factor influencing maternal energy allocation to offspring. We then examined how maternal condition at the time of gametogenesis affects the relationships among early life-history traits and survivorship during early development of the coral reef fish Pomacentrus amboinensis. 3. Maternal condition did not affect the number of embryos that hatched or the number of hatchlings surviving to a set time. 4. We found no significant difference in egg size in relation to the maternal physiological state. However, eggs spawned by supplemented mothers were provisioned with greater energy reserves (yolk-sac and oil globule size) than nonsupplemented counterparts, suggesting that provision of energy reserves rather than egg size more closely reflected the maternal environment. 5. Among offspring originating from supplemented mothers, those with larger yolk-sacs were more likely to successfully hatch and survive for longer periods after hatching. However, among offspring from nonsupplemented mothers, yolk-sac size was either inconsequential to survival or offspring with smaller yolk-sac sizes were favoured. Mothers appear to influence the physiological capacity of their progeny and in turn the efficiency of individual offspring to utilize endogenous reserves. 6. In summary, our results show that the maternal environment influences the relationship between offspring characteristics and survival and suggest that energy-driven selective mechanisms may operate to determine progeny viability.

Temperature-induced shifts in selective pressure at a critical developmental transition

Selective mortality within a population, based on the phenotype of individuals, is the foundation of the theory of natural selection. We examined temperature-induced shifts in the relationships among early life history traits and survivorship over the embryonic and larval stages of a tropical damselfish, Pomacentrus amboinensis. Our experiments show that temperature determines the intensity of selective mortality, and that this changes with ontogeny. The size of energy stores determined survival through to hatching, after which egg size became a good indicator of fitness as predicted by theoretical models. Yet, the benefits associated with egg size were not uniform among test temperatures. Initial egg size positively influenced larval survival at control temperature (29 degrees C). However, this embryonic trait had no effect on post-hatching longevity of individuals reared at the higher (31 degrees C) and lower (25 degrees C) end of the temperature range. Overall, our findings indicate that the outcome of selective mortality is strongly dependent on the interaction between environment conditions and intrinsic developmental schedules.

MOTHERS MATTER: CROWDING LEADS TO STRESSED MOTHERS AND SMALLER OFFSPRING IN MARINE FISH

Most marine populations are sustained by the entry of juveniles that have survived the larval phase, during which time most die. The number of survivors depends strongly on the quality of the eggs produced by spawning females, but it is not known how the social conditions under which breeding occurs influence the quality of larvae produced. Here I show that the density of females interacting with breeding mothers directly influences the size of larvae produced, through a stress-related mechanism. On the Great Barrier Reef of Australia, breeding pairs of a damselfish, Pomacentrus amboinensis, were isolated on habitat patches, and additional females that could not access the spawning site were added at four densities (0, 1, 3, or 6 females). Additional females increased aggressive interactions by mothers and increased the levels of the stress hormone, cortisol, in their ovaries, leading to reduced larval size. Neither egg output nor yolk size of the larvae was influenced by female density. Pairs breeding in isolation produced the largest larvae; current theory suggests that these larvae should contribute most to subsequent population replenishment events. This social mechanism may influence which females effectively contribute to the next generation and may promote resilience in patchy or isolated populations.

Coral decline threatens fish biodiversity in marine reserves

The worldwide decline in coral cover has serious implications for the health of coral reefs. But what is the future of reef fish assemblages? Marine reserves can protect fish from exploitation, but do they protect fish biodiversity in degrading environments? The answer appears to be no, as indicated by our 8-year study in Papua New Guinea. A devastating decline in coral cover caused a parallel decline in fish biodiversity, both in marine reserves and in areas open to fishing. Over 75% of reef fish species declined in abundance, and 50% declined to less than half of their original numbers. The greater the dependence species have on living coral as juvenile recruitment sites, the greater the observed decline in abundance. Several rare coral-specialists became locally extinct. We suggest that fish biodiversity is threatened wherever permanent reef degradation occurs and warn that marine reserves will not always be sufficient to ensure their survival.

Selective predation for low body condition at the larval-juvenile transition of a coral reef fish

Mortality is known to be high during the transition from larval to juvenile life stages in organisms that have complex life histories. We are only just beginning to understand the processes that influence which individuals survive this period of high mortality, and which traits may be beneficial. Here we document a field experiment that examines the selectivity of predation immediately following settlement to the juvenile population in a common tropical fish, Pomacentrus amboinensis (Pomacentridae). Newly metamorphosed fish were tagged and randomly placed onto replicated patches of natural habitat cleared of resident fishes. After exposure to transient predators for 3 days, fish were recollected and the attributes of survivors from patch reefs that sustained high mortality were compared to individuals from patch reefs that experienced low mortality. Seven characteristics of individuals, which were indicative of previous and present body condition, were compared between groups. Predation was found to be selective for fish that grew slowly in the latter third of their larval phase, were low in total lipids, and had a high standardized weight (Fulton's K). Traits developed in the larval phase can strongly influence the survival of individuals over this critical transition period for organisms with complex life cycles.

Consumption of coral propagules after mass spawning enhances larval quality of damselfish through maternal effects

The synchronized spawning of corals in many parts of the Indo-Pacific represents a huge injection of biological material into the waters around reefs. Much of this material is consumed by fishes and filter-feeding invertebrates in the 5 or so days following spawning. The present study is the first to document the effect of the consumption of coral propagules on a population of facultatively planktivorous fish and the transference of physiological condition across generations. The study compares two populations of the damselfish Pomacentrus amboinensis that fed to differing degrees on coral propagules for 5 days after the annual mass spawning of corals at Lizard Island, Great Barrier Reef, Australia. Wind blew coral slicks over the outer lagoon to the inner lagoon some 1.5 km away. While coral propagules were abundant in the water column in the windward location, they were scarce by the time the water mass reached the inner lagoon. Behavioral observations 2-5 days after coral spawning showed that a significantly higher proportion of P. amboinensis was feeding on coral propagules in the windward location than in the inner lagoon location. Windward location females consumed coral propagules almost exclusively and had fuller guts than females from the inner lagoonal location. Five days after the mass coral spawning, windward location females had a higher condition factor and a larger liver mass relative to body mass compared to females within the inner lagoon or females from both locations 2 months later. Fish eggs laid by the windward location females soon after coral spawning yielded larvae that had 25% larger yolk sacs and 100% larger oil globules than did larvae produced from the females from the inner lagoon location, or larvae produced at either location prior to or well after coral spawning in 2 previous years. Larger yolk sacs and oil globules have been shown to have direct survival benefits in the transition from endogenous to exogenous feeding. A feeding experiment conducted on patch reefs showed that diet supplementation of breeding females with a high lipid food for just 5 min per day was sufficient to significantly increase yolk-sac sizes of newly hatched larvae. Evidence suggests that females gain a fitness advantage from feeding on coral spawn and that this is passed on to their offspring.

Influence of cortisol on developmental rhythms during embryogenesis in a tropical damselfish

Newly-spawned teleost eggs can vary widely in their maternal endowment of a variety of hormones, including cortisol. Field and laboratory experiments have shown that initial egg cortisol concentrations directly influence the size at hatching of the benthic spawning damselfish, Pomacentrus amboinensis. The present study examines the mechanism by which cortisol influences larval size at hatching by investigating the growth and developmental rhythms throughout embryogenesis. Newly spawned eggs of P. amboinensis were collected from natural benthic nests, and half of each clutch was incubated in a moderate level of cortisol (2.7 x 10(-6) M, equivalent to a concentration of 0.79 pg/egg). Cortisol was found to have no affect on the rate of cell-pulsations up to epiboly (18 hr post-fertilization), with cells pulsing at a mean rate of 56-60 pulses/min. Cortisol had an affect on the relative growth rate from the start of gastrulation to knot formation. Growth in the cortisol-supplemented embryos was pulsed, with periods of fast growth punctuated by long periods of stasis. Overall growth rates during this period were lower in the cortisol-supplemented embryos despite their higher growth during active periods. Pulse rates of somite cells and contraction rhythms of myotomes and the heart were twice as high in cortisol-supplemented embryos than controls. Despite this, cortisol-supplemented eggs developed at the same rate as controls and hatched at the same time. This study suggests that the maternal endowment of cortisol to eggs plays a vital role in determining the embryonic rhythms by which embryos grow and may be directly influencing metabolism.

Larval growth predicts the recruitment success of a coral reef fish

While growth rates of pelagic larvae have been argued to be one of the principal determinants of the recruitment success of temperate marine fishes, it is not known if this is the case in the tropics. Here, we use larval growth histories derived from otoliths of a Caribbean reef fish to show that monthly variation in the intensity of settlement and recruitment of pelagic juveniles onto reefs is positively correlated with variation in growth rates 1-2 weeks after larvae begin feeding. Our results suggest that the processes thought to underlie recruitment of marine fishes in temperate regions may also operate in the tropics and contrasts with current research on the causes of recruitment variability in coral reef fishes, which emphasises the role of larval transport.

Vancomycin-resistant enterococci in stool specimens submitted for Clostridium difficile cytotoxin assay

The prevalence of, and clinical risk factors associated with, vancomycin-resistant enterococcal colonization were investigated in patients suspected of having Clostridium difficile infection. Stools submitted for C difficile cytotoxin testing were screened for vancomycin-resistant enterococci (VRE). Isolates were speciated and characterized further by antibiotic susceptibility testing, DNA fingerprinting, and DNA:DNA hybridization for detection of specific vancomycin resistance genes. Of the 79 evaluable patients identified during a 3-month period, 16.5% were VRE-positive. The VRE isolates were genetically heterogeneous, although all carried the vanA gene. DNA fingerprinting data suggest that patient-to-patient transmission occurred, implicating colonized patients as potential reservoirs for VRE transmission. A positive C difficile cytotoxin assay and diabetes mellitus were the only identifiable risk factors associated with VRE colonization. Patients at risk for C difficile infection therefore may serve as reservoirs for VRE.