Food availability affects growth in a coral reef fish

Summary Pomacentrus amboinensis is common on small patch reefs within One Tree Lagoon (Great Barrier Reef), where it preferentially settles onto deep reefs. A preliminary experiment, in which juveniles were transplanted to identical reef structures at two sites, within two depth strata, indicated that juvenile growth and survivorship were better in deeper water. The hypothesis that this difference was due to food availability was tested by a supplemental feeding experiment, carried out at another two randomly chosen sites, within the same two depth strata. Fish were fed each day over a one month period, during which no mortality was observed. The growth rates of juveniles were markedly higher on all food-supplemented reefs, when compared to controls. Growth differed between depth strata, but there was no interaction between the food x depth factors, which would have suggested a greater effect of food supplementation in either habitat. Thus, although the difference between depths cannot be attributed to food availability, the results have a more general significance. Food appears to be a limiting resource (in terms of growth) in both the marginal shallow habitat, and the more suitable deeper habitat.     

Larval sensory abilities and mechanisms of habitat selection of a coral reef fish during settlement

In most animals, siblings from a given reproductive event emerge over a very short period of time. In contrast, many species of birds hatch their young asynchronously over a period of days or weeks, handicapping last-hatched chicks with an age and size disadvantage. Numerous studies have examined the adaptive significance of this atypical hatching pattern, but few have attempted to explain the considerable intrapopulation variation that exists in hatching asynchrony. I explored proximate determinants of hatching asynchrony by monitoring 112 Burrowing Owl (Athene cunicularia) nests in the grasslands of southern Saskatchewan, Canada, over 4 years. Age disparities between first- and last-hatched siblings (i.e., hatching spans) varied considerably, ranging between 1 and 7 days (mode = 4 days). These hatching spans increased with increased hatching success. Hatching spans also increased with larger clutches, but the increase was less than predicted given the increased time required to lay more eggs. Hatching span was unrelated to number of prey cached in the nest during egg laying (an index of food availability), and was unaltered by a year of super-abundant prey. Furthermore, pairs given extra food during laying had hatching spans equal to those of unsupplemented control pairs. These results were inconsistent with both the energy constraint and facultative manipulation hypotheses, which predict that hatching asynchrony should vary with the level of food during laying, when incubation onset is determined. Burrowing Owls were apparently free of food limitation early in breeding, yet may not have been able to optimize hatching spans because food conditions during laying were largely unrelated to food conditions during brooding. Thus, one of the premises for facultative manipulation of hatching asynchrony—that laying females are able to forecast post-hatch food conditions—may not have been met for this population of Burrowing Owls.     

Temporal settlement patterns of larvae of the broadcast spawning reef coral Favites chinensis and the broadcast spawning and brooding reef coral Goniastrea aspera from Okinawa, Japan

The faviid corals, Favites chinensis and Goniastrea aspera are widely distributed in the Indo-Pacific region. Both corals are hermaphroditic broadcast spawners, but G. aspera is also known to brood planula larvae in Okinawa. This study investigated the temporal settlement patterns of planula larvae of the scleractinian corals F. chinensis and G. aspera that developed from spawned gametes, and planula release and settlement of brooded larvae of G. aspera from Okinawa, Japan. Some of the broadcast-spawned larvae of F. chinensis and G. aspera had very short pre-competency periods of 1–2 and 2–3 days after spawning, and relatively long maximum settlement-competency periods of 56–63 and 63–70 days after spawning, respectively. These pre-competency periods are among the shortest reported for larvae of broadcast spawning coral species, and appear to be negatively correlated with seawater temperature. F. chinensis larvae tended to settle rapidly with 34–39% of larvae settling in the first week after spawning, while broadcast-spawned G. aspera larvae had a slower settlement pattern with 11–15% of larvae settling in the first week after spawning. Brooded larvae of G. aspera settled more rapidly, with settlement rates of 27–31% within the first 24 h and 45–65% within the first week after the start of the experiment. The production of planula larvae with rapid settlement capabilities may enable F. chinensis and G. aspera to establish and maintain populations in shallow reef sites at Okinawa. The release of the brooded planulae for up to 2 months may explain why G. aspera is locally more dominant on shallow reefs in Okinawa than F. chinensis. On a broader scale, the longer settlement competency periods of some of the broadcast-spawned larvae of these species increase their potential for longer-distance dispersal and may partly explain the wide biogeographic distribution of these species in the Indo-Pacific region.     

Experimental confirmation of aggressive mimicry by a coral reef fish

A number of potential mimetic relationships between coral reef fishes have been described, but the underlying mechanisms are poorly understood. Similarities in colour between species have often been attributed to aggressive mimicry (where predators resemble models in order to deceive prey), however this has not been tested. The fang blenny, Plagiotremus rhinorhynchos is a specialized predator that feeds on tissues of other fishes. Some individuals appear to mimic the harmless cleaner wrasse Labroides dimidiatus in order to deceive fish visiting cleaning stations, thereby increasing access to food. In this study, the ecological relationship between the mimic and model was examined at Kimbe Bay (Papua New Guinea) and the hypothesis that colour similarities represent facultative aggressive mimicry was experimentally evaluated. Some juveniles exhibited a striking resemblance to the juvenile colouration of the cleaner wrasse, but only when in close proximity to the wrasse and only when similar in size. As predicted for mimics, P. rhinorhynchos co-occurred with L. dimidiatus, but was rare relative to the model. Among site comparisons showed that the abundance of mimetic type blennies was positively correlated with the abundance of juvenile cleaner wrasses. Approximately 50% of all P. rhinorhynchos were found 1 m from the nearest L. dimidiatus, a distance significantly shorter than expected if they were not associated. A cleaner wrasse removal experiment was carried out to test whether the colour displayed by the blenny and its foraging success were contingent upon the presence of a model. In all cases, removal of the model prompted a rapid colour change to a general non-mimetic colouration in P. rhinorhynchos. Removal of L. dimidiatus also resulted in a ~20% reduction in the average foraging success of the blenny compared to controls, supporting the hypothesis that the blenny is a facultative aggressive mimic of the cleaner wrasse.     

Grazing by a small fish affects the early stages of a post-settlement stony coral

Short-term experiments were used to isolate the detrimental effects of grazer disturbance on young corals, and determine the stage of development at which recruits are no longer susceptible to this disturbance. Artificial substrata containing an algal matrix and coral recruits of different life stages were exposed to grazing by epilithic algal matrix (EAM) feeding combtoothed blennies, Salarias fasciatus. Single polyp recruits were vulnerable to grazer disturbance, while multi-polyp recruits (ca. 6–8 polyps) survived with evidence of minor damage in the form of tissue and polyp loss. The result indicates that blennies, although small and possessing weak dentition, can negatively influence the survival of young coral recruits. The protruding structure of micro-nubbins, representing juvenile corals were not damaged, suggesting that coral achieving that size and form can escape such damage. Communicated by Ecology Editor Prof. Peter Mumby     

Experimental test of the effect of maternal hormones on larval quality of a coral reef fish

Maternal hormones can play an important role in the development of fish larvae. Levels of the stress hormone, cortisol, in females are elevated by social interactions and transferred directly to the yolk of eggs, where they may influence developmental rates. In some vertebrates, prenatal exposure to high levels of testosterone determine early growth rates, social status and reproductive success. The present study examined whether post-fertilization exposure of eggs of the tropical damselfish, Pomacentrus amboinensis (Pomacentridae), to natural levels of cortisol or testosterone directly affects larval morphology at hatching. Maternal and egg levels of cortisol and testosterone varied widely among clutches of eggs from local populations around Lizard Island on the Great Barrier Reef. The morphology of larvae produced by these local fish populations also varied widely and differed significantly among sites (e.g., standard length: 2.6–3.4 mm; yolk sac area: 0.01–0.13 × 10⁻² mm²). Laboratory experiments showed that elevated cortisol levels in the egg reduced larval length at hatching, while slight elevations in testosterone increased yolk sac size. The influence of testosterone, and to a smaller extent cortisol, on larval morphology differed among egg clutches. These differences were partly explained by differences in initial egg hormone levels. Morphological changes induced by experimental hormonal regimes encompassed the entire range of variability in body attributes found in field populations. It is unclear whether cortisol influences growth alone or development rate or both. Testosterone appears to influence yolk utilization rates, and has no significant effect on growth, in contrast to its role in later developmental stages. Maternally derived cortisol and testosterone are important in regulating growth, development, and nutritive reserves of the embryo and larvae of this fish species. Factors that influence the maternal levels of cortisol and testosterone may have a major impact on larval mortality schedules and, therefore, on which breeding individuals contribute to the next generation. Received: 19 August 1998 / Accepted: 16 November 1998     

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 (Fultons 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 represents a significant trophic link between corals and reef fish

Mass spawning of corals provides a large seasonal pulse of high-energy prey that potentially benefits reef fish that are capable of capturing and digesting coral propagules. This study examines the range of fish species that consume coral propagules and also tests whether reef fish experience a significant increase in physiological condition when feeding on coral propagules. Thirty-six species of diurnal reef fish were seen to consume coral propagules released during mass coral spawning. Stomach content analyses of three reef fish species (Pomacentrus moluccensis, Abudefduf whitleyi, and Caesio cunning) revealed that both P. moluccensis and A. whitleyi feed almost exclusively on coral propagules during mass coral spawning. Fish feeding extensively on coral propagules also amassed considerable lipid stores, which could greatly improve the quality and survivorship of their progeny. In contrast, C. cunning consumed only very small quantities of coral propagules, and showed no detectable change in lipid stores during the course of the study. This study provides the first direct evidence that reef fish benefit from mass coral spawning, and reveals a potentially significant trophic link between scleractinian corals and reef fish. Accepted: 9 June 2000     

Food restriction affects maternal investment but not neonate phenotypes in a viviparous lizard

Food availability significantly affects an animal's energy metabolism,and thus its phenotype,survival,and reproduction.Maternal and offspring responses to food conditions are critical for understanding population dynamics and life-history evolution of a species.In this study,we conducted food manipulation experiments in field enclosures to identify the effect of food restriction on female reproductive traits and postpartum body condition,as well as on hatchling phenotypes,in a lacertid viviparous lizard from the Inner Mongolian desert steppe of China.Females under low-food availability treatment (LFT) had poorer immune function and body condition compared with those under high-food availability treatment (HFT).The food availability treatments significantly affected the litter size and litter mass of the females,but not their gestation period in captivity or brood success,or the body size,sprint speed,and sex ratio of the neonates.Females from the LFT group had smaller litter sizes and,therefore,lower litter mass than those from the HFT group.These results suggest that female racerunners facing food restriction lay fewer offspring with unchanged body size and locomotor performance,and incur a cost in the form of poor postpartum body condition and immune function.The flexibility of maternal responses to variable food availability represents an important life strategy that could enhance the resistance of lizards to unpredictable environmental change.     

Long-term cleaner fish presence affects growth of a coral reef fish

Cleaning behaviour is considered to be a classical example of mutualism. However, no studies, to our knowledge, have measured the benefits to clients in terms of growth. In the longest experimental study of its kind, over an 8 year period, cleaner fish Labroides dimidiatus were consistently removed from seven patch reefs (61-285 m(2)) and left undisturbed on nine control reefs, and the growth and parasite load of the damselfish Pomacentrus moluccensis determined. After 8 years, growth was reduced and parasitic copepod abundance was higher on fish from removal reefs compared with controls, but only in larger individuals. Behavioural observations revealed that P. moluccensis cleaned by L. dimidiatus were 27 per cent larger than nearby conspecifics. The selective cleaning by L. dimidiatus probably explains why only larger P. moluccensis individuals benefited from cleaning. This is the first demonstration, to our knowledge, that cleaners affect the growth rate of client individuals; a greater size for a given age should result in increased fecundity at a given time. The effect of the removal of so few small fish on the size of another fish species is unprecedented on coral reefs.     

Parental care of kestrels living in stable and varying environmental conditions

We studied parental behaviour of two populations of Eurasian kestrels (Falco tinnunculus) living in environments with different reproductive constraints: a northern Italy population, resident, with relatively long reproductive season and almost stable prey availability, and a western Finland population, long-distance migrant, with short reproductive season and fluctuating availability of main prey (voles). The Finnish population was studied during a low vole availability year. We aimed to test the supposition that in variable environments the few pairs capable of breeding during low prey availability years show high-quality parental care. To estimate the quality of parental care in both Finnish and Italian pairs we measured the difference in the number of prey delivered by the male and by the female within each pair. The parental care, measured by that parameter, was greater among Italian pairs during both the incubation and early nestling periods. However, this was not the case in the late nestling period, when food requirements of offspring are greater. Moreover, the Finnish and Italian pairs produced about the same number of fledglings. Therefore, our data supported the initial supposition and we conclude that the few Finnish pairs capable of breeding in a low vole availability year show parental care similar to Italian pairs.     

Influence of recruit condition on food competition and predation risk in a coral reef fish

Settlement rate is considered to be a major determinant of the population structure of coral reef fishes. In this study, the effects of larval physiological condition on survival, predation risk and competitive ability are assessed for a small damselfish, Pomacentrus moluccensis. New settlers were collected and fed for 5 days to produce high and low condition (measured as lipid) treatment fish. In a field experiment, pairs (one high and one low condition fish) were transplanted to corals. Persistence over 2 weeks was much higher (100% vs. 25%) in high condition fish. In mixed groups in the laboratory, high condition fish were both aggressively dominant and consumed more of a limiting prey source than low condition fish. In addition, low condition fish were shown to be at much higher risk of predation. All of the low condition fish but only 33% of high condition fish in mixed groups were consumed by fish predators, and in a separate experiment, 73% of feeding strikes by predators were directed at low condition fish. Quality of new settlers can have an important influence on subsequent juvenile survival. The mechanisms for this effect are likely to include a combination of effects of condition on food competition and predation risk.     

Food supply controls the body condition of a migrant bird wintering in the tropics

The hypothesis that migratory bird populations are limited during the non-breeding season is increasingly supported by empirical studies that also suggest consequences that carry-over into subsequent seasons. Although variation in food supply is the best supported explanation for non-breeding season limitation, the ecological mechanisms and physiological consequences are not well understood. We both supplemented and reduced Ovenbird (Seiurus aurocapilla) food availability on replicate plots in Jamaica in each of 3 years to determine the direct role of food in limiting the physical condition of Ovenbirds. Annual variation in rainfall and food supply created a natural experiment in parallel with manipulations. Sex and age-classes of Ovenbirds did not respond differently in terms of body condition to either food manipulation or natural variation in environmental conditions, suggesting that this population is not structured by strong dominance relationships. Ovenbird body mass, fat, and pectoralis muscle shape were positively and predictably related to manipulated food availability. Feather regrowth rate also responded positively to food supplementation and negatively to food reduction in the drier of 2 years. Prior to manipulation, annual variation in body mass corresponded to annual variation in food supply and rainfall, providing additional, correlational evidence of food limitation. Since multiple intercorrelated body condition indices of Ovenbirds responded directly to food supply, and since food supply influenced body condition independently of other habitat features, we argue that food is a primary driver of non-breeding season population limitation. Moreover, since these effects were observed during the late non-breeding period, when individuals are preparing to migrate, we infer that food availability likely initiates carry-over effects.     

Filial cannibalism in a non-brood cycling marine fish

Synopsis Direct observation and the analysis of stomach contents indicate that custodial male Cortez damselfish, Stegastes rectifraenum, regularly eat some of the eggs they fertilize and guard. The pattern of forced starvation and brood cycling by custodial males that is seen in sticklebacks and other fishes for which filial cannibalism is known in the field does not exist for this species. These results indicate that filial cannibalism is likely to be more common among fishes than previously thought, because this behavior is not restricted to species that brood cycle.     

Influence of depth on sex-specific energy allocation patterns in a tropical reef fish

The effect of depth on the distribution and sex-specific energy allocation patterns of a common coral reef fish, Chrysiptera rollandi (Pomacentridae), was investigated using depth-stratified collections over a broad depth range (5–39 m) and a translocation experiment. C. rollandi consistently selected rubble habitats at each depth, however abundance patterns did not reflect the availability of the preferred microhabitat suggesting a preference for depth as well as microhabitat. Reproductive investment (gonado-somatic index), energy stores (liver cell density and hepatocyte vacuolation), and overall body condition (hepato-somatic index and Fulton’s K) of female fish varied significantly among depths and among the three reefs sampled. Male conspecifics displayed no variation between depth or reef. Depth influenced growth dynamics, with faster initial growth rates and smaller mean asymptotic lengths with decreasing depth. In female fish, relative gonad weight and overall body condition (Fulton’s K and hepato-somatic index) were generally higher in shallower depths (≤10 m). Hepatic lipid storage was highest at the deepest sites sampled on each reef, whereas hepatic glycogen stores tended to decrease with depth. Depth was found to influence energy allocation dynamics in C. rollandi. While it is unclear what processes directly influenced the depth-related patterns in energy allocation, this study shows that individuals across a broad depth gradient are not all in the same physiological state and may contribute differentially to the population reproductive output. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Zooplankton capture by a coral reef fish: an adaptive response to evasive prey

Synopsis High-speed cinematography and video using modified Schlieren optics and laser illumination helped elicit details of prey capture mechanisms used by Chromis viridis while feeding on calanoid copepods and Artemia. Chromis viridis is capable of a ram-jaw, low-suction feeding, as well as a typical suction feeding behavior described for other species of planktivores. By adjusting the degree of jaw protrusion and amount of suction used during a feeding strike, this fish can modulate its feeding strikes according to the prey type being encountered. The ram-jaw feeding mode enables C. viridis to capture highly evasive calanoid copepods within 6 to 10 msec. The use of specialized feeding behavior for evasive prey and the ability to vary feeding behavior are adaptations for feeding on evasive prey.     

The role of microhabitat preference and social organization in determining the spatial distribution of a coral reef fish

Both habitat preferences and social organization can influence the spatial distributions of individuals. We explored effects of individual behavior and social organization on distributions of arc-eye hawkfish (Paracirrhites arcatus) in lagoons of French Polynesia. Analysis of habitat selectivity data obtained during surveys revealed that the most highly preferred microhabitat of arc-eye hawkfish was large Pocillopora coral with an open branching morphology. However, such corals were rare and most hawkfish occupied smaller, less preferred Pocillopora. Indeed, total abundance of Pocillopora explained nearly two thirds of the lagoon-wide variation in abundance of hawkfish and the derived relationship between the numbers of hawkfish and Pocillopora predicted 86% of the spatial variation in hawkfish abundance during subsequent surveys. In contrast, large, open-branched Pocillopora explained little of the spatial variation in abundance of hawkfish. Individual behavior and social organization significantly impacted use of the most highly preferred Pocillopora. During a colonization experiment set up outside hawkfish home ranges, all colonizers resided on the most highly preferred corals. Following addition of the most highly preferred Pocillopora corals to areas occupied by hawkfish, only the largest, socially dominant individuals obtained access to added corals, spending significantly more time and increasing both aggressive acts and prey attacks from these substrates. These results illustrate the importance of understanding the modulating effects of social behavior on habitat use and explain why most hawkfish individuals do not occupy their most preferred microhabitat type.