Eric T. Schultz

Phenotypic similarity and the evolutionary significance of countergradient variation

Countergradient variation is a geographical pattern of genotypes (with respect to environments) in which genetic influences on a trait oppose environmental influences, thereby minimizing phenotypic change along the gradient. Phenotypic similarity across changing environments ought to be of intense interest because it belies considerable genotypic change. When it occurs in characters that are positively associated with fitness, countergradient variation conflicts with the hypothesis that local adaptation to one environment trades off against performance in another environment. Cases of countergradient variation therefore offer unique insight into the mechanisms that produce and maintain phenotypic similarity and/or differences along environmental gradients.

Latitudinal differences in somatic energy storage: adaptive responses to seasonality in an estuarine fish (Atherinidae: Menidia menidia)

Abstract This study focuses on the seasonal accumulation and depletion of somatic energy in the Atlantic silverside (Menidia menidia), an annual estuarine fish. Previous research revealed that northern silversides are subject to strong size-dependent winter mortality, while southern fish suffer no appreciable winter mortality. To examine whether there was geographic differentiation in allocation strategies, we compared temporal patterns of energy storage and utilization among three populations along this gradient in seasonality. The comparative design used monthly or biweekly samples of fish collected in the wild, as well as samples of fish from each population reared in a common environment, where genetic differences can be clarified. Somatic energy stores were quantified via gravimetric analysis of neutral storage lipids and lean tissue. Analysis revealed that small individuals maintained relatively low levels of lipid reserves, which may account for their lower survival in winter. Wild fish in the north rapidly accumulated large somatic reserves, which were depleted over the winter and then increased again during the subsequent spring breeding season. In wild southern fish, relatively small reserves accumulated slowly until breeding commenced in the spring. The common-environment comparison of somatic storage patterns revealed a genetic basis for among-population differences in reserve accumulation rates, but no differences in the amount of reserves stored. We conclude that the overwinter depletion of somatic reserves has a significant selective impact on energy accumulation and allocation strategies in seasonal environments.

Fish Schools: An Asset to Corals

Schools of juvenile haemulid fish feed in sea grass beds at night. By day they rest over coral heads, where they excrete substantial quantities of ammonium and particulate nitrogen and phosphorus into the nutrient-poor waters. The percentages of these nutrients contributed by the fish were comparable to those from other sources. Coral heads with resident fish schools grew faster than those without resident schools, indicating that fish may be more beneficial to the corals than has been assumed.

Social transmission of behavioural traditions in a coral reef fish

Abstract Traditional behaviours involve the non-genetic transmission of social information across age classes or generations. French grunts (Haemulon flavolineatum) exhibit social traditions of daytime schooling sites and twilight migration routes. Individuals transplanted to new schooling sites and allowed to follow residents at the new sites used the new migration routes and returned to the new sites in the absence of resident fish. Control fish with no opportunity to learn showed no such directionality or return. This is the first demonstration of apparent pre-cultural behaviour in free-living fish. Our observations suggest additional classes of behaviour and taxonomic groups in which pre-cultural activities are likely to have evolved.

The allometry of energy reserve depletion: test of a mechanism for size-dependent winter mortality

Abstract We experimentally tested the hypothesis that energy reserve depletion varies inversely with size in the fish Menidia menidia, an estuarine fish known to exhibit size-dependent winter mortality. Individuals in two size groups were starved at two winter temperatures (4°and 8°C) and sacrificed at a range of time intervals (up to 127 days). Lipid levels and lean tissue were analyzed to estimate somatic energy storage. As predicted, energy depletion was greater at high temperatures, and proportionally greater in small than in large fish. After 60 days of starvation at 4°C, small fish retained an average of 67% of their original energy reserves (vs 53% at 8°C), while large fish retained an average of 80% (vs 66% at 8°C). At 4°C, fish that were fed depleted their energy reserves as rapidly as unfed fish, but at 8°C, fish that were fed maintained reserves at higher levels than unfed fish. A high proportion of unfed fish (56% at 4°C, 27% at 8°C) died before they were to be sacrificed. Survival probability did not vary with size, nor was it influenced by the amount of energy reserves. The rate of energy depletion (equivalent to routine metabolic rate) decreased gradually over time, particularly in small fish. Routine metabolism did not conform to a single scaling relationship. Within each temperature-size group, the routine rate declined more rapidly than metabolically active mass (lean mass). At 8°C, the difference between size groups in energy depletion rate conformed closely to the expected allometry exponent of 0.8. In contrast, at 4°C, the estimated allometry exponent increased over the experiment (−0.19 to 2.5). We conclude that strategies to minimize energy loss may often modify bioenergetic scaling relationships.

Adaptive variation in energy acquisition and allocation among latitudinal populations of the Atlantic silverside

Abstract Understanding the evolution of growth rate requires knowledge of the physiology of growth. This study explored the physiological basis of countergradient variation (CnGV) in somatic growth across latitudinal populations of the Atlantic silverside, Menidia menidia. Energetics of northern (Nova Scotia, Canada) and southern (South Carolina, USA) genotypes were compared across resource levels, temperatures, and fish sizes to identify trade-offs to rapid growth. Offered unlimited resources, genotypes differed in both energy acquisition and allocation. Food consumption, growth, and efficiency of northern genotypes were consistently higher than in southern genotypes, across temperatures and body sizes. Feeding metabolism (specific dynamic action; SDA) was proportional to meal size, differing between genotypes to the extent that food consumption differed. Given limited resources, northern and southern genotypes displayed similar growth, efficiency, routine activity, and SDA across temperatures and fish sizes. Routine metabolism was equal at 17°C and 22°C, yet was significantly higher in northern fish at 28°C. Growth rates in M. menidia do not appear to trade off across environments or body sizes, i.e., at no temperature, ration, or size do southern fish outgrow northern conspecifics. Nor does submaximal growth result from increased costs of maintenance, tissue synthesis, or routine activity. Based on our findings, we propose that CnGV consumption and growth in M. menidia likely result from trade-offs with other energetic components, namely sustained and burst swimming.

Pterois volitans and Pterois miles: two valid species

Specimens of the nominal species Pterois volitans from the Indian and Pacific Oceans were examined. Based on meristic (number of dorsal and anal rays) and morphometric (length of pectoral fin, size of spots on vertical fins) evidence, specimens from the Indian Ocean are referred to P. miles. Character differences in specimens of P. volitans from Western Australia and the southern Pacific are evident, but the status of these groups remains unclear, pending the collection of more material.

Recruitment of coral reef fishes to Bermuda: Local retention or long-distance transport?

The benthic marine fauna of isolated oceanic islands may be self-seeding, or alternatively may be continually supplied with recruits advected from elsewhere by ocean currents. Estimates of the time required for transport of larvae from elsewhere was combined with information on the larval durations of fishes inhabiting the reefs of Bermuda, to test the hypothesis that other populations are sources for recruits to Bermuda. Specifically, we tested the prediction that transport occurs frequently enough to sustain local populations of reef fishes. Transport of larvae was modelled as a 2-step process in several numerical simulations. The first step of transport was assumed to occur via the Gulf Stream. The second step, in which larvae are transported across the northern Sargasso Sea to Bermuda, was modelled in several different ways, Involving either cold-core rings thrown off by the Stream, or mixtures of Gulf Stream and Sargasso water entrained by cold-core rings. Travel distances and speeds were estimated from a variety of sources, focusing on satellite imagery of sea-surface temperature Pelag~c larval durations (PLDs) for reef f~sh on Bermuda were analyzed from the daily age record in the otoliths of 6 wrasse and 1 parrotfish species. When PLDs were compared with transport times, it was determined that transport events would occur too infrequently to sustain Bermudian populations of reef fishes. The PLDs for most of these species are similar to results obtained elsewhere in their geographic range. It is evident that these populations must be mainly replenished via a pool of larvae that are spawned locally and retained in the vicinity of the Bermuda islands.

THE EFFECT OF BIRTH DATE ON FITNESS OF FEMALE DWARF PERCH, MICROMETRUS MINIMUS (PERCIFORMES: EMBIOTOCIDAE)

Lifetime reproductive success may vary considerably with birth date. I measured phenotypic selection on female birth date in a viviparous teleost fish (Embiotocidae: Micrometrus minimus) by sampling birth‐date cohorts over time in Tomales Bay, California. Four episodes of selection were measured: survival from birth to first reproduction, reproductive success in the first breeding season, survival to second reproduction, and reproductive success in the second season. Birth date had a significant impact on fitness in the first two episodes. Early born females were more successful in their first breeding season than late born females (directional selection on birth date), but early born females were less likely to survive the period between birth and first reproduction, relative to females born in the middle of the season (stabilizing selection on birth date). The final two episodes of selection had no detectable effect on birth date. Because of the relationship between birth date and survival in the first year, overall selection on female birth date was stabilizing.

Energetic constraints and size-based tactics : the adaptive significance of breeding schedule variation in a marine fish (Embiotocidae : Micrometrus minimus)

Female breeding schedules in the viviparous dwarf surfperch (Micrometrus minimus) vary systematically with size: small females conceive and bear young later in the season than large females. We test two general hypotheses for this pattern: (1) energy reserves impose a proximate energetic constraint on the timing of reproduction, and thus small females breed late because they carry less stored energy; (2) small females postpone reproduction as an adaptive size-specific tactic to maximize reproductive success. The hypotheses are not mutually exclusive, and both appear to apply to the dwarf surfperch. Among yearling fish, those that conceive their broods early in the season do so at higher reserve levels than females that breed late in the season. Late breeding places less demand on stored energy, and postponement enables individuals with low initial reserves to breed without drawing energy stores down to lethal low levels. However, lipid reserves do not explain all of the size-based variation in breeding schedule. In fish of both age classes, small size causes a delay in breeding, independent of lipid reserves. This is consistent with our second hypothesis, that small females postpone as a tactic. A potential tactical payoff is offspring number: because females grow throughout gestation, and fecundity increases with size, larger broods can be produced by delaying conception. We show that small females realize greater proportional increases in fecundity for equal delays in breeding.