Behavioral acquisition of the hindgut fermentation system by hatchling Iguana iguana

Summary Hatchling green iguanas (Iguana iguana) spend the first month of life obtaining their hindgut fermentation systems before settling into their characteristic habitat. Newly hatched iguanas consume soil within the nest chamber, establishing populations of soil microbes in the hindgut, before digestive activity begins in the stomach. Hatchlings dig their way out of the nest chamber during the first week after hatching; once on the surface, they may eat both soil and plant materials. The rudimentary microbial fermentation system acquired through soil consumption supports faster growth, and presumably more effective degradation of plant materials, than microbes obtained without contact with soil. During the second and third weeks of life, hatchling iguanas disperse away from the nesting area. They travel into the forest canopy, associate with older consepcifics, and obtain a more complex and effective microbe community by consuming the feces of their seniors. Approximately a month after hatching, iguanas settle into low vegetation in open habitats, and may continue to associate in groups.     

Effects of simulated sublethal predation on the growth and regeneration rates of a spionid polychaete in laboratory flumes

Most spionid polychaetes switch from deposit feeding to suspension feeding as current speed and the flux of suspended food increase. Growth rates of juvenile Polydora cornuta are strongly affected by flow and can be as rapid as 60% day⁻¹ in moderate currents. Feeding palps that extend above the sediment–water interface during suspension feeding are especially vulnerable to sublethal predation, but individuals with damaged posteriors are also common. We performed a series of laboratory flume experiments to test the effects of sublethal tissue damage on the growth and regeneration rates of P. cornuta juveniles. Replicated experiments were conducted at three flow speeds in counter-rotating annular flumes containing field-collected sediment and a nonliving algal slurry as deposited and suspended food. In the first set of experiments, we removed 2, 1, or 0 of worms’ two feeding palps and measured the relative growth rates of worm bodies and palps after 3 days in the flumes. Worms that lost both palps grew significantly slower than the other two groups, but the growth rate of worms that had one undamaged palp was not significantly different from worms that had two undamaged palps. Faster flow speeds significantly increased rates of body growth, and there was a significant interaction between flow and the effect of palp loss. During the 3-day experiments, damaged palps fully regenerated and often grew larger than they were prior to being removed. Damaged palps also grew significantly faster than undamaged palps. The second set of experiments tested the effects of removing a worm’s posterior region (~18% of body volume). The growth rates of these damaged and undamaged worms did not differ significantly. By the end of a 3-day flume experiment, damaged worms had grown 6× larger than they were prior to the posterior damage. The rapid regeneration of damaged palps and posterior tissue in moderate flows that allow suspension feeding suggests that sublethal predation on spionids might be more frequent than previously estimated and will have little impact on the growth of juvenile recruits.     

Social dominance in migrant and resident turkey vultures at carcasses: evidence for a despotic distribution?

We placed carcasses in three different vegetation types in the heterogeneous savannas of central Venezuela to investigate the role of social dominance in habitat use by flocking migrant and resident turkey vultures (Cathartes aura meridionalis and C. a. ruficollis). Migrants foraged primarily in savanna habitats while residents foraged almost exclusively in gallery forest. In the gallery forest residents discovered carrion first significantly more often than migrants, despite there being equal densities of residents and migrants foraging over this habitat. Because residents fed in smaller groups than migrants at carcasses they had higher feeding rates. There was also a negative relationship between group sizes of residents and migrants. The feeding rate of residents declined in response to increased group size of migrants, but group size of residents had no effect. Migrant group size also had a greater effect on resident feeding rates than king vulture presence or absence. When the effect of migrant and resident group size on feeding rates in migrants was compared, the most significant factor was migrant group size. A second analysis showed that both resident group size and presence or absence of king vultures had a significant effect on feeding rates in migrants. Rates of agonistic encounters in migrant and resident turkey vultures increased weakly in relation to group size. However, there was an increase in residents' encounter rate with migrants in relation to increased migrant group size; there was no difference in resident encounter rates with other residents in relation to resident group size. Migrants dominated residents in almost all agonistic interactions over carcasses. We suggest that savanna habitats were less attractive to residents for foraging because they held larger groups of migrants.     

Malaria infection and host behavior: a comparative study of Neotropical primates

Parasites are ubiquitous in populations of free-ranging animals and impact host fitness, but virtually nothing is known about the factors that influence patterns of disease risk across species and the effectiveness of behavioral defenses to reduce this risk. We investigated the correlates of malaria infection (prevalence) in Neotropical primates using data from the literature, focusing on host traits involving group size, body mass, and sleeping behavior. Malaria is spread to these monkeys through anopheline mosquitoes that search for hosts at night using olfactory cues. In comparative tests that used two different phylogenetic trees, we confirmed that malaria prevalence increases with group size in Neotropical primates, as suggested by a previous non-phylogenetic analysis. These results are consistent with the hypothesis that larger groups experience increased risk of attack by mosquitoes, and counter to the hypothesis that primates benefit from the encounter-dilution effect of avoiding actively-seeking insects by living in larger groups. In contrast to non-phylogenetic tests, body mass was significant in fewer phylogeny-based analyses, and primarily when group size was included as a covariate. We also found statistical support for the hypothesis that sleeping in closed microhabitats, such as tree holes or tangles of vegetation, reduces the risk of malaria infection by containing the host cues used by mosquitoes to locate hosts. Due to the small number of evolutionary transitions in sleeping behavior in this group of primates, however, this result is considered preliminary until repeated with a larger sample size. In summary, risk of infection with malaria and other vector-borne diseases are likely to act as a cost of living in groups, rather than a benefit, and sleeping site selection may provide benefits by reducing rates of attack by malaria vectors.Electronic Supplementary Material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Disentangling the effects of group size and density on shoaling decisions of three-spined sticklebacks (<Emphasis Type="Italic">Gasterosteus aculeatus</Emphasis>)

Many animals live in groups most of their life. One function of this behaviour is an increased predator protection whereas larger groups provide better protection than smaller ones. A causal explanation is that due to a higher number of shoal members the individual risk of being predated will decrease (“dilution effect”). Additionally, shoaling leads to increased predator confusion. This “confusion effect” can be strengthened by an increased group density, which often correlates with group size. Many studies found that individuals prefer the larger of two groups. However, whether this preference is due to a larger group size or because of an increased density of the larger group remained unclear. To disentangle these factors we gave three-spined sticklebacks (Gasterosteus aculeatus) the choice between shoals of (1) different group size and density, (2) different group size, but equal density and (3) equal group sizes, but different densities. As expected, test fish preferred the larger and denser shoal over the smaller, less dense one. This preference was lost when shoal size differed but density was kept constant. When shoal size was equal but density differed, test fish preferred the less dense shoal. However, this was only the case when test fish chose between two relatively dense shoals. On the other hand, when overall density was low, test fish did not discriminate between shoals of different densities. This result may be explained in terms of predator avoidance. The results show that shoaling preferences might not always be influenced by a higher number of group members but also by the density and cohesiveness of the respective groups. An erratum to this article can be found at     

Distribution and early life history of <Emphasis Type="Italic">Kaupichthys</Emphasis> leptocephali (family Chlopsidae) in the central Indonesian Seas

Leptocephali of the widely distributed tropical marine eels of the genus Kaupichthys (family Chlopsidae) were collected around Sulawesi Island during a sampling survey in the Indonesian Seas in late September and early October 2002, and the otolith microstructure of 24 of the 59 specimens captured was examined to learn about the larval growth rates and spawning times of these small sized eels. Leptocephali ranging in size from 25 to 60 mm were collected in Makassar Strait and the Celebes Sea, but they were most abundant in the semi-enclosed Tomini Bay of northeast Sulawesi Island. The Kaupichthys leptocephali examined had 39–161 otolith growth increments. Their back-calculated hatching dates indicated that five age groups were present and each group appeared to have been spawned around the full moon of previous months. Average growth rate estimates of the first two age groups were 0.65 and 0.54 mm/day for the 27.4–30.4 and 37.6–45.6 mm age classes. The growth rates of the oldest three age groups (52.0–60.8 mm) appeared to have slowed down after they reached their approximate maximum size. An increase in increment widths at the outer margin of the otoliths of those larger than 53 mm suggested that the process of metamorphosis had begun even though there were few external morphological changes indicating metamorphosis. It is hypothesized that chlopsid leptocephali have an unusually short gut that may not need to move forward during early metamorphosis. The presence of four age classes in Tomini Bay suggests that the Togian Islands region may be productive habitats for Kaupichthys juveniles and adults.     

Effects of sand deposition on scleractinian and alcyonacean corals

The ability of corals to withstand experimental sand deposition was investigated for two experimental periods (17 h and 6 wk) in eight scleractinia (Favia favus, Favites pentagona, Platygyra daedalea, Gyrosmilia interrupta, Galaxea fascicularis, Cyphastrea chalcidicum, Favites abdita, Goniopora dijboutensis) and five alcyonacea (Lobophytum depressum, L. venustum, Sinularia dura, S. leptoclados, Sarcophyton glaucum) collected in 1992 from Natal, South Africa. Scleractinia were active sediment shedders, alcyonacea passive, relying on water motion and gravity. Short-term sand clearing efficiency was primarily dependent on corallum shape. Sand application led to hydrostatic inflation of polyps in scleractinia and the entire colony in alcyonacea as well as to increased tentacular action in the scleractinian Gyrosmilia interrupta. Under continuous sand application, inflation remained while other activities, such as tentacular motion, ceased completely. In scleractinia and alcyonacea, tissue necroses appeared after the first week of continuous sand application. Death of entire colonies and partial bleaching of continually sandcovered areas were observed in alcyonacea only. Different grain sizes of sand had no influence on clearing reaction or efficiencies.     

Competition and group size in Thomas’s langurs (Presbytis thomasi): the folivore paradox revisited

Among primates, group size is highly variable. The standard ecological model assumes that better predation avoidance as group size increases favours living in larger groups, whereas increased travel costs and reduced net food intake due to within-group competition for resources set the upper limit. Folivorous primates, however, tend to defy this generalisation in that some live in small groups despite low costs of feeding competition. To resolve this ’folivore paradox’, it has been suggested that folivore group size is limited by social factors such as male harassment or infanticide, or that females can disperse more easily and thus maintain group size near optimum levels. In this paper, we examine the effects of group size on home range size, day-journey length, activity budget and diet in wild Thomas’s langurs (Presbytis thomasi), which live in one-male multi-female groups with a limited life cycle. We examined only data from the stable middle tenure phase when factors such as the strength of the breeding male or the way in which groups were formed did not influence ranging and activities. During this phase, group size affected day-journey length and home range size, and had a minor effect on diet, but did not influence time spent feeding or resting, allogrooming or birth rates. Hence the upper limit to group size during the middle tenure phase in Thomas’s langurs is not set by feeding competition. The folivore paradox is not due to frequent female dispersal in Thomas’s langurs. The timing of female dispersal is not as expected if it serves to keep group sizes near the ecological optimum, and groups seem to be below this optimum. Instead, female reproductive success is presumably maximised in small to mid-sized groups because larger groups show a clear trend to experience higher risk of take-over, often accompanied by infanticide. Because females can redistribute themselves among nearby groups when groups reorganise each time a new male starts up a new group, females can keep the group small. Thus, a social factor, risk of infanticide, seems to provide the selective advantage to small group size in Thomas’s langurs. Received: 29 July 1999 / Revised: 17 November 1999 / Accepted: 15 October 2000     

Variation in pelagic larval growth of Atlantic billfishes: the role of prey composition and selective mortality

Atlantic blue marlin (Makaira nigricans) and sailfish (Istiophorus platypterus) larvae were collected from 10 monthly cruises (June–October 2003 and 2004) across the Straits of Florida to test (1) whether growth differed between the more productive western region near the Florida shelf, and the less productive eastern region toward the Bahamas, and (2) whether growth was related to prey consumption. Examination of larval sagittal otoliths revealed that instantaneous growth and daily growth during the first 2–3 weeks of life did not vary significantly between the two regions for either species. However, recent growth during the last two full days prior to collection was greater in the west for blue marlin larvae. Recent growth of blue marlin larvae <9 mm SL (primarily zooplanktivorous) was significantly related to prey composition (faster growth when higher proportions of Farranula copepods were consumed). Western larvae grew faster and had higher proportions of Farranula in their guts. Trends for sailfish larvae were not significant. In both species, comparison of early growth between <9 and ≥9 mm SL size groups indicated that growth trajectories diverged around 5–8 mm SL, the time when billfish larvae become capable of piscivory. Significantly faster growth of larger (older) larvae suggests that mortality was selective for fast growers and that the transition to piscivory may be a critical point in the early life of billfish.     

Highly dynamic fission–fusion species can exhibit leadership when traveling

Leadership by specific individuals is thought to enhance the fitness of followers by allowing them to take advantage of the knowledge or skills of key individuals. In general, consistent leadership is expected to occur primarily in stable groups of related individuals where the benefits enhance the inclusive fitness of a leader. Societies with less stability in group composition (i.e., fission–fusion groups) are less likely to feature unshared decision making. However, in situations where frequent interactions among individuals occur (e.g., small population size and small range of movement) and/or the complexity of the environment requires substantial experience and knowledge, consistent leadership might be expected. We tested if a highly dynamic fission–fusion population of bottlenose dolphins (Tursiops truncatus), inhabiting a complex environment, exhibited leadership when traveling. A small number of specific individuals led group travel more often than expected by chance, and were more likely to initiate successful direction changes of groups than following individuals. The number of leaders in a group remained relatively constant across a wide range of group sizes and was not affected by the number of potential leaders (i.e., those that had led previously) present in the group. Together, these results suggest that leadership can occur in species with high rates of group fission and fusion. Therefore, the loss of key individuals could have disproportionate effects on population dynamics.     

Relationships between spacing behavior and growth rates: A field study of lizard feeding territories

Summary Relationships between spacing behavior and growth rates were investigated in a field experiment with juvenile lizards, Anolis aeneus. The behavioral variable most closely related to juvenile growth was distance moved per unit time. This variable had a curvilinear relationship with growth, such that juveniles moving approximately 400 cm/h grew more rapidly than those traveling either larger or shorter distances per unit time. Daily fluctuations in arthropod abundance were also related to growth rates, with restricted growth during periods of low food availability. Temporal fluctuations in prey and distance traveled per unit time had independent effects on growth; together these two variables accounted for 43% of the variance in growth rate for the juveniles in this study.Territory size, overlap and social status appeared to affect growth indirectly, by influencing distance traveled per unit time. Optimal travel distances of around 400 cm/h were most likely when a juvenile had a relatively exclusive territory of about 0.5 m². High ranking juveniles were more apt to achieve this spacing pattern than were low ranking juveniles, but some high ranking juveniles had very large territories, extensive overlap with subordinates, supraoptimal travel distances and relatively low growth rates. Low ranking juveniles tended to fall into two groups: subordinates, with a small home range overlapping that of a more dominant individual and low travel distances, and floaters, with a large home range overlapping several more dominant individuals and high travel distances. Although a few low ranking juveniles achieved travel distances permitting high growth rates, most had either supra or suboptimal travel distances and relatively low growth rates.     

Group size in wedge-capped capuchin monkeys Cebus olivaceus and the reproductive success of males and females

Summary The effect of variation in group size on age-specific survivorship and fecundity rates were examined in a population of wedge-capped capuchin monkeys Cebus olivaceus during a 10 year study. Life tables were constructed separately for four large (15 individuals) and four small groups (<15 individuals). Female reproductive success, and its relative contribution to population growth, was much higher in large groups, primarily through higher age-specific fecundity. Age-specific survivorship was similar in groups of different sizes. The reproductive success of the single breeding male in a group was much higher in large than small groups. Compared to small groups, breeding males in large groups had a longer breeding tenure, and access to greater numbers of reproductive females with a higher average fecundity. Differences in female reproductive success apparently resulted from variation in access to monopolizable fruit trees. Large groups predictably displaced small groups during intergroup encounters. Group rank depended on the number of males resident in groups. The large number of non-breeding males in large groups results from their longer average residency time. I explain the longer residency of males in large groups by the higher average reproductive success of breeding males in these groups.     

Foraging strategies of the marine iguana,Amblyrhynchus cristatus

Summary Two foraging strategies were found in marine iguanas (Amblyrhynchus cristatus); (1) subtidal feeding: the animals swam out to sea and dived for algae in the subtidal zone; (2) intertidal feeding: the animals foraged around low tide in the intertidal zone on more or less exposed algae. Most marine iguanas were very consistent in their foraging strategy and so could be classified as subtidal feeders (SFs) or intertidal feeders (IFs). Feeding strategy was weight-related (Fig. 1), not sexspecific. Animals 1,200 g were IFs, animals >1,800 g SFs. Some iguanas in between followed a mixed foraging strategy. SFs foraged independently of the tides, IFs always around low tide (Figs. 2, 3). Feeding time patterns of IFs and SFs are described (Table 1). Sea motion seemed to have little effect on the foraging pattern of SFs, but strongly influenced that of IFs (Fig. 2). The smaller a marine iguana, the faster it cooled when immersed in water (Fig. 4). The difference between water temperature and core temperature of animals returning from foraging was significantly greater in IFs than SFs (Fig. 5). SFs swimming in very cold water regulated their body temperature to prevent excessive cooling. Possible costs and benefits of the two foraging strategies are discussed. Only part of a marine iguana population lives really amphibiously and only ca. 5% of a 24 h day is spent close to or in the water. All social activities, including mating, take place on land. These life history characteristics preclude those adaptations to an amphibious way of life that would at the same time reduce the iguanas' ability to be maximally active at their typical terrestrial body temperature of 35° C.     

Females prefer larger leks: field experiments with ruffs (Philomachus pugnax)

Summary Preference by females for choosing mates at male aggregations has been hypothesized as the primary selective pressure favoring the formation of leks, but alternative hypotheses account for lek formation without invoking female preference. Observational studies to determine whether male mating success increases with lek size, as predicted under the female preference hypothesis, have produced inconsistent results, possibly due to covariation of lek size with other variables or to male-male or intersexual conflict over lek size. We tested whether females prefer larger leks in a field experiment with ruffs (Philomachus pugnax), a lekking sandpiper, in which male group size, composition, and location were controlled. Wild females chose the larger of two adjacent groups often enough such that males in larger groups had significantly higher per capita rates of female visitation (Table 3). Such behavior would probably lead to higher per male mating rates at larger leks, which is generally considered a necessary condition for female choice to select for lek display (Fig. 2). Lek size in nature will reflect both female preference for larger leks and competition among males, which may favor smaller lek size. All else being equal, however, female ruffs preferred to visit larger groups strongly enough to maintain lekking by males.     

Allocation in reproduction is not tailored to the probable number of matings in common toad (<Emphasis Type="Italic">Bufo bufo</Emphasis>) males

The theory of life history evolution assumes trade-offs between competing fitness traits such as reproduction, somatic growth, and maintenance. One prediction of this theory is that if large individuals have a higher reproductive success, small/young individuals should invest less in reproduction and allocate more resources in growth than large/old individuals. We tested this prediction using the common toad (Bufo bufo), a species where mating success of males is positively related to their body size. We measured testes mass, soma mass, and sperm stock size in males of varying sizes that were either (1) re-hibernated at the start of the breeding season, (2) kept without females throughout the breeding season, or (3) repeatedly provided with gravid females. In the latter group, we also estimated fertilization success and readiness to re-mate. Contrary to our predictions, the relationship between testes mass and soma mass was isometric, sperm stock size relative to testes mass was unrelated to male size, fertilization success was not higher in matings with larger males, and smaller males were not less likely to engage in repeated matings than larger males. These results consistently suggest that smaller males did not invest less in reproduction to be able to allocate more in growth than larger males. Causes for this unexpected result may include relatively low year-to-year survival, unpredictable between-year variation in the strength of sexual selection and low return rates of lowered reproductive investment.     

Spacing and group coordination in a nocturnal primate, the golden brown mouse lemur (Microcebus ravelobensis): the role of olfactory and acoustic signals

In order to remain stable, dispersed social groups have to solve two fundamental problems: the coordination of movement and cohesiveness within a group and the spacing between the groups. Here, we investigate mechanisms involved in intra-group coordination and inter-group spacing using the golden brown mouse lemur, Microcebus ravelobensis, as a model for a nocturnal, solitary foraging mammal with a dispersed social system. By means of radiotelemetry and bioacoustics we studied the olfactory and vocal behaviour during nocturnal dispersal and reunion of five sleeping groups.All groups used 3–17 sleeping sites exclusively, suggesting a sleeping site-related territoriality and competition for them. The occurrence of olfactory and vocal behaviour showed an asymmetrical temporal distribution. Whereas marking behaviour was observed exclusively during dispersal, a particular call type, the trill, was used by all groups during reunions. Interestingly, these trills carried group-specific signatures.Our findings provide the first empirical evidence for nocturnal primates in a natural environment that olfactory signals represent an important mechanism to regulate the distribution of different groups in space, whereas acoustic signals control intra-group cohesion and coordination.     

Growth and mortality rates of the fan mussel <Emphasis Type="Italic">Pinna nobilis</Emphasis> in Lake Vouliagmeni (Korinthiakos Gulf,Greece): a generalized additive modelling approach

The temporal patterns and the effect of shell size and depth on growth and mortality rates of the endangered fan mussel Pinna nobilis were investigated in the marine Lake Vouliagmeni (Korinthiakos Gulf, Greece). A total of 160 individuals were tagged and monitored monthly for a period of 17 months. At each visit, the size of the tagged individuals (shell width, w) was measured in situ and recorded. Any mortality event was also recorded and attributed to natural causes or (illegal) fishing. Growth and mortality rates were modeled with generalized additive models, which are non-parametric flexible models that free the researcher from the limiting concept of a strict parametric shape. The use of GAMs allowed the exploration of shapes of growth and mortality response curves in relation to predictor variables and allowed the fitting of statistical models that better agree with ecological theory. Growth rates had a seasonal pattern, with an extended period of very slow growth between late autumn and early spring, i.e., during the cold season, another short period of slow growth during August (when water temperatures reached their maximum values exceeding 29°C), and a peak in growth rates during late spring–early summer, probably related to an optimum combination of temperature and food availability. Growth rates varied with shell size, with a peak at w ∼4.5 cm, followed by a sharp decline to an approximately constant level, with sizes ranging from 9 to 15 cm, and a further decline with larger sizes down to almost zero for w > 20 cm. Growth rates did not vary substantially with depth. Although P. nobilis is a protected species in the EU and its fishing is strictly prohibited, fishing mortality was very high in Lake Vouliagmeni (much greater than natural mortality), especially during the hot season when the lake was crowded by summer visitors. The fan mussels were poached exclusively by free-diving and due to the high turbidity of the lake’s water, fishing mortality was higher in shallow areas (and mostly for large individuals) and was practically zero at depths >9 m. Due to fishing mortality, a size segregation of P. nobilis was observed in the lake: large individuals were restricted to deeper areas, while young and small individuals were more abundant in shallow areas where there was preferential recruitment. Natural mortality was strikingly size dependent and P. nobilis suffered high natural mortality during the first year of life; the probability of death by natural causes quickly diminished as the fan mussels grew in size. No depth-related differences in natural mortality were found.     

The effects of alginate microspheres on phytoremediation and growth of Lemna minor in the presence of Cd

Today, the removal of increased amount of contaminant concentrations in nature such as metals, nanoparticles, has become an essential issue to struggle with. In this paper, both the toxic effects of non-essential Cd on Lemna minor in presence of alginate microspheres and metal removal capacity of Cd-exposed L. minor and microspheres were investigated. Three test groups were constructed: group (L) containing Lemna, group (M) containing alginate microsphere and group (L?+?M) containing alginate microsphere and Lemna. Five different levels of Cd concentrations were added into these groups. Estimation models were constructed to estimate Cd removal capacities of L. minor and microspheres, and time-concentration-dependent growth (GRC) rates of both microsphere-containing and non-containing groups were modelled by regression analysis. It was seen that microspheres alone only had no positive or negative effects on Lemna growth. It was observed that, for all test groups the time-dependent growth rate has both increase–decrease trends in general. The toxicity effect of Cd on Lemna growth was reduced by adding microspheres into medium. Furthermore, the results stated that test groups containing only microspheres (M, 0–97.56%) and consortium groups (M?+?L, 0–98.26%) were slightly more successful in Cd removal process compared to Lemna (L) groups (L, 0–97.32%).     

Growth patterns in oysters, Crassostrea virginica, from different estuaries

We analyzed a data set collected over 15 yr, containing growth data from strains of eastern oysters, Crassostrea virginica (Gmelin, 1791), initiated from parent populations in Long Island Sound, Delaware Bay, and lower Chesapeake Bay. The long-term growth data proved to be a powerful tool for examining patterns of growth differentiation among separated populations of C. virginica. The oyster strains had been grown in a common environment in lower Delaware Bay for up to seven generations. We found that the oyster strains with origins in Long Island Sound were significantly larger over several generations than oyster strains from Delaware Bay and Chesapeake Bay. Chesapeake Bay oyster strains were larger than Delaware Bay oyster strains at 1.5 yr old, but Delaware Bay oysters were larger at 2.5 yr. Year-to-year variation in environmental conditions had a strong significant effect on absolute oyster size and the relative sizes of the oyster strains. Persistent differences between oyster strains from different origins over several generations support a hypothesis that these estuarine populations have experienced long-term genetically-based population differentiation. This result is consistent with hypotheses of population differentiation of oysters based on observations of local reproductive timing. Received: 12 August 1997 / Accepted: 26 May 1998