Using external morphology as a proxy for stomach size in Hemigrapsus sanguineus

Stomach morphology can provide insights into an organism's diet. Gut size or length is typically inversely related to diet quality in most taxa, and has been used to assess diet quality in a variety of systems. However, it requires animal sacrifice and time-consuming dissections. Measures of external morphology associated with diet may be a simpler, more cost-effective solution. At the species level, external measures of the progastric region of the carapace in brachyuran crabs can predict stomach size and diet quality, with some suggestion that this approach may also work to examine individual diet preferences and specialization at the individual level; if so, the size of the progastric region could be used to predict trends in diet quality and consumption for individuals, which would streamline diet studies in crabs. Here, we tested whether external progastric region size predicts internal stomach size across latitude and time of year for individuals of the invasive Asian shore crab Hemigrapsus sanguineus. We found that the width of the progastric region increased at a faster rate with body size than stomach width. In addition, the width of the progastric region followed different trends across sites and over time compared to stomach width. Our results therefore suggest that the progastric region may not be used as a proxy for stomach size variation across individuals.

Shift from income breeding to capital breeding with latitude in the invasive Asian shore crab Hemigrapsus sanguineus

Organisms vary in the timing of energy acquisition and use for reproduction. Thus, breeding strategies exist on a continuum, from capital breeding to income breeding. Capital breeders acquire and store energy for breeding before the start of the reproductive season, while income breeders finance reproduction using energy acquired during the reproductive season. Latitude and its associated environmental drivers are expected to heavily influence breeding strategy, potentially leading to latitudinal variation in breeding strategies within a single species. We examined the breeding strategy of the Asian shore crab Hemigrapsus sanguineus at five sites spanning nearly 10° of latitude across its invaded United States range. We hypothesized that the primary breeding strategy of this species would shift from income breeding to capital breeding as latitude increases. We found that though this species' breeding strategy is dominated by capital breeding throughout much of the range, income breeding increases in importance at lower latitudes. This latitudinal pattern is likely heavily influenced by the duration of the foraging and breeding seasons, which also vary with latitude. We also found that reproductive characteristics at the northern and southern edges of the invaded range were consistent with continued range expansion. We suggest that the reproductive flexibility of the Asian shore crab is a key facilitator of its continued invasion success. Our results highlight the influence of latitude on the breeding strategy of a species and emphasize the need for further research regarding the ecological importance and implications of flexibility in breeding strategies within species.

Past energy allocation overwhelms current energy stresses in determining energy allocation trade-offs

Regeneration of lost appendages is a gradual process in many species, spreading energetic costs of regeneration through time. Energy allocated to the regeneration of lost appendages cannot be used for other purposes and, therefore, commonly elicits energetic trade-offs in biological processes. We used limb loss in the Asian shore crab Hemigrapsus sanguineus to compare the strength of energetic trade-offs resulting from historic limb losses that have been partially regenerated versus current injuries that have not yet been repaired. Consistent with previous studies, we show that limb loss and regeneration results in trade-offs that reduce reproduction, energy storage, and growth. As may be expected, we show that trade-offs in these metrics from historic limb losses far outweigh trade-offs from current limb losses, and correlate directly with the degree of historic limb loss that has been regenerated. As regenerating limbs get closer to their normal size, these historical injuries get harder to detect, despite the continued allocation of additional resources to limb development. Our results demonstrate the importance of and a method for identifying historic appendage losses and of quantifying the amount of regeneration that has already occurred, as opposed to assessing only current injury, to accurately assess the strength of energetic trade-offs in animals recovering from nonlethal injury.

Predicting diet in brachyuran crabs using external morphology

Morphological traits have often been used to predict diet and trophic position of species across many animal groups. Variation in gut size of closely related animals is known to be a good predictor of dietary habits. Species that are more herbivorous or that persist on low-quality diets often have larger stomachs than their carnivorous counterparts. This same pattern exists in crabs and in most species, individuals exhibit external markings on the dorsal side of their carapace that appear to align with the position and size of their gut. We hypothesized that these external markings could be used as an accurate estimate of the crab's cardiac stomach size, allowing an approximation of crab dietary strategies without the need to sacrifice and dissect individual animals. We used literature values for mean diet and standardized external gut size markings taken from crab photographs across 50 species to show that percent herbivory in the diet increases non-linearly across species of brachyuran crab with the external estimate of gut size. We also used data from dissections in four species to show that external gut markings were positively correlated with gut sizes, though the strength of this correlation differed across species. We conclude that when rough approximations of diet quality such as percent herbivory will suffice, measuring external carapace markings in crabs presents a quick, free, non-lethal alternative to dissections. Our results also provide important insights into tradeoffs that occur in crab morphology and have implications for crab evolution.

Developmental Change in Predators Drives Different Community Configurations

AbstractTheoreticians who first observed alternative stable states in simple ecological models warned of grave implications for unexpected and irreversible collapses of natural systems (i.e., regime shifts). Recent ecosystem-level shifts engendering considerable economic losses have validated this concern, positioning bistability at the vanguard of coupled human-environment systems management. While the perturbations that induce regime shifts are known, the ecological forces that uphold alternative stable states are often unresolved or complex and system specific. Thus, the search continues for general mechanisms that can produce alternative stable states under realistic conditions. Integrating model predictions with long-term zooplankton community experiments, we show that the core feature of ontogenetic development, food-dependent maturation, enables a single community to reach different configurations within the same constant environment. In one configuration, predators regulate prey to foster coexistence, while in the other, prey counterintuitively exclude their predators via maturation-limiting competition. The concordance of these findings with the unique outcome and underlying mechanism of a general model provides empirical evidence that developmental change, a fundamental property of life, can support bistability of natural systems.

Deadly competition and life-saving predation: the potential for alternative stable states in a stage-structured predator-prey system

Predators often undergo complete ontogenetic diet shifts, engaging in resource competition with species that become their prey during later developmental stages. Theory posits that this mix of stage-specific competition and predation, termed life-history intraguild predation (LHIGP), can lead to alternative stable states. In one state, prey exclude predators through competition (i.e. juvenile competitive bottleneck), while in the alternative, adult predators control prey density to limit competition and foster coexistence. Nevertheless, the interactions leading to these states have not been demonstrated in an empirical LHIGP system. To address this gap, we manipulated densities of cannibalistic adult cyclopoid copepods (Mesocyclops edax) and their cladoceran prey (Daphnia pulex) in a response-surface design and measured the maturation and survival of juvenile copepods (nauplii). We found that Daphnia reduced and even precluded both nauplii maturation and survival through depletion of a shared food resource. As predicted, adult copepods enhanced nauplii maturation and survival through Daphnia consumption, yet this positive effect was dependent on the relative abundance of Daphnia as well as the absolute density of adult copepods. Adult copepods reduced nauplii survival through cannibalism at low Daphnia densities and at the highest copepod density. This work demonstrates that predation can relax a strong juvenile competitive bottleneck in freshwater zooplankton, though cannibalism can reduce predator recruitment. Thus, our results highlight a key role for cannibalism in LHIGP dynamics and provide evidence for the interactions that drive alternative stable states in such systems.

Trait-mediated functional responses: predator behavioural type mediates prey consumption

The predator functional response (i.e. per capita consumption rate as a function of prey density) is central to our understanding of predator-prey population dynamics. This response is behavioural, depending on the rate of attack and time it takes to handle prey. Consistent behavioural differences among conspecific individuals, termed behavioural types, are a widespread feature of predator and prey populations but the effects of behavioural types on the functional response remain unexplored. We tested the effects of crab (Panopeus herbstii) behavioural type, specifically individual activity level, on the crab functional response to mussel (Brachidontes exustus) prey. We further tested whether the effects of activity level on the response are mediated by the presence of toadfish (Opsanus tau) predation threat in the form of waterborne chemical cues known to reduce crab activity level. The effects of crab activity level on the functional response were dependent on crab body size. Individual activity level increased the magnitude (i.e. slope and asymptote) of the type II functional response of small crabs, potentially through an increase in time spent foraging, but had no effect on the functional response of large crabs. Predation threat did not interact with activity level to influence mussel consumption, but independently reduced the slope of the type II functional response. Overall, this study demonstrates size-specific effects of a behavioural type on a predator-prey interaction, as well as a general pathway (modification of the functional response) by which the effects of individual behavioural types can scale up to influence predator-prey population dynamics.

The role of individual behavior type in mediating indirect interactions

Trait-mediated indirect interactions (TMII) play an important role in structuring natural communities, and numerous studies have experimentally demonstrated their presence in a variety of systems. However, these studies have largely examined the presence or absence of traits that are responsible for these interactions, without considering natural variation between individuals in the extent to which these traits are manifested. We used a well-documented TMII to investigate the importance of individual behavior type for determining the strength of the TMII. The toadfish Opsanus tau has an indirect positive influence on bivalve survival because the mud crab Panopeus herbstii, a consumer of bivalves, reduces foraging effort in the presence of toadfish. We quantified variation in the strength of persistent individual mud crab responses to toadfish and resulting variation in the strength of TMII. We demonstrate that the strength of this TMII is strongly influenced by mud crab size and behavior type, strengthening with the intensity of response of individual mud crabs to toadfish predator cues. Further, we demonstrate that the spatial distribution within intertidal oyster reefs of crabs with different behavior types is not random; mud crabs inhabiting subtidal areas, where predator cues are more persistent, are significantly less responsive to toadfish cues than mud crabs from intertidal areas. This spatial behavioral structure should lead to spatial variation in the strength of TMII. Given the widespread importance of TMII and the broad occurrence of individual personality or behavior types across numerous taxa, these results should be generally applicable. The distribution of behavior types within a population may therefore be a useful metric for improving our ability to predict the strength of TMII.