Allometric relationships of reproductive output were compared in 20 species from 7 families of brachyuran crabs from the east and west coasts of North America, using regression analysis of log reproductive parameters versus log body weight. Comparisons of crabs spanning 4 orders of magnitude in body weight indicated that female body size was the principal determinant of reproductive output: 95% of the variance in brood weight, 79% of the variance in the number of eggs per brood, 63% of the variance in annual brood weight, and 74% of the variance in annual fecundity were explained by body weight. Brood weight exhibited an isometric constraint to about 10% of body weight. Allometric limitations on space available for yolk accumulation in the body cavity appeared to be the main constraint on brood size. Ovum size increased significantly, but only slightly, with increasing body size. There was a significant trade-off between ovum size and the number of eggs per brood. There was no significant relationship between the number of broods per year and body size. The number of eggs per brood was significantly better than brood weight as a predictor of the number of broods produced per year by a species, indicating that demographic pressure on fecundity rather than energetic considerations is the primary selective mechanism influencing annual reproductive effort. Each of the 7 families of crabs exhibited trends toward distinct patterns for the suite of co-adapted reproductive traits. However, no interspecific reproductive patterns were apparent with respect to the variables of feeding type, salinity tolerance, habitat, and geographic range represented by the 20 species.

Allometric constraints and variables of reproductive effort in brachyuran crabs

Gammarus spp. (Crustacea: Amphipoda) are widespread throughout a diverse range of marine, freshwater and estuarine}brackish habitats, often dominating benthic macroinvertebrate communities in terms of both numbers and}or biomass. Gammarus spp. are the dominant macroinvertebrate prey items of many fish, whether as a seasonal food source or a year-round staple. Selective predation by fish on Gammarus spp. is often linked to parasitism and the body size of the prey. Gammarus spp. populations are under increasing threat from both pollution and replacement}displacement by introduced species. Loss of populations and species invasions}replacements could have significant impacts on native predator species if the predator(s) cannot successfully adapt their feeding patterns to cope with non-indigenous Gammarus prey species. Despite this, many fish predation studies do not identify Gammarus prey to species level. This lack of precision could be important, as Gammarus spp. exhibit wide variations in physiochemical tolerances, habitat requirements, abilities to invade and susceptibility to replacement. Although rarely acknowledged, the impacts of nonpiscean predators (particularly macroinvertebrates) on Gammarus prey species may frequently be stronger than those exerted by fish. A major aim of this review is to ascertain the current importance of Gammarus as a prey species, such that the implications of changes in Gammarus spp. populations can be more accurately assessed by interested groups such as ecologists and fisheries managers.We also review the dynamics of Gammarus spp. as prey to a diverse array of mammals, birds, amphibians, insects, flatworms, other crustaceans such as crabs and crayfish and, perhaps most importantly, other Gammarus spp.

The dynamics of predation on Gammarus spp. (Crustacea: Amphipoda)

SUMMARY. The seasonal variation in population density of Gammarus pulex was studied in a Dorset chalk stream. The numbers increased markedly in June and July and reached a maximum of c. 10000m−2 in September whilst the most rapid decline in density occurred in October-November and reached a minimum of 820 m−2 in February. The animals occurred in greater densities in habitats containing Ranunculus or Callitriche than in those devoid of vegetation. The population structure was determined monthly and was split into juveniles (length <4mm), immature males, immature females, mature males and ovigerous females. The percentage of juveniles (39–76) was always the highest of any of the categories. Ovigerous females were found at all times of the year. The sex ratio varied with the time of year both for immatures and matures, although there was approximately a 1:1 ratio for the mature individuals. Seasonal variation in biomass showed a maximum of 7.l g dry wt m−2 in September and a minimum of 1.4 g dry wt m−2 in March. Production was calculated by two methods giving values of 12.9 g dry wt m−2 year−1 and 12.8 g dry wt m−2 year−1.

Life‐history and production of the amphipod Gammarus pulex in a Dorset chalk stream

Geographic variation in size of sexually mature females was compared in five species of crabs at sites spanning about 10 degrees of latitude along the east and west coasts of North America. Populations were sampled along the west coast at six sites for Pachygrapsuscrassipes, four sites for Hernigrapsus nudus, eight sites for H. orgenensis, and two sites for Scyra acutifrons, and along the east coast at eight sites for Panopeus herbstii. Four of the five species showed significant geographic variation in size at onset of maturity, mean size, and size frequency distribution. P. herbstii and P. crassipes exhibited latitudinal variations and marked changes in population structure of mature females at biogeographic boundaries; P. herbstii matured at larger sizes at latitudes below Cape Hatteras; while P. crassipes matured at smaller sizes below Point Conception. H. nudus showed variation in the size distribution of mature females, but little variation in size at onset of maturity. For H. oregonensis and S. acutifrons, size at maturity varied on a local scale among neighboring populations. For S. acutifrons, a difference of about seven molts before the terminal molt at puberty was deduced to produce the observed variation in maturity. Its molt increment percentage was constant across all sizes of crabs and did not differ between two populations with disparate size structures. For H. oregonensis variation in molt increment and in the number of molts appears to account for differences in size at maturity among populations. Variation in maturation within H. oregonensis populations occurs over one to two molts, and a difference of about two molts accounts for differences in size at onset of maturity among populations. Geographic variations in growth and size structure of mature females in these species are likely to have major consequences for population fecundity and demography. Because size and age at maturity are critical determinants of brachyuran reproductive output (Hines, 1982) and the rate of population growth (Cole, 1954; Murphy, 1968; Stearns, 1976), it is important to consider patterns of variation in the size at maturity in brachyuran crabs. Intra-specific geographic variations in life histories are well-documented in many groups of crustaceans: mole crabs (Hippidae) (Efford, 1969; Wenner et a1., 1987), stomatopods (Reaka, 1979), barnacles (Barnes and Barnes, 1965; 1968), copepods (McLaren et a1., 1969; Lonsdale and Levinton, 1985), mysids (Lasenby and Langford, 1972), caridean shrimp (Alon and Stancyk, 1982), lobsters (Annala et a1., 1980; Morizur et a1., 1981), amphipods (Steele and Steele, 1975), and hermit crabs (Childress, 1972; Bach et a1., 1976; Fotheringham, 1976; 1980; Bertness, 1981). Growth and size at maturity are important components in the dynamics of reproductive stocks and thus have significant indirect effects on population recruitment in many commercially fished crustaceans (Cobb and Phillips, 1980). Despite their economic and ecological importance, geographic variation in growth and size at maturity has been documented in only a few species of brachyuran crabs. At least five species from four brachyuran families (Cancridae, Grapsidae, Majidae, and Portunidae) exhibit geographic variation in size at maturity, although much of the documentation is derived incidentally from studies designed for other purposes. Cancer magister (Cancridae) is an extensively studied species, but there does not appear to be significant differences in growth rates, size at maturity, number of molts to maturity, or maximum size among populations from central California to Washington along the west coast of North America (Mackay and

Geographic variation in size at maturity of brachyuran crabs

Patterns of reproductive parameters found among the gammaridean amphipod crustaceans were examined quantitatively. Highly significant positive correlations were found in pairwise comparisons of brood size, egg size, minimum size of reproductive females, and mean size of reproductive females. Significantly higher values of minimum and mean size of reproductive females and brood size were found for comparisons of: 1) winter versus summer populations of the same amphipod species, 2) brackish-water versus freshwater and marine species, 3) species of the family Gammaridae versus the families AmpeUscidae and Haustoriidae, 4) single-brooded versus multiple-brooded species (brood size only), and 5) infaunal versus epifaunal species. Reproductive patterns appear to be more specific within amphipod family groupings than is true for some other organisms, potentially because of the habitat Specificity of the gammaridean families examined.

Reproductive patterns of gammaridean amphipods

Available data on some aspects of the biology of 10 species of Gammarus are summarized and compared. It is concluded that the main adaptation allowing these species to occupy their extensive geographic range from the North Pole to the Gulf of Mexico is variation in body size. The large size of northern species (G. wilkitzkii and G. setosus) results in the production of a single, large, well-timed brood, which is released within the short period of optimum conditions. The medium-sized species (G. oceanicus, G. duebeni, G. finmarchicus, and G. obtusatus) produce several medium-sized broods spread through the longer optimum season. The small species (G. lawrencianus, G. tigrinus, G. stoerensis, and G. mucronatus) can produce a large number of small broods per female and also mature second generations in the much longer and warmer optimum season characteristic of the southern environment.

The biology of Gammarus (Crustacea, Amphipoda) in the northwestern Atlantic. XI. Comparison and discussion

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The biology of Gammarus (Crustacea, Amphipoda) in the northwestern Atlantic. XI. Comparison and discussion