Frequency of moulting by shore crabs Carcinus maenas (L.) changes their colour and their success in mating and physiological performance

Male shore crabs, Carcinus maenas [Linnaeus, 1758], compete aggressively for access to receptive females to mate. Size is the single most important factor for the outcome of these conflicts, large males with carapace width (CW) over 60 mm being much more likely to gain access to receptive females than smaller males. To compete aggressively, large male shore crabs decrease moulting frequencies and may potentially terminate moulting to enter a state of anecdysis, in which further growth is suspended to increase reproductive output. This change from a “growth” strategy to a “reproduction” strategy results in the creation of two morphs, which can roughly be separated by their colouration. As the new exoskeleton created during moult is always green in appearance, crabs tend to be green in appearance during periods where they grow rapidly and moult frequently. Green crabs are found in all size classes. However, as the exoskeleton becomes older, the colour gradually changes to a darker red colour, and large crabs that have spent an extended period in intermoult are therefore often red in appearance. Also, the exoskeleton of red crabs exhibits a higher incidence of epibionts and wear and tear. Red crabs can also be found in all size classes, but their relative proportion in the population increases dramatically in size classes above 60-mm CW. Size for size, the red morph has a thicker carapace and larger master chelae than the green morph. Also, the reproductive indices (RI) for red crabs are higher than for green crabs, and they experience higher mating success. However, this mating success appears to be achieved at the expense of a lower physiological tolerance, green crabs being better adapted to deal with changes in the surrounding environment. This increased tolerance is not only observed with regard to natural variations in the habitats where shore crabs live, but green crabs also appear to be more tolerant to variations caused by anthropogenic pollution. Consequently, the shift from growth to reproduction exerts a profound effect on the behaviour, physiology and ecotoxicology of male shore crabs. The present paper reviews the studies conducted so far, proposes a mechanism by which some of these differences between the two morphs are created and discusses their ecological and ecotoxicological significance.     

Climate-induced changes in bottom-up and top-down processes independently alter a marine ecosystem

Climate change has complex structural impacts on coastal ecosystems. Global warming is linked to a widespread decline in body size, whereas increased flood frequency can amplify nutrient enrichment through enhanced run-off. Altered population body-size structure represents a disruption in top-down control, whereas eutrophication embodies a change in bottom-up forcing. These processes are typically studied in isolation and little is known about their potential interactive effects. Here, we present the results of an in situ experiment examining the combined effects of top-down and bottom-up forces on the structure of a coastal marine community. Reduced average body mass of the top predator (the shore crab, Carcinus maenas) and nutrient enrichment combined additively to alter mean community body mass. Nutrient enrichment increased species richness and overall density of organisms. Reduced top-predator body mass increased community biomass. Additionally, we found evidence for an allometrically induced trophic cascade. Here, the reduction in top-predator body mass enabled greater biomass of intermediate fish predators within the mesocosms. This, in turn, suppressed key micrograzers, which led to an overall increase in microalgal biomass. This response highlights the possibility for climate-induced trophic cascades, driven by altered size structure of populations, rather than species extinction.     

Taxonomic and biological notes on a species of Iphitime (Polychaeta, Eunicida) associated with Pagurus prideaux from western Norway

On the basis of 658 specimens collected over an 11 year period, the symbiotic eunicid polychaete, Iphitime paguri Fage & Legendre, 1934, is redescribed, and its biology in its presumed preferred host, Pagurus prideaux Leach, 1815, is outlined. The main ecological part is based on sampling of Pagurus on five occasions throughout a year in Raunefjorden, southwest of Bergen. Iphitime paguri turned out to have a strong sexual dimorphism, both in outer appearance and in the morphology of the jaws. Both juveniles and mature individuals of both genders are described. The infestation rate of Pagurus prideaux was very high (as opposed to the low reported infestation rate in Pagurus bernhardus, generally regarded as the host of this species) and the symbiotic polychaetes turned out to have a distinct pattern of host utilization in different life stages. Ripe males and females were, at certain times of the year, generally found more or less intertwined in the apex of the gastropod shell inhabited by its host. Small larval stages were found in the same place. Intermediate size groups were found in the gill chamber and in a sulcus on the carapax of the pagurid host.     

THE COST OF RELIABLE SIGNALING: EXPERIMENTAL EVIDENCE FOR PREDICTABLE VARIATION AMONG MALES IN A COST-BENEFIT TRADE-OFF BETWEEN SEXUALLY SELECTED TRAITS

Claw size of male fiddler crabs, Uca perplexa appears to be a target of female choice that increases the likelihood a female will initially approach a male. Here we show that a behavioral display trait, the maximum height that the tip of the claw reaches during a courtship wave, is a strong correlate of the subsequent likelihood that a female will visit a male's burrow (which is a prerequisite for a burrow mating). We experimentally manipulated claw mass, to test whether there is a trade-off between claw mass and wave height. Males with a metal weight added to their claw showed a large reduction in wave height, whereas control males (plastic added) showed no net change in wave height. There is therefore a trade-off between these two sexually selected traits (claw size and wave display). More importantly, the greater the initial wave height the smaller the subsequent decline in wave height. Assuming that variation in wave height is an index of quality, this variation in the cost-benefit trade-off is consistent with the requirements of a signaling system that conforms to the handicap principle when fitness is the multiplicative product of different fitness components. We conclude by discussing the ongoing difficulties in testing the handicap principle.     

Demineralization of crab shells by chemical and biological treatments

To achieve demineralization of crab shell waste by chemical and biological treatments, lactic acid and lactic acid bacterium were applied. In 5.0 and 10% lactic acid, pH rapidly decreased from 6.8 to 4.2 and from 4.5 to 2.4 at day 3, respectively, and thereafter the pH remained at an almost constant level. In a 10% lactic acid bacterium inoculum, pH lowered to 4.6 at day 5. Relative residual ash content rapidly decreased to 49.1 and 16.4% in 5 and 10% lactic acid treatments, respectively, for the initial 12 h. In 2.5, 5 and 10% lactic acid bacterium inoculums, relative residual ash content rapidly decreased to 55.2, 40.9 and 44.7%, respectively, on the first day. Residual dry masses were 76.4, 67.8 and 46.6% in 2.5, 5 and 10% lactic acid treatments, respectively, for the initial 12 h. After a one-time exchange of the lactic acid solution, in the 5.0% lactic acid treatment, residual dry mass rapidly decreased from 66.0 to 41.4%. In 2.5, 5 and 10% lactic acid bacterium inoculums, residual dry masses decreased to 67.6, 57.4 and 59.6% respectively, on the first day. Protein contents after demineralization ranged from 51.3–54.7% in the chemical treatments and decreased to 32.3% in the lactic acid fermentation process. A negative relationship was shown between pH and demineralization rate in lactic acid and lactic acid bacterium treatments. These results suggest that lactic acid fermentation can be an alternative for demineralization of crab shells, even though the rate and efficiency of the demineralization is lower than the chemical treatment.