Red Crab Research Articles

Influence of cadmium accumulation and dietary status on fatty acid composition in two colour forms of shore crabs, Carcinus maenas

The effects of cadmium exposure and dietary status on cadmium accumulation, fatty acid (FA) content and profiles were investigated in two colour forms of the shore crab Carcinus maenas. Groups of shore crabs were either starved or fed with blue mussels, Mytilus edulis, during a 40 d exposure period to 2 or 6 μM Cd2+ (as CdCl2). Starved green individuals accumulated more cadmium in haemolymph and hepatopancreas than did red crabs and green crabs fed during the experiments. In the red colour form, no difference in cadmium accumulation was observed between starved and fed individuals. In both colour forms, hepatopancreas contained more FA than gills and muscle. The FAs often present in the largest amounts in the tissues were 16:0, 16:1ω7, 18:1ω7, 18:1ω9, 20:4ω6, 20:5ω3 and 22:6ω3. However, saturated (SAFA) and mono-unsaturated fatty acids (MUFAs) were dominant in hepatopancreas, whereas poly-unsaturated fatty acids (PUFAs) were dominant in gills and muscles. At the beginning of the experiment, the total FA content in the hepatopancreas was 111.6 mg g−1 (dry weight) for red crabs and 78.4 mg g−1 for green shore crabs. During the experiment, however, the FA content decreased in red crabs. This decrease was more pronounced for starved individuals than for fed individuals. Also, the decrease in FA content was more pronounced in crabs exposed to 6 μM cadmium compared to crabs exposed to 2 μM or crabs not exposed to cadmium. No change in FA content was observed in green shore crabs, irrespective of diet and cadmium exposure. For both colour forms, no change in FA content was observed for gills and muscle. In red crabs, a decrease was observed for all FAs in the hepatopancreas. This decrease, however, was more pronounced for SAFAs and MUFAs than for PUFAs, indicating that the metabolism of FAs during starvation and cadmium exposure is selective. The experiments indicate that green colour forms of shore crabs are more tolerant of natural stress such as starvation and anthropogenic stress, e.g. cadmium exposure, than are red colour forms of shore crabs.

Respiratory gas transport, metabolic status, and locomotor capacity of the Christmas Island red crab Gecarcoidea natalis assessed in the field with respect to dichotomous seasonal activity levels.

Red crabs, Gecarcoidea natalis, exhibit seasonal activity patterns: low activity during the dry season when they shelter in burrows to avoid dehydration, and high activity during the wet season. Red crabs were examined in situ in the rainforest of Christmas Island to determine if there were underlying seasonal differences in the capacity for exercise and associated metabolism. During both seasons, free-ranging (FR) crabs engaged in their normal activities and, together with crabs induced to exercise for 5 min, were sampled for haemolymph and muscle tissue. Respiratory gases in the haemolymph and key metabolites were measured to assess differences in metabolic status of FR and exercised crabs. Actively foraging FR crabs during the wet season exhibited a relative haemolymph hypoxia (2.9 kPa) and accumulated an extra 3 mmol. litre(-1) of CO(2) compared to the relatively inactive FR crabs during the dry season. Wet-season crabs appeared to be in a state of relative respiratory acidosis compared to dry-season animals. This hypercapnia may arise as a consequence of a relative hypoventilation in animals with a relatively higher metabolic rate during the wet season. Oxygenation of pulmonary and arterial haemolymph was similar and remained high after 5 min of exercise, indicating that the gills and lungs functioned similarly in gas exchange in both FR and exercised crabs. During exercise, venous O(2) reserves decreased and red crabs experienced a mixed respiratory/metabolic acidosis. Similar changes, after 5 min of enforced exercise, in metabolite concentrations, pH and respiratory gas status in the haemolymph during both sampling seasons suggest that the crabs maintain similar capacity to increase exercise during the wet and the dry seasons, despite the differences in underlying physiological status. This is important since after prolonged inactivity during the dry season, with the arrival of moonsoonal rains, red crabs must engage in their annual breeding migration.

Hydroacoustical evidence of autumn inshore residence of the pelagic red crab Pleuroncodes planipes at Punta Eugenia, Baja California, Mexico

A previously published conceptual model of the life history of the red crab Pleuroncodes planipes (Galathei- dae) (Stimpson, 1860) reports that the center of distribution of this micronektonic crustacean is located at Bahia Magdalena, on the southwest coast of Baja California (25 to 24° 20' N). During spring, the highest abundance of mature females is found nearshore with abundance peaks of larval stages dur- ing February and March. Larval drift offshore is associated with a strong Ekman transport system, and the virtual disap- pearance of the adult pelagic and benthic population from the neritic region at Bahia Magdalena in late summer and autumn can be explained by an inshore-offshore migration. However, huge benthic-pelagic concentrations of this crus- tacean occur nearshore during autumn at Punta Eugenia (27 to 29° N). Circadian cycles of the spatial distribution of these aggregations were recorded with a single beam echosounder Simrad EY-200 (200 kHz) along Baja California during Octo- ber 1994, March, June, and October 1995, and March and June 1996. Data obtained from Isaacs-Kidd midwater net trawls during October 1994 along Baja California were used to identify the red crab aggregations using hydroacoustics. The target strength of the red crab ranged between -50 and -54 dB. The largest aggregations were found in Bahia Mag- dalena during summer (June 1995 and 1996; 24 to 25° N) and nearshore at Punta Eugenia (27 to 28° N) during autumn and spring (March 1995 and 1996, October 1994 and 1995). These observations suggest that the conceptual model of inshore- offshore adult population migration is valid for Bahia Mag- dalena, but apparently not for Punta Eugenia at least during autumn. At Punta Eugenia an abrupt change in the orienta- tion of the shoreline enhances upwelling events and promotes semipermanent eddies, thus supporting the inshore residence of this adaptive omnivorous-detritivorous crustacean.

Morphometric, functional and sexual maturity of the deep-sea red crab Chaceon affinis inhabiting Canary Island waters: chronology of maturation

Although Chaceon affinis is practically unknown to Canarian fishermen, relatively abundant quantities of deep-sea red crab C. affinis have been found in all trap surveys conducted around the Canary Islands in the last 10 yr, in which waters deeper than 550 m have been examined. From July 1994 to May 1996, monthly samples were taken from north-eastern waters off Tenerife using a variety of trap designs. Crabs were caught at depths ranging from 550 to 1200 m on muddy- rocky bottoms. A variety of measurements were taken from each specimen, and most of these showed linear and isometric growth relative to carapace width (CW). Male chela width (ChW) and female abdomen width (AW) did not show such evident allometry as other crab species. Changes in growth of these features, considered as secondary sexual characters, were used to discriminate morpho- metrically immature crabs from mature crabs, using hierarchical cluster analysis. Different morpho- metric groups were distinguished, for both males and females, and male ChW and female AW were plotted against CW. Regressions were calculated separately for each cluster to test the discrimination statistically. Morphometric size at maturity was initially estimated and compared with data on gonad stage. This comparison revealed a high correlation in males between morphometric and sexual maturity, which is reached at about 129 mm CW. However, most females become morphometrically mature before their ovaries mature and before copulation (at sizes of ~99, 108 and 113 mm CW, respectively).

Langostilla (Pleuroncondes planipes) as a Source of Protein Hydrolysate and Carotenoprotein

Langostilla (swimming red crab) is an abundant resource from the Mexican Pacific Ocean that is not yet commercially harvested. Use of langostilla meal as a feed ingredient for protein and pigment in salmonid, crustacean, and poultry diets is limited because of the high ash content. An alternative use of this resource is production of protein hydrolysates with high protein and carotenoprotein content but low ash. This work is aimed at finding methods to produce protein hydrolysates and carotenoproteins from langostilla crab. Dried, whole langostilla contains about 43% protein, 8% lipids, and 7.1 mg% of astaxanthin. Protein hydrolysis by several added enzymes was monitored with a pH stat. The highest degree of hydrolysis and recovery of protein (73% of crude protein) was obtained with Alcalase treatment. The molecular weight distribution of the Alcalase hydrolysate was lower than the starting material as shown by SDS-PAGE. To obtain carotenoproteins, langostilla was homogenized with EDTA and 0.1% trypsin. About 62% of the pigment is recovered as carotenoprotein by EDTA-trypsin extraction. However, only 24% of the crude protein is recovered as protein in the carotenoprotein fraction.

Monopolization of litter processing by a dominant land crab on a tropical oceanic island.

Litter processing by macroinvertebrates typically involves suites of species that act together to determine rates of breakdown and decomposition. However, tropical oceanic islands and coastal fringes on continents are often dominated by one or a few species of omnivorous land crabs that consume leaf litter. We used an exclusion experiment, together with other leaf removal and litter decomposition studies, to assess the role of a single dominant species, the red crab (Gecarcoidea natalis), in litter dynamics in rain forest on Christmas Island, Indian Ocean. In the presence of red crabs, litter cover and biomass varied seasonally, from almost complete cover and high biomass at the end of the dry season to almost total absence of litter at the end of each wet season. When crabs were excluded from both the shaded understory and light gaps in rain forest, litter increased rapidly to almost complete cover, which was then maintained year round. Leaf tether experiments, and measures of litter input and standing crops, indicated that red crabs monopolize litter processing, removing between 39 and 87% of the annual leaf fall from the forest floor. Rates of litter turnover were over twice as high in the presence of land crabs: the decomposition constant, k, was 2.6 in the understory exclusion plots, but rose to 6.0 in the presence of crabs. Red crabs occur at biomass densities (114 g m-2) far greater than those reported elsewhere for entire litter faunas. They significantly reduced the abundance of other litter invertebrates, but we did not detect any change in the relative frequencies of the major invertebrate groups (mites, collembolans, pulmonate snails, ants, psocopterans, and spiders). Wherever omnivorous land crabs are abundant, their activities may be paramount in litter decomposition and in regulating the rate of nutrient cycling. In monopolizing litter processing, they may also be important physical "ecosystem engineers", translocating organic matter and nutrients into the soil and reducing available habitat for other animals.

Foraging behaviour of an excavating predator, the red rock crab (Cancer productus Randall) on soft-shell clam (Mya arenaria L.)

We investigated the influence of soft-shell clam (Mya arenaria) burial depth on red rock crab (Cancer productus) foraging behaviour. In laboratory tanks, crabs searched for and excavated clams buried at depths of 5, 10, and 15 cm. Handling time of prey was partitioned into excavation time, breaking time, and eating time. Prey excavation formed 23% of mean total handling time (mean 8.2 min total 35.7 min−1) at 5 cm prey depth, 62% at 10 cm depth (mean 49.3 min total 79.5 min−1), and 73% at 15 cm depth (mean 93.5 min total 128.2 min−1). Mean excavation time for clams increased at a rate of 8.4 min cm−1 of depth; clam profitability (energy intake per unit handling time (J s−1)) decreased with burial depth. Predation rates decreased with prey burial depth. Often crabs gave up attempts to excavate a clam, and abandonment frequency and excavation time before abandonment increased significantly (p<0.05) with burial depth. Our findings confirm earlier studies which indicate that burial depth affords an important refuge from crab predation, significantly increasing prey handling time and decreasing the profitability of individual prey.

Red crabs in rainforest on Christmas Island, Indian Ocean: No effect on the soil seed bank

Red land crabs (Gecarcoidea natalis) are both abundant and widespread in intact rainforest on Christmas Island, Indian Ocean, occurring at densities of ~ 1 crab m–2. We used an on‐going exclusion experiment to test the hypothesis that through their activities as seed predators and soil disturbers, these omnivores are important determinants of the density and relative species abundance of seeds in the soil seed bank of undisturbed primary rainforest. After 6.5 y of continuous crab exclusion, there was no significant difference in the density of seeds in the top 3 cm of soil between the control (3671 ± 517 seeds/0.19 m2) and exclusion plots (3285 ± 801 seeds/0.19 m2), nor was there any significant treatment difference in relative species composition. There were also no significant treatment differences when Muntingia calabura, which accounted for 91.9% of all seeds in the seed bank, was excluded from the analyses. We attribute the lack of a significant impact of red crabs to their probable inability to handle the very small seeds which occurred in the seed bank, and the low level of soil disturbance at the study site. We conclude that the observed impact of red crabs on the abundance and diversity of successional species in natural light gaps on Christmas Island is due to their impact on germinating seedlings alone, rather than any additional impact on the soil seed bank.

In situ studies on physiology and behaviour in two colour forms of the shore crab Carcinus maenas in relation to season

Heart rate and locomotor activity were monitored continuously in situ in male green and red colour forms of the shore crab Carcinus maenas along with simultaneous recordings of light intensity, temperature, depth and salinity in the shallow, weakly tidal Isefjord, Denmark. Experiments were conducted during 2 seasons, summer and late autumn. Circatidal and/or circadian rhythmicity in heart rate and/or locomotor activity was observed in 87 % of all crabs during summer and in 55 % of all crabs during autumn. During summer, 45 % of the green crabs and 17 % of the red crabs had significantly higher heart rates during high tide than during low tide. During autumn, 30% of the green crabs had significantly higher heart rates during high tide than during low tide (p <0.05 This was never observed in red crabs. Furthermore, 45 % of green and 42 % of red crabs expressed a diurnal variation in heart rate during summer. During autumn, this was observed in 40 % of the green and 30 % of the red crabs. For both colour forms, heart rate levels were significantly higher during summer than during autumn (p <0.001). For the temperature interval 4 to 25°C, Q 10 for heart rate was 2.14. No significant difference was observed between colour forms. A tight coupling was found between heart rate and locomotor activity, heart rate increased at times with increased locomotor activity. Temperature was the abiotic variable found to exert the largest influence on heart rate and locomotor activity, followed by depth, light and salinity.

Strategies for migration in the terrestrial christmas island red crab gecarcoidea natalis: intermittent versus continuous locomotion

The terrestrial red crab Gecarcoidea natalis undertakes an annual breeding migration and must sustain locomotion for prolonged periods. The migrating crabs must travel a specific distance in a fixed time and can either walk at a constant speed or walk faster for short periods and then pause to feed or rest. To simulate the potential differences between continuous and intermittent locomotion during the migration, red crabs were sampled after walking at a voluntary speed for 5 or 20 min without pausing or after 20 min of enforced walking intermittently at approximately twice that speed. The respiratory and metabolic status of the crabs was investigated during the different exercise regimes to assess which strategy might be more advantageous during the migration. The gills and lungs appeared to function similarly in gas exchange, and the PO2 in the haemolymph was 8.2 kPa which fully saturated the haemocyanin with O2. The uptake of O2 by red crabs was diffusion-limited and the diffusion coefficient (Ldiff) varied from 0.53 in resting crabs to 0. 8 post-exercise. Post-exercise, red crabs experienced a mixed respiratory/metabolic acidosis which was greatest (0.2 pH units) in crabs walking intermittently, i.e. at a higher speed. Haemolymph l-lactate concentrations peaked at 5 mmol l-1 immediately post-exercise in the intermittent exercise group, whereas after 20 min of continuous exercise haemolymph l-lactate continued to increase, reaching a maximum of 2.5 mmol l-1 at 1 h post-exercise. l-Lactate recovered slowly to basal levels within 5 h. The maximum rate of l-lactate clearance from the haemolymph was only 1.75 mmol l-1 h-1, and short pauses in exercise were insufficient for substantial l-lactate reoxidation. Exercise regimes in the laboratory were within the locomotor speeds determined for migrating red crabs, which overall have a mean walking speed close to their aerobic limit but periodically pause and also exceed this limit by three- to fourfold.

The functioning of the haemocyanin of the terrestrial christmas island red crab gecarcoidea natalis and roles for organic modulators

Gecarcoidea natalis is a land crab that migrates annually several kilometres to breed. The O2-binding properties of haemocyanin in G. natalis were investigated in vitro to test the idea that the O2-binding properties of the haemocyanin of land crabs are not dependent on circulating modulators and to provide a model of haemocyanin functioning during exercise. The affinity of the haemocyanin for O2 decreased with increasing temperature (change in the heat of oxygenation; capdelta H=-59 kJ mol-1). The haemocyanin of G. natalis apparently differs from that of other terrestrial crabs in showing haemocyanin O2 modulation by both organic and inorganic molecules. Haemocyanin O2-affinity was not affected by Mg2+ but was sensitive to changes in Ca2+ concentration ( capdelta logP50/ capdelta log[Ca]=-0.61, where P50 is the partial pressure of O2 required for half-maximal O2 binding). The Bohr factor was modest ( &phgr; =-0.26+/-0.03, N=4, in whole haemolymph at 25 degreesC) and there was no specific effect of CO2 on the O2-binding properties of the haemocyanin. An increase in urate concentration increased haemocyanin O2-affinity, but the effect was linear ( capdelta logP50/ capdelta [urate]=-0.06) and not logarithmic as is the case in other species. The effect of l-lactate on the haemocyanin O2-affinity in G. natalis was unique among the crustaceans, because an increase in l-lactate concentration decreased the haemocyanin O2-affinity. The effect of l-lactate on haemocyanin O2-affinity ( capdelta logP50/ capdelta log[lactate]) was time-dependent and decreased from a maximum of 0.044 on day 1 to 0.001 after 4 days of storage at 4 degreesC. The presence of an unknown dialysable and unstable factor in the haemolymph is postulated to explain the time-dependent effect of l-lactate on haemocyanin O2-binding properties. Model oxygen equilibrium curves constructed for in vivo conditions showed that the reverse effect of l-lactate was advantageous by decreasing the O2-affinity of the haemocyanin beyond that predicted by the Bohr shift alone and assisted in O2 off-loading at the tissues. This effect of lactate can only provide an advantage if the gas-exchange organs maintain arterial O2 loading and thus is dependent on lung function in land crabs and must have occurred coincident with the evolution of these other features.

Red and green colour forms in the common shore crab Carcinus maenas (L.) (Crustacea: Brachyura: Portunidae): theoretical predictions and empirical data

The shore crab Carcinus maenas (L.) is known to exhibit a range of carapace colours from green through orange to red. Recent studies have demonstrated that the different colours reflect increasing lengths of intermoult duration, with clear physiological and ecological differences between red and green crabs. It was hypothesized that these different colour forms represent different ‘life-strategies’. However, to date, there has been no study on population dynamics which attempts to account for these differences in colour. In order to test the red/green hypothesis, theoretical predictions were made about the occurrence of red males within a population, and the occurrence of different male colour forms was investigated in a population of shore crabs in the German Wadden Sea. The general differences in size and the coverage of epibionts and the seasonal changes in abundance between red and green males were investigated. These empirical data were compared with the theoretical predictions about red males which supported the hypothesis that red and green forms represent separate phases with different ‘life-strategies’ in the life cycle of Carcinus maenas.

The red-crab bloom off the west coast of Baja California, México

Hydroacoustic data indicate that the benthic red crab Pleuroncodes planipes (Stimpson, 1860) rises from the bottom towards the surface at dusk from very compacted patches settled on the continental shelf. The behaviour is fast and abrupt, lasting 32 mm in standard carapace length, they become strictly benthic (Boyd, 1967). The seasonal inshore-offshore movements of benthic P.planipes are well studied (AuriolesGamboa, 1992), as well as the inshore-offshore larval drift (G6mez-Gutierrez and Sanchez-Ortiz, 1997). However, the diel movements of this red crab are not well understood. Although migration from the bottom to mid-water during the night is a common behaviour (Boyd, 1967; Kato, 1974), P.planipes may also be found on the surface during the day (Alvarifio, 1976; Blackburn, 1977). We have observed surface swarms arranged in bands at Magdalena Bay (24°N) during summer, like those observed with Munida gregaria (New Zealand) by Jillet and Zeldis (1985) aggregated by internal waves and surface Langmuir circulation. Boyd (1967) reported that the red crabs do migrate towards the sea surface during the night; however, Aurioles-Gamboa (1992) observed that only a fraction of the benthic red crabs migrate towards the surface. Pelagic and benthic phases can be found simultaneously only inshore and during the seasonal reproductive months (spring, February-March), the benthic phase may migrate into the pelagic zone for reproduction (Gomez-Gutierrez and Sanchez-Ortiz, 1997). Our personal observations using hydroacoustic records indicate horizontal movements and dispersion during the night in the water column. During the day, however, the pattern is less regular; the red crabs may be found compacted and very close to © Oxford University Press 2009 C-f.Robinson and J.G6mez-Gutierrez the bottom, compacted and in continuous layers in deep water, in dense groups in mid-water or near to the surface. One of the problems when assessing pelagic stocks and behaviour of anchovies and sardines in the west coast of Baja California is the sudden, huge appearance of red crabs in middle water during dusk in areas where they were absent during the day. Although hydroacoustically it is possible to discriminate between the anchovy's target strength and the red crab's (MacLennan and Simmonds, 1992; Robinson et al., 1995), the problem remains when it is necessary to obtain mid-water net samples to validate the observations. A few minutes of trawl are more than enough for a net to be lost or damaged due to the presence of these crabs. Their sudden appearance may be an expansion of the red crabs observed in the middle water layer, they are very active swimmers during the night, or perhaps they rise from the bottom. The aim of this study is to find out from where, and how, the red-crab swarms appear suddenly in the middle water. In March 1995, considered as the peak of reproduction of the red crab, three transects each 18 km long, perpendicular to the coast and separated 36 km from each other, were defined from the RV 'El Puma' off the west coast of Baja California near Punta Eugenia, between 27°30'N, 115°00'W and 26°00'N, 114°00'W (Figure 2). Continuous hydroacoustic survey of the water column was obtained on each transect, starting from the neritic zone, heading to the oceanic zone and returning to the neritic zone -4 h later. Observations started on 18 March in transect one at 10:35 h and finished the next day at 09:00 h. Transect two started on 19 March at 12:00 h and finished the next day at 08:00 h. Transect three started on 20 March at 11:30 h and finished the next day at 07:00 h (standard Pacific time). A Simrad EY-200, single-beam echosounder with a working frequency of Pleuroncodes planipes Fig. 1. Adult of the red crab Pleuroncodes planipes (Stimpson, 1860).

Control of Seedling Recruitment by Land Crabs in Rain Forest on a Remote Oceanic Island

We tested the hypothesis that an omnivorous land crab determines the dynamics of seedling recruitment in rain forest on Christmas Island, Indian Ocean. The red crab (Gecarcoidea natalis) occurs at densities of ∼1 crab/m2 and consumes seeds, seedlings, and leaf litter. A crab-exclusion experiment below closed canopy and in light gaps showed that red crabs markedly reduced seedling recruitment. Overall, seedling emergence was 29-fold and 21-fold greater on the exclusion plots in the understory and in gaps, respectively. In control plots, mean seedling densities never exceeded 15 seedlings/25 m2 in the understory and 3 seedlings/25 m2 in gaps. Species richness of seedlings was significantly higher in the absence of crabs, averaging >10 and 12 species per plot in the understory and gaps, respectively. On control plots, mean species richness was less than one species per plot. Two-dimensional ordination (using nonmetric multidimensional scaling) showed that red crabs had a large impact on relative species abundances. Analyses of similarity confirmed that significant differences existed between both treatments and habitats. Seed removal and seedling transplant experiments suggested that most rain forest species are rapidly eaten by red crabs. Seedlings of many of these vulnerable species (e.g., Maclura cochinchinensis, Planchonella nitida, Schefflera elliptica) were abundant on exclusion plots. However, they were rare on control plots, and none persisted for >2 mo. Persistent seedlings were limited to two species (Inocarpus fagifer and Tristiropsis acutangula), both of which were resistant as seeds and seedlings to crab predation. Although red crabs are the major biotic determinants of seedling recruitment, this impact was not reflected in the overhead canopy. Some tree species unable to recruit in the presence of crabs were common as adults. Shifting mosaics in crab densities over space and time may offer the best explanation for recruitment of vulnerable species. Our results show that a dominant consumer has large effects on plant community structure on a remote oceanic island, consistent with the viewpoint that strong control by a single species is more likely in simple communities.

CONTROL OF SEEDLING RECRUITMENT BY LAND CRABS IN RAIN FOREST ON A REMOTE OCEANIC ISLAND

We tested the hypothesis that an omnivorous land crab determines the dynamics of seedling recruitment in rain forest on Christmas Island, Indian Ocean. The red crab (Gecarcoidea natalis) occurs at densities of ∼1 crab/m2 and consumes seeds, seedlings, and leaf litter. A crab-exclusion experiment below closed canopy and in light gaps showed that red crabs markedly reduced seedling recruitment. Overall, seedling emergence was 29-fold and 21-fold greater on the exclusion plots in the understory and in gaps, respectively. In control plots, mean seedling densities never exceeded 15 seedlings/25 m2 in the understory and 3 seedlings/25 m2 in gaps. Species richness of seedlings was significantly higher in the absence of crabs, averaging >10 and 12 species per plot in the understory and gaps, respectively. On control plots, mean species richness was less than one species per plot. Two-dimensional ordination (using nonmetric multidimensional scaling) showed that red crabs had a large impact on relative species abundances. Analyses of similarity confirmed that significant differences existed between both treatments and habitats. Seed removal and seedling transplant experiments suggested that most rain forest species are rapidly eaten by red crabs. Seedlings of many of these vulnerable species (e.g., Maclura cochinchinensis, Planchonella nitida, Schefflera elliptica) were abundant on exclusion plots. However, they were rare on control plots, and none persisted for >2 mo. Persistent seedlings were limited to two species (Inocarpus fagifer and Tristiropsis acutangula), both of which were resistant as seeds and seedlings to crab predation. Although red crabs are the major biotic determinants of seedling recruitment, this impact was not reflected in the overhead canopy. Some tree species unable to recruit in the presence of crabs were common as adults. Shifting mosaics in crab densities over space and time may offer the best explanation for recruitment of vulnerable species. Our results show that a dominant consumer has large effects on plant community structure on a remote oceanic island, consistent with the viewpoint that strong control by a single species is more likely in simple communities.

Erratum

Green, P. T. (1997)Red crabs in rain forest on Christmas Island, Indian Ocean: activity patterns, density and biomass. Vol. 13 Part 1: 17–38. On page 29 the phrase: “but a significant correlation existed across sites (r=0.81, n=44, P&lt;0.001).” should read: “but a significant correlation existed across sites (r=-0.69, n=44, P&lt;0.001).”On page 33 the phrase “but the same correlation accounted for 81% of total variation when all sites were analysed together.” should read: “but the same correlation accounted for 48% of total variation when all sites were analysed together.”

A review of the offshore trawl fishery for crustaceans on the east coast of South Africa

This paper reviews current knowledge on the trawl fishery for crustaceans on the east coast of South Africa. The shallow water fishery targets penaeid prawns, particularly Penaeus indicus Milne‐Edwards, while the deep water fishery targets pink prawns, Haliporoides triarthrus Stebbing, langoustines, Metanephrops mozambicus (Jenkins), and rock lobster, Palinurus delagoae, Barnard. Between 1988 and 1993, annual catches of targeted species averaged about 350 t. There was a total bycatch of about 1700 t per annum, most of which was discarded. Retained bycatch was dominated by the red crab, Chaceon macphersoni, as well as fish and cephalopods. Although trawling effort declined during the period under review, this trend has recently reversed.

Red crabs in rain forest on Christmas Island, Indian Ocean: activity patterns, density and biomass

ABSTRACTThe red crabGecarcoidea natalisis the most abundant land crab on Christmas Island, Indian Ocean, and is found wherever native rain forest is still intact. The largest crabs grow to &gt; 120 mm carapace width and reach &gt; 500 g in mass. Red crabs are diurnally active, and moisture appears to be the most important factor governing surface activity – activity was nil below 77% RH, relatively low at 87% RH, and high above 95% RH. The crabs retreat to the interior of their burrows during dry periods, but emerge rapidly in response to rain. Because of their dependence on moisture for surface activity, red crabs show greater levels of activity during the wet season. Annual breeding migrations and moulting activity decreased the density of surface-active crabs at the main study sites for several weeks at the start of the wet season, and fruit and leaf fall caused dramatic local increases in the density of active red crabs. Density and biomass at the main study sites averaged 1.3 crabs m-2and 1454 kg ha-1, respectively, and 1.2 crabs m-2and 1137 kg ha-1at a total of five sites across the island. Rough calculations suggest that the island-wide population is at least 100 million individuals. Red crabs are the dominant consumer on the forest floor, and consume seeds, seedlings and leaf litter. Because they are both abundant and widespread, these animals have the potential to be the most important determinant of pattern and process in rain forest on Christmas Island.

Seasonal and Bathymetric Changes in Feeding Habits of the Benthic Red Crab Pleuroncodes Planipes (Decapoda, Anomura, Galatheidae) Off the Pacific Coast of Baja California Sur, Mexico

Stomach contents of benthic red crabs (Pleuroncodes planipes) were analyzed to document number of items and composition. The crabs were captured along a series of latitudinal transects covering depths from 50 to 250 m, during March (12 stations) and September 1990 (8 stations). The number of food items decreased significantly from March to September at 150 and 200 m depth, the only strata compared since red crabs are usually absent from the coast above 100 m in summer. The particulate organic matter (POM) varied from 60 to 70% of total food items. The zooplankton fraction, mainly crustaceans, foraminiferans, and radiolarians, was second in importance. Phytoplankton was the next most important group, and was represented by 17 genera in March and 11 in September. The mean number of diatoms in stomachs per station decreased significantly (73.5 versus 7) from March to September. The number of genera/station showed the same pattern (5.36 versus 3.6). The most abundant (number of cells) and frequent (presence over number of stations) diatom genera in March were Nitzschia, Melosira and Cocconeis in that order, accounting for 74% of the relative importance index. In September, Cyclotella, Nitzschia and Melosira accounted for 42% of the importance index. The stomach contents in benthic red crabs indicate a decreasing availability of food in the area from winter-spring through the summer. This pattern correlates well with the diminishing intensity of the coastal upwelling system and weakening of the California Current. The amount of stomach contents in red crabs decreased as the density of the crustacean species increased.

Size and Weight Relationships for the Golden Crab, Chaceon fenneri, and the Red Crab, Chaceon quinquedens, from the Eastern Gulf of Mexico

Carapace length, carapace width, and weight relationships are discussed for the golden crab, Chaceon fenneri, and the red crab, Chaceon quinquedens, from the eastern Gulf of Mexico. Males of both species were significantly larger than females in comparisons of means of all measured parameters. Relationships between carapace length and carapace width, carapace length and weight, and carapace width and weight were similar between Atlantic and Gulf of Mexico populations for both species.