The ultrastructure of the sinus gland of Carcinus maenas (Crustacea: Decapoda)

Summary The sinus gland of Carcinus maenas consists of the swollen axonal endings of the neurosecretory cells of the major ganglia and acts as a storage release centre for the membrane bound neurosecretory material. These neurosecretory granules fall into five different types based on size and electron density. Their contents are released by exocytosis of the primary granules or smaller units budded from the primary granules.I thank Professor E. Naylor for his constant advice and Professor E. W. Knight-Jones, Department of Zoology, University College, Swansea, for the provision of laboratory facilities. I am grateful to the Science Research Council for the financial support. Finally, I thank the Electron Microscope Unit, Southampton General Hospital, where the work was completed.     

Predatory behaviour of juvenile shore crab Carcinus maenas (L.)

Juvenile shore crabs Carcinus maenas (L.) were observed feeding on rock barnacles Semibalanus balanoides (L.) on a Bay of Fundy rocky shore. This previously unreported predatory behaviour was further investigated in the laboratory. When given a choice of three common and abundant gastropods, Nucella lapillus (L.), Littorina littorea (L.), and Littorina obtusata (L.), and the rock barnacle Semibalanus balanoides, juvenile shore crabs of both sexes ate mainly barnacles and consumed proportionately more barnacles than gastropods compared with adults, which ate mainly gastropods. The rock barnacle is an abundant and readily available food source which may be important in sustaining the juvenile crab through periods of moults and rapid growth. As the shore crab attains a certain age (size), it must forage lower on the shore as gastropods become more important in its diet.     

The structure of the cuticle of the shore-crab Carcinus maenas (L.)

The cuticle of Carcinus has been critically examined by light microscopy and found to present features differing from the model proposed by Bouligand (1965), elaborated by Neville and his collaborators (see references), and re-stated by Bouligand (1971). The laminae have the appearance of discrete and separable sheets connected by interlaminar fibres, and between them long unorientated macrofibres pursue a sinuous course and pass from one interlaminar zone to another. Together with the interlaminar connecting fibres the macrofibres appear to account for the plumose appearance of the interlaminar zones.     

Carapace repair in the green crab,Carcinus maenas (L.)

Formation of a circular hole 8–10 mm in diameter in the calcified layers of the carapace from crabs in stage C₄ of the molt cycle stimulates the tissue under and adjacent to the injury to deposit a unique calcified cuticular material below the intact membranous layer. Deposition was followed for 69 days using light microscopic histology, histochemistry, and scanning electron microscopy. Quantitative analyses of CaCO₃ were conducted using atomic absorption spectrophotometry and Gran titration. Spatial distribution of CaCO₃ was determined with X-radiography. A scab is formed by day two under the injury. At four days the epithelium changes from squamous to columnar and deposits a PAS-positive layer with an irregular lamellar fine structure, followed by highly organized lamellae structurally similar to normal exocuticle. Histochemically, however, these lamellae resemble normal endocuticle. CaCO₃ is evident external to the outermost lamellae by day eleven as a fused mass of aragonite granules. The lamellar region calcifies proximally from the outer surface and is amorphous CaCO₃. Repair cuticle is approximately 20%CaCO₃ by weight.     

The cuticle of the crabs Cancer pagurus L. and Carcinus maenas (L.)

Fine closely-packed parallel fibres pass obliquely, at an angle of about 10° to the horizontal, through the cuticles of Cancer and Carcinus . They lie on axes parallel to the four faces of an obtuse pyramid, and have no counterpart in the model proposed by Bouligand (1965, 1971, 1972).
Replication of vertically broken surfaces of cuticle on cellulose acetate film shows that the laminae of the cuticle are discrete structural entities which preserve their identity around the angle formed by two vertical faces meeting at right-angles. This does not conform to the requirements of the Bouligand model.     

Multi-scale phenotype-substrate matching: Evidence from shore crabs (Carcinus maenas L.)

Phenotype-environment correlations can be caused by various overlapping ecological processes (e.g. differential mortality, phenotypic plasticity and adaptation) operating over a range of spatial and temporal scales. However, multi-scale studies of the relationship between specific traits and environmental variables are rare, even though such a holistic view may have greater predictive value than the more typical single-scale observations. Here, we investigate the relationship between cryptic colouration and environmental characteristics at a range of spatial scales for the shore crab (Carcinus maenas L.), a common intertidal species. Using existing data sets at macro-scale (10,000s m²) and meso-scale (100s m²) and previously unpublished data at a micro-scale (<1 m²) we demonstrate that phenotype-substrate associations in this species are sampling-scale invariant. The association is strongest at the micro-scale, possibly because the advantages of cryptic colouration for visual predator avoidance are dependent on close concordance between prey phenotype and the surrounding substrate. By contrast, correlations at greater spatial scales may reflect longer term processes such as phenotypic plasticity or the accumulated effects of predator mediated differential mortality.     

Physiological responses to emersion in the intertidal green crab,Carcinus maenas (L.)

The green shore crab, Carcinus maenas, undergoes on average 6?h periods of emersion during each low-tide cycle during the summer months. Under those conditions, the crab is cut off from its normal water environment and is exposed to potential stress from a suite of environmental and physiological changes: dehydration, compromised gas exchange and resultant internal hypoxia and hypercapnia, thermal stress, and ammonia toxicity. This study examined the comprehensive responses of the green crab in water and to a 6?h emersion period laboratory simulation of a tidal cycle followed by a 1?h re-immersion period, measuring indicators of dehydration, hemolymph osmolality, oxygen uptake, hemolymph acid–base status, heart and ventilatory rate, and hemolymph ammonia and ammonia excretion. Green crabs showed physiological responses of varying magnitude to 6?h of emersion. Individuals were found in the field exclusively under rocks and large clumps of seaweed where temperatures were approximately half those of exposed surfaces and relative humidity was about twice as high as ambient air. During emersion, crabs lost less than 5% of their wet weight, and hemolymph osmolality did not increase significantly. Oxygen uptake continued in air at about 50% of the control, aquatic values; and the gills continued to be ventilated by the scaphognathite, albeit at lower rates. Hemolymph lactate concentrations increased, indicating a shift to a greater reliance on anaerobic metabolism to support energetic needs. A slight acidosis developed in the hemolymph after 1?h of emersion, but it did not increase thereafter. Ammonia concentrations in the hemolymph were unchanged, as ammonia was volatilized by the gills and excreted into the air as NH₃ gas. These results show that the green crab copes with emersion by seeking refuge in microhabitats that mitigate the changes in the physical parameters of intertidal emersion. Physiologically, desiccation is avoided, cardio-respiratory processes are maintained at reduced levels, and hemolymph acid–base balance is minimally affected. Ammonia toxicity appears to be avoided by a shift to excreting NH₃ gas directly or indirectly to air.     

Effects of mercury on ion and osmoregulation in the shore crab Carcinus maenas (L.)

The effects of lethal mercury concentrations on regulation of hemolymph electrolytes in the shore crab Carcinus maenas were investigated. In most experiments mercury exposure reduced hemolymph osmolality and Na+, Cl- and K+ levels to 55-90% of controls in 48 hr. Mercury exposure augmented calcium levels to 120-300% of controls. The effect of mercury on all of the electrolytes varied during the year, especially on magnesium levels which showed significant increases or decreases, or remained unaffected in different experiments.     

Heterochely and handedness in the shore crab Carcinus maenas (L.) (Crustacea: Brachyura)

A large sample ofadult male Carcinus maenas was 79% right-handed and 21% left-handed. A separate sample of 207 intact adult males was divided into left-handed and right-handed crabs and four measurements were taken from all major and minor chelae. Correlation and regression analyses against carapace width on log-transformed data showed that major chelae of right-handed crabs grow proportionately higher with increasing size and the ideal mechanical advantage increases; concurrently, the fingers of the minor chelae grow proportionately longer. The data for left-handed crabs showed greater variability, especially for minor chelae, providing evidence for the concept that left-handedness arises by reversal of handedness following loss of the major chela from the right-hand side. Records of handedness in large samples of non-ocypodid heterochelous brachyuran crabs show a preponderance of right-handedness.     

The effect of size and temperature on the predation of cockles Cerastoderma edule (L.) by the shore crab Carcinus maenas (L.)

Laboratory studies demonstrated that shore crabs Carcinus maenas (L.) can consume <40 cockles Cerastoderma (= Cardium) edule (L.) per individual · day⁻¹. Various predation techniques used by the crabs are reported. The time required to open and consume individual cockles increased exponentially with prey size. Small (<l5-mm shell height), easily broken cockles appeared to be the most profitable in terms of energy acquisition per unit of handling time, the optimal size of prey increasing with predator size. With unlimited prey available, however, crabs selected prey of mean size smaller than these predicted optima, and much below the maximum size they were capable of opening. Feeding rates, both in terms of cockles ingested or energy intake per day, rose steeply with increasing temperature, but the size range of prey consumed remained unchanged. These data strongly suggest that Carcinus maenas is a potentially important predator of small cockles, particularly during the wanner summer months.     

The effect of environmental ammonia concentrations on the ion-exchange of shore crabs,Carcinus maenas (L.)

1. Ion exchange characteristics (ion-permeability, net, passive and active ion fluxes) of adult shore crabs were studied in relation to experimentally-increased external ammonia concentrations.2. Moderately-elevated ammonia concentrations (up to about 1 mmol/1 NH₄⁺) induce an increase in the ion permeability and the salt fluxes across the body wall of Carcinus maenas. At still higher NH₄⁺ concentrations, ion-permeability and ion fluxes are reduced again.3. Active salt influx generally follows the same pattern as observed for whole animal permeability, probably because both parameters are strongly related to gill perfusion and gill ventilation.4. Prolonged exposure to elevated ammonia concentrations in the environment is unfavourable, not because of the NH₄⁺ toxicity, but because of the higher energy requirements associated with the higher salt fluxes.     

Parasites of the shore crab Carcinus maenas (L.): implications for reproductive potential and invasion success

The European shore crab, Carcinus maenas, is one of the most successful marine invasive species. Its success has been in part attributed to the loss of parasites, rekindling an interest in host-parasite interactions and impacts on host fitness in this crab. In the present study, we investigated C. maenas populations from Europe, South Africa and Australia for parasites, and assessed their impact on the fitness of male crabs. For the shore crab, testes weight along with success in mating competition is traded off against other life-history traits. We therefore used this parameter as an indicator both for reproductive fitness and a possible resource trade-off in response to parasite infestation. In the native range, crabs infested with Sacculina carcini showed significantly lower testes weight than uninfected crabs. However, helminth parasites did not generally cause reduced testes weights. Crab populations from South Africa and Australia were either parasitized at very low prevalences, or were completely parasite free. However, no population level effect of this parasite release was reflected in testes weight. These findings do not support a severe fitness impact of helminth parasites on C. maenas, which questions the role of parasites on its population dynamics, both in the native area and for invasive success.