Fouling of gastropods: a role for parasites?

A sample of mud snails Hydrobia ulvae (Prosobranchia) from an intertidal population revealed that the shells of trematode-infected specimens were especially likely to be fouled with epibionts. Experimentally trematode-infected Biomphalaria glabrata (Pulmonata) appeared to be especially prone to develop epigrowth in comparison with uninfected conspecifics as well. These findings suggest an interaction between trematode infections and epibiosis in aquatic gastropods. The two most likely explanations for this are (1) that trematode infections weakens the snails' natural defences against epibionts, or (2) that the defences against epibionts also are effective against invading trematodes, causing snail specimens with a particularly good fouling defence to be less likely to become infected.     

Intraspecific morphological variation related to the moult-cycle in colour forms of the shore crab Carcinus maenas

The common shore crab Carcinus maenas exhibits a range of carapace colours from green through orange to red, green forms showing some differences of distribution from red/orange forms. To test the hypothesis that colour differences were moult-related, large numbers of Carcinus were collected intertidally and subtidally in summer when moulting is most prevalent, and their moult stages determined. Red and orange coloration was found only to occur in the larger size classes in crabs in prolonged intermoult, but not solely in the largest crabs in terminal anecdysis. Red crabs were characterized by a heavier load of epibionts and a strong, thicker carapace. In contrast, green crabs were found over the entire size range and appeared to be actively moulting forms; they carried fewer epibionts and had a thinner integument than red forms. The significance of morphological differences between red/orange and green forms of Carcinus maenas is considered in relation to previously reported behavioural, physiological and ecological differences between the colour forms.     

Epibionts of the great spider crab, Hyas araneus (Linnaeus, 1758), in the Barents Sea

Hyas araneus is the most common brachyuran crab in the coastal Barents Sea. Its epibionts were occasionally examined in 1940–1950s. To obtain modern information about associated organisms living on the crabs and compare new data with previous findings, the species composition, infestation indices, and spatial distribution of macro-epibionts colonizing H. araneus were examined in Dalnezelenetskaya Bay, a small semi-open gulf, in summer 2008–2010. A total of 41 taxa were observed on 48 crabs collected from depths ranging from 5 to 28 m. Red algae Ptilota plumosa (prevalence 39.6%) and Palmaria palmata (37.5%) as well as the copepod Harpacticus uniremis (39.6%) and the tube-dwelling worm Placostegus tridentatus (35.4%) predominated on the crabs. Infestation indices were similar in male and female crabs except for the turbellarian worm Peraclistus oophagus. Comparison with the previous study showed some changes in H. araneus fouling community that may be attributed to the shift of climatic regime or differences in sampling procedures. Presence of the amphipod Ischyrocerus commensalis, a new epibiont of H. araneus in the study area may be associated with introduction of the red king crab Paralithodes camtschaticus. Infestation indices of common epibionts depend on the crab shell conditions. Localization of epibionts strongly depended on the settlement patterns of larvae and host-associate relationships. Typical fouling organisms were found predominantly on the carapaces or limbs, while the majority of mobile species were recorded on the gills. Colonization of great spider crabs is beneficial for the epibiont species, whereas some negative effects for the basibiont could not be excluded.     

Herbivory by crabs and the control of algal epibionts on Caribbean host corals

Summary A short-term experiment was conducted to examine the relationships among the branching coral Porites porites, algal epibionts, and a facultative crab associate Mithrax sculptus in Belize, Central America. Initial field observations suggested that coral colonies supporting resident crabs generally had lower algal cover than colonies without crabs. The hypothesis was tested that Mithrax significantly depresses host coral algal cover and thereby indirectly affects host survivorship and growth. Crab accessibility to an array of coral colonies, similarly covered with algal epibionts, was manipulated in three treatments. Results strongly support the hypothesis, with significant differences in algal cover (primarily Dictyota spp.) noted among treatments after only one month. Caged heads with crabs included and uncaged natural controls allowing crabs free access averaged less than 10% cover, whereas mean algal cover exceeded 75% where crabs were excluded. The uncaged treatment, in which crabs were allowed free access to Porites heads was not significantly different from the crab inclusion treatment. Collectively, these results demonstrate that under natural conditions, crabs can have pronounced effects on host corals by reducing fouling algal epibionts. Furthermore, these facultative coral associates may have more important, albeit localized effects on Caribbean corals than has been suggested previously.     

Fouling community of the red king crab, <Emphasis Type="Italic">Paralithodes camtschaticus</Emphasis> (Tilesius 1815), in a subarctic fjord of the Barents sea

We examined the species composition of red king crab (Paralithodes camtschaticus) fouling communities in Dolgaya Bay, a small fjord of the Barents Sea, in August 2005 and 2006. In total, there were 13 species observed on 301 crabs collected from water depths of 5–90 m. Barnacles (Balanus crenatus; prevalence 42.9%) and blue mussels (Mytilus edulis; 11.6%) were the most common epibionts, while amphipods (Ischyrocerus commensalis) were the most common symbionts (28.6%). Infestation rates in Dolgaya Bay were different from those in an “open” area of the Barents Sea (Dalnezelenetskaya Bay), probably due to differences in hydrodynamic conditions. Differences in infestation prevalence and intensity were detected neither between male and female crabs nor between crabs collected at 5–35 m versus 90 m depths. Prevalence of common fouling species increased with host size. Amphipods I. commensalis colonized the carapace and limbs in Dolgaya Bay less frequently than in Dalnezelenetskaya Bay, probably due to interspecific competition with barnacles occupying the dorsal parts of the host. Juvenile barnacles and mussels dominated the fouling communities on the crabs. The age of barnacles did not exceed 2–4 months. However, the presence of 4-year-old mussels suggests that these older mollusks have been directly transferred from mussel beds to the hosts. Our results indicate that colonization by epibionts and symbionts is generally not disadvantageous for the crab hosts, except for some possible negative impacts of amphipods occupying the gills.     

The effect of epibionts on the susceptibility of the red seaweed Cryptonemia seminervis to herbivory and fouling

Epibiosis or fouling on living organisms can have direct and indirect detrimental effects, in particular on photosynthetic organisms such as seaweeds. It thus seems reasonable to hypothesize that macroalgae have been selected for the presence or induction of antifouling (AF) defences. The red seaweed Cryptonemia seminervis is usually found in nature with an elevated cover of epibionts. To assess the effect of epibiosis on the susceptibility of this seaweed to herbivory and fouling, the abundance of fouling was evaluated and compared to herbivore consumption (by amphipods and sea urchins) of fouled (bryozoan and sponge) and non-fouled C. seminervis. Attachment of the mussel Perna perna to surfaces treated with extracts from seaweeds with and without epibionts was also assessed. Epibiosis corresponded to ca. 51% of the blade surface of C. seminervis, sometimes covering as much as 90% and up to 51% of the thallus weight, encompassing mainly the bryozoan Membranipora membranacea and an unidentified sponge. Algae colonized by M. membranacea were preferred compared to algae devoid of epibionts, a 'shared doom' effect, either by the amphipod Elasmopus brasiliensis or by the urchin Lytechinus variegatus (p < 0.01). Sponge epibiosis also increased consumption by both herbivores (p < 0.001), suggesting that epibionts may act as lures to herbivores, attracting consumers that otherwise would not feed significantly on the seaweed. Foods containing extracts from fouled C. seminervis were preferred by urchins over the alga devoid of epibionts. However, extracts from fouled alga inhibited mussel attachment when compared to epibiont-free alga. Differences might be a direct detrimental effect of the presence of epibionts. On the other hand, epibiosis may induce the production of AF defences in C. seminervis.     

Host specificity of <Emphasis Type="Italic">Sacculina carcini</Emphasis>, a potential biological control agent of the introduced European green crab Carcinus maenas in California

The European green crab, Carcinus maenas, is an introduced marine predator established on the west coast of North America. We conducted laboratory experiments on the host specificity of a natural enemy of the green crab, the parasitic barnacle Sacculina carcini, to provide information on the safety of its use as a possible biological control agent. Four species of non-target, native California crabs (Hemigrapsus oregonensis, H. nudus, Pachygrapsus crassipes and Cancer magister) were exposed to infective larvae of S. carcini. Settlement by S. carcini on the four native species ranged from 33 to 53%, compared to 79% for green crabs. Overall, cyprid larvae tended to settle in higher numbers on individual green crabs than on either C. magister or H. oregonensis. However, for C. magister this difference was significant for soft-shelled, but not hard-shelled individuals. Up to 29% of the native crabs arrested early infections by melanizing the rootlets of the parasite. Most native and green crabs settled on by S. carcini became infected, especially when settled on by >3 cyprids. Infected green crabs died at more than twice the rate of uninfected green crabs. In contrast to green crabs, all infected native crabs died without producing an externa (reproductive sac). At high settlement intensities, infected native crabs frequently exhibited neurological symptoms (twitching, loss of movement) before death. These results indicate that use of S. carcini as a biological control agent could result in the death of native crabs. The magnitude of this effect would be proportional to the density of infected green crabs in the environment and the probability that cyprids would contact native crabs in the wild. Potential benefits of biological control should be assessed in relation to these potential non-target effects.     

High serotonin levels due to the presence of the acanthocephalan Hexaglandula corynosoma could promote changes in behavior of the fiddler crab Uca spinicarpa

Between February and June 2010, 113 fiddler crabs Uca spinicarpa were collected from the Chuburna lagoon system on the northern coast of the Yucatan Peninsula, México. Of the 68 crabs gathered outside their burrows, 13 were infected with 25 cystacanths of Hexaglandula corynosoma (intensity of infection from 1 to 5) and the remaining 55 crabs were uninfected. The other 45 crabs were found inside their burrows and only one was found infected with 1 cystacanth of H. corynosoma. Serotonin (5-HT) levels were higher in the group of crabs infected with H. corynosoma in contrast to the group of uninfected crabs and the group of those infected with other parasites. A redundancy analysis corroborated a positive relationship between 5-HT and the intensity of infection with H. corynosoma. In contrast, dopamine levels remained similar among different groups of crabs.     

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.     

Effects of artificial epibionts on byssogenesis,attachment strength,and movement in two size classes of the blue mussel,Mytilus edulis

Blue mussels (Mytilus edulis) can alter the strength of byssal attachment and move between and within mussel aggregations on wave‐swept shores, but this movement ability may be limited by epibiont fouling. We quantified the effects of artificial epibiont fouling on the production of byssal threads, attachment strength, and movement in two size classes of blue mussels. In a factorial experiment, large epibiont‐covered mussels produced more functional byssal threads (i.e., those continuous from animal to substrate) after 24 h than large unfouled and small fouled mussels, but not more than small unfouled mussels. Small unfouled mussels formed and released more byssus bundles compared to any other treatment group, which indicates increased movement. Conversely, epibiont fouling resulted in decreased numbers of byssus bundles shed, and therefore reduced movement in small mussels. Epibiont‐covered mussels started producing byssal threads sooner than unfouled mussels, while small mussels began producing byssal threads earlier compared to large mussels. Mean attachment strength from both size classes increased by 9.5% when mussels were artificially fouled, and large mussels had a 34% stronger attachment compared to small mussels. On the other hand, a 2.3% decrease in attachment strength was found with increasing byssus bundles shed. Our results suggest that fouling by artificial epibionts influences byssal thread production and attachment strength in large mussels, whereas epibionts on small mussels impact their ability to move. Mussels are able to respond rapidly to fouling, which carries implications for the dynamics of mussel beds in their intertidal and subtidal habitats, especially in relation to movement of mussels within and among aggregations.     

Aspects of the biology of the deep-sea spider crab, Dorhynchus thomsoni (Crustacea: Brachyura)

Pelagic first crab stages of the deep-sea spider crab, Dorhynchus thomsoni , are reported from mid-water samples taken at a variety of localities in the Porcupine Seabight and the Rockall Trough. Some of these samples were from hauls made near the surface over depths of 3000 m or more.
An analysis of these pelagic samples, and of benthic catches of D. thomsoni held by the Institute of Oceanographic Sciences and the British Museum (Natural History), indicate that most females and at least some males undergo the puberty prior to the third crab instar, these individuals therefore probably pass through a total of only three larval and three post-laval instars. There is at least a possibility that some individuals moult after the moult of puberty, unlike any other spider crabs.     

The effect of epibionts on the susceptibility of the red seaweed Cryptonemia seminervis to herbivory and fouling

Epibiosis or fouling on living organisms can have direct and indirect detrimental effects, in particular on photosynthetic organisms such as seaweeds. It thus seems reasonable to hypothesize that macroalgae have been selected for the presence or induction of antifouling (AF) defences. The red seaweed Cryptonemia seminervis is usually found in nature with an elevated cover of epibionts. To assess the effect of epibiosis on the susceptibility of this seaweed to herbivory and fouling, the abundance of fouling was evaluated and compared to herbivore consumption (by amphipods and sea urchins) of fouled (bryozoan and sponge) and non-fouled C. seminervis. Attachment of the mussel Perna perna to surfaces treated with extracts from seaweeds with and without epibionts was also assessed. Epibiosis corresponded to ca. 51% of the blade surface of C. seminervis, sometimes covering as much as 90% and up to 51% of the thallus weight, encompassing mainly the bryozoan Membranipora membranacea and an unidentified sponge. Algae colonized by M. membranacea were preferred compared to algae devoid of epibionts, a ‘shared doom’ effect, either by the amphipod Elasmopus brasiliensis or by the urchin Lytechinus variegatus (p < 0.01). Sponge epibiosis also increased consumption by both herbivores (p < 0.001), suggesting that epibionts may act as lures to herbivores, attracting consumers that otherwise would not feed significantly on the seaweed. Foods containing extracts from fouled C. seminervis were preferred by urchins over the alga devoid of epibionts. However, extracts from fouled alga inhibited mussel attachment when compared to epibiont-free alga. Differences might be a direct detrimental effect of the presence of epibionts. On the other hand, epibiosis may induce the production of AF defences in C. seminervis.     

Saliva can mediate HIV-1-specific antibody-dependent cell-mediated cytotoxicity

HIV is not usually transmitted by saliva from HIV-1-infected individuals. Antiviral substances in saliva responsible for this may include HIV-1-specific antibody-dependent cell-mediated cytotoxicity (ADCC). We evaluated saliva ADCC titers of 62 HIV-1-infected women from the Women's Interagency HIV Study (WIHS) and 55 uninfected individuals. HIV-1-infected women were less likely to have ADCC activity in saliva than in serum or cervical lavage fluid (CVL). 24% of HIV-1-positive women and a similar percentage of uninfected women had HIV-1-specific saliva ADCC activity. A significant amount of saliva ADCC activity in infected women was HIV-gp120-specific. These studies demonstrate that HIV-specific ADCC activity can be present in saliva. This activity may contribute to host defence against initial infection with HIV.     

Relationships among size, development, and Batrachochytrium dendrobatidis infection in African tadpoles

The fungal pathogen Batrachochytrium dendrobatidis contributes to the global decline of amphibians. Although mortality from B. dendrobatidis infections occurs primarily in postmetamorphic individuals, infected tadpoles may suffer reduced growth and developmental rates as a result of oral chytridiomycosis, possibly affecting adult fitness. We conducted a field study in which we examined South African tadpoles for oral chytridiomycosis and compared the body sizes of infected and uninfected individuals of 2 species, Heleophryne natalensis and Strongylopus hymenopus. Presence of B. dendrobatidis was determined by microscopic inspection of mouthparts. Infection prevalence was high in both species, 62.5 and 38.6%, respectively, and infected individuals were significantly larger in both species. The inclusion of developmental stage in the analysis of S. hymenopus body size eliminated the relationship between body size and infection status, suggesting that differences in body size were not due to differences in growth, but to differences in developmental stage of infected larvae. These results suggest that larvae at more advanced developmental stages are more likely to be infected with B. dendrobatidis and that infection in larval amphibians may be dependent on time or developmental status of larvae. Contrary to the results of past studies, there was no evidence that oral chytridiomycosis resulted in decreased growth of tadpoles, despite the occurrence of oral abnormalities in infected individuals of 1 species. Because tadpole performance can subsequently affect the health of anuran populations and because tadpoles can act as reservoirs of infection, the study of B. dendrobatidis in larval amphibians is important to understanding the effects of this emerging disease.     

Behaviour of juvenile mud crabs Scylla serrata in aquaculture: Response to odours of moulting or injured crabs

Behaviour of juvenile mud crabs, Scylla serrata (70–90 mm carapace width, CW) were observed in response to odours of moulting and injured conspecifics and food (pilchard) under controlled flow conditions using bioassay technique. This study was undertaken to better understand the role that chemical cues have in mediating the attraction of cannibal crabs to moulting crabs in an aquaculture growout facility and thus aid in more successful production. In response to moult odour juvenile S. serrata spent 5.6 ± 1.9% of the time in locomotion but this did not differ significantly (P > 0.05) from that of control odours (seawater, 2.6 ± 1.2%), however a tactile response was observed for moult odour not seen in controls. Juveniles exposed to the odour of injured conspecifics (12.2 ± 2.3% of time in locomotion) and to food (22.7 ± 3.1%) differed significantly from the seawater control (P < 0.05). The possibility that the active agent in moult water is relatively dilute was considered, however varying concentrations of food and crushed conspecific odour failed to demonstrate concentration dependant behavioural results. Variation in the form of size (carapace width) and sex was revealed, so too was preliminary evidence of behavioural differences between crab hatchings. For example, larger crabs (around 80 mm CW) increased tactile investigation of the odour inlet pipe in response to the crab-based odours of moulting (3.5 ± 1.7% of the time) and injured conspecifics (2.6 ± 1.0%), though response to food remained constant over all sizes (10.0% of the time). In another experiment, larger female crabs maintained a high frequency of tactile response to the odour of injured conspecifics. In males, the response was attenuated in larger individuals. Despite no differences being found between moult odour and controls some evidence exists to suggest that a small proportion of crabs were responding to the moult odour and that this still has the potential to cause a dramatic cumulative reduction in growout survival. Variation in size and subsequent behaviour of individuals poses the question of whether behaviour contributes to growth rate and might be a focus of genetic selection for more uniform growth.     

Incidence of bacteremia in stressed and unstressed populations of the blue crab, Callinectes sapidus

The incidence of bacteremia in the blue crab, Callinectes sapidus, is reported to be in excess of 80%. Because these results have been controversial, a field study was initiated to determine the effect of commercial capture and handling stresses on the incidence and levels of infection in blue crabs. The majority (75%) of "unstressed" crabs which were captured individually and bled immediately upon removal from the water were bacteremic, with a geometric mean level of infection of 14 CFU/ml of hemolymph. Crabs collected by crab pot, confined within these pots for as long as 24 h, and sampled immediately after removal from the water had a similar mean level of infection. Crabs subjected to the stresses of commercial capture, handling, and transport showed a higher incidence of infection (91%) and a mean infection level of 46 CFU/ml. Injuries sustained by crabs during commercial handling are thought to be associated with the higher incidence of infection. Vibrio spp. were primarily responsible for progressive infections in commercially stressed crabs and were the predominant bacterial type in heavily infected crabs. Our results indicated that uninjured healthy crabs do not have sterile hemolymph but instead harbor low-level bacterial infections.     

Incidence of bacteremia in stressed and unstressed populations of the blue crab, Callinectes sapidus.

The incidence of bacteremia in the blue crab, Callinectes sapidus, is reported to be in excess of 80%. Because these results have been controversial, a field study was initiated to determine the effect of commercial capture and handling stresses on the incidence and levels of infection in blue crabs. The majority (75%) of "unstressed" crabs which were captured individually and bled immediately upon removal from the water were bacteremic, with a geometric mean level of infection of 14 CFU/ml of hemolymph. Crabs collected by crab pot, confined within these pots for as long as 24 h, and sampled immediately after removal from the water had a similar mean level of infection. Crabs subjected to the stresses of commercial capture, handling, and transport showed a higher incidence of infection (91%) and a mean infection level of 46 CFU/ml. Injuries sustained by crabs during commercial handling are thought to be associated with the higher incidence of infection. Vibrio spp. were primarily responsible for progressive infections in commercially stressed crabs and were the predominant bacterial type in heavily infected crabs. Our results indicated that uninjured healthy crabs do not have sterile hemolymph but instead harbor low-level bacterial infections.     

Morphology and biology of Polysaccus japonicus (Crustacea, Rhizocephala, Akentrogonida, Polysaccidae, fam. n.), a parasite of the ghost-shrimp Callianassa japonica

The morphology and biology of Polysaccus japonicus Høeg & Lützen, 1993 is described. Nearly all infected hosts are adults, the females of which become castrated. Each bears 4–50 (female) externae on the underside of the abdomen. The externae are interconnected via a root system within the host's perineural blood sinus. The externae pass through three moults. Cypris larvae inject spermatogonia into the immature externa's mantle cavity, which is where spermiogenesis occurs later. Multiplication of the male cells is probably supported by secretion from paired mantle cavity glands, presumably homologous to the receptacles of kentrogonid rhizocephalans. Late during the 2nd intermoult the visceral mass ruptures to release the ova into the mantle cavity where they become fertilized. The third moult results in opening of the mantle aperture which initiates a period of vigorous peristalsis of the muscular mantle. Following emission of the cypris larvae the externae perish leaving distinct scars. The root system always contains buds of various sizes and stage of development. When the scarred host moults, the most advanced buds emerge to give rise to a new generation of externae. This is thought to be repeated for the rest of the host's life.
The antennule of the cyprid has a relatively very long and slender 3rd segment, a unique construction among Akentrogonida. Because of this and the presence of special mantle cavity glands, a new family, Polysaccidae, is erected, which in some characters is intermediate between the Kentrogonida and the Akentrogonida.     

Parasitic mites influence fitness components of their host, the land snail Arianta arbustorum

Abstract. Parasites can influence the population dynamics of their hosts by affecting life-history strategies and behavior. The hematophageous mite Riccardoella limacum lives in the lung cavity of terrestrial gastropods. We used correlational and experimental approaches to investigate the influence of parasite infection on the behavior and life-history traits of the simultaneously hermaphroditic land snail Arianta arbustorum , a common host of R. limacum. Naturally infected individuals of A. arbustorum , collected in the wild, showed a decreased activity compared with uninfected snails. The reproductive output, expressed as the number of eggs deposited in a reproductive season, was reduced in mite-infected hosts. However, the hatching success of the eggs laid by parasitized snails was slightly higher than that of uninfected individuals. We also examined winter survival in 361 adults of A. arbustorum collected from four natural populations. The prevalence of mite infection ranged from 44.8% to 70.1% in three populations (snails in the fourth population were not infected). Winter survival was reduced in infected snails in two out of three populations. Furthermore, experimentally infected snails from an uninfected population showed a reduced winter survival compared with control snails. Our results indicate that parasite pressure imposed by members of R. limacum may influence life history in A. arbustorum.     

Untersuchungen zur Häutungsphysiologie der dekapoden Krebse am Beispiel der StrandkrabbeCarcinus maenas

Under constant laboratory conditions, juvenile shore crabs moult at fixed intervals which depend upon their body size. During one moult every crab exhibits increases of the same relative amounts, independent of its absolute size. Basing on the predictable duration of the intermoult period, the morphological changes in the structure of the cuticle and the development of limb-buds, the intermoult period could be divided into 21 different stages. After studying the moulting rhythm in constant milieu, the influence of the following exogenous and endogenous factors upon the moulting rhythm and growth of normal and of eye-stalkless individuals was investigated: temperature, photoperiod, loss of pereiopods, feeding, and presence of larger specimens. From these investigations it became evident that the moulting rhythm is regulated by growth. The crabs are able to moult only after achieving a minimum of tissue growth. So long as this minimum growth is not achieved, a moult-inhibiting hormone is secreted and moulting is prevented. If the moult-inhibiting hormone is absent, moulting hormone is secreted and initiates a moult. Under dangerous conditions, the crabs are able to delay the next moult. Under unfavourable conditions they consume less food than normal. Therefore, the amount of tissue growth which is the necessary prerequisite for moulting is delayed, and continued release of moult-inhibiting hormone prevents the moult. Under conditions favourable for moulting, or demanding moult (e. g. after loss of many pereiopods) the crabs accelerate the moult. Temperature influences the moulting rhythm by indirect effects on the metabolic rate. During further investigations, the variation of the following parameters were determined quantitatively: content of moulting hormone in whole crabs; content of aminoacids, protein, glucose, Na⁺, K⁺, Mg⁺⁺ and Ca⁺⁺ in the hemolymph; pH and osmotic pressure in the hemolymph; and Ca⁺⁺ content in skeleton and whole crabs. All parameters mentioned — excepting pH and K⁺ content of the hemolymph — vary characteristically during the intermoult period. The titre of moulting hormone has 4 different maxima. Of all parameters, only the content of animoacids and protein in the hemolymph vary in the same way as the titre of the hormone. From these results the following conclusions are drawn: The moulting hormone not only initiates the moulting process, but controls it at several stages. Only protein metabolism seems to be under direct control of the moulting hormone which stimulates protein-synthesis. Chitin formation, regeneration, apolysis and ecdysis are indirectly controlled by the moulting hormone through protein metabolism. As in most of the other processes mentioned, the calcification of the new cuticle is not under the direct influence of the moulting hormone. The conclusion ofDigby (1966) that calcification in crabs is an electrochemical process, is confirmed.