Colin L. McLay

Contingencies in the behaviour of the crab Heterozius rotundifrons

We recorded the responses of individual intertidal crabs, Heterozius rotundifrons, to stimuli presented singly and in combinations in the laboratory. Undisturbed crabs did not respond to the introduction of odour from a crushed conspecific but did respond strongly to food odour. Undisturbed crabs responded equally to food odour alone and a combination of food and odour from a crushed conspecific. When tactile stimulation was applied, as when the crab is grasped by a predator, individual H. rotunidfrons assumed a rigid, appendage-extended posture for several minutes. Tests with predatory fish showed that this catatonic posture is a very effective predator-defence mechanism. The duration of the catatonic state was decreased by the addition of food odour but increased by the addition of alarm odour (crushed conspecific) or the combination of alarm and food odours. Thus, which chemical stimulus was dominant was reversed by tactile input (i.e. dominance was contingent upon context). The effect of alarm odour on food odour responses lasted 4 h. Visual input in the form of a shadow passing over the crabs, either before or after tactile induction of the catatonic state, also increased the duration of that state. However, the duration of the catatonic state following exposure to both cues associated with danger (shadow+alarm odour) was similar to that of the control level. The crabs appeared to switch strategies when three cues associated with danger (tactile grasping, alarm odour and shadows) were detected, either simultaneously or over a 4-h period. The results illustrate the highly contingent nature of the behaviour of these crabs. Copyright 2000 The Association for the Study of Animal Behaviour.

Human-Mediated Spread of Alien Crabs

The introduction and spread of alien species is now recognized as one of the most significant modifiers of biodiversity. In the absence of their normal predators and parasites, alien crabs often establish high population densities and tend to compete fiercely with local fauna for food and shelter. A total of 73 species of brachyuran and crab-like anomuran decapods are known as alien species, of which 48 (65.8%) have become established. Three groups stand out with their high number of alien species: namely the Portunoidea (swimming crabs, such as Carcinus maenas), Grapsoidea (shore crabs, such as Hemigrapsus takanoi) and Majioidea (spider crabs, such as Pyromaia tuberculata). Canals, ballast water and hull fouling are the primary vectors/routes by which crabs are spread. Transfer of crabs with shellfish, combined with the live seafood trade, are also important. The Mediterranean Sea has the highest number of alien brachyuran species as many have invaded through the Suez Canal, making the Mediterranean the meeting place of Atlantic and Indo-West Pacific faunas. We used egg size as an indicator of life history strategies and a comparison of established alien species with a matched control group of crabs shows that mean egg size of alien crabs is smaller, but it shows wide variation. The Erythrean invaders from the Red Sea are a representative sample of aliens that shows the same pattern even though their transfer agent was a canal rather than shipping. Deliberate transfers to establish new fisheries has been successful in some cases (e.g., Paralithodes camtschaticus to the Barents Sea), but some species are still expanding their range and so their ultimate effects are unknown. The impact of most aliens remains uncertain, but recent work on Hemigrapsus sanguineus provides an excellent model of the kind of experimental field work that needs to be done. The current focus of attention on coastal aliens has resulted in the unfortunate agreement, at the international level, that ballast water can be dumped with impunity on the high seas, without any knowledge of its impact.

Handling times, prey size and species selection by Cancer novaezelandiae (Jacquinot, 1853) feeding on molluscan prey

Abstract Handling times, consumption rates, and prey selection of the New Zealand cancrid crab Cancer novaezelandiae Jacquinot were investigated in relation to mollusc prey of different size, species, geometric shape, and anti-predatory adaptations. In the laboratory, male crabs of three size classes (55–65, 80–90 and 105–115 mm carapace width) were offered four mollusc species, bivalves Mytilus edulis aoteanus Powell and Austrovenus stutchburyi Wood, and gastropods Cominella maculosa Martyn and Turbo smaragdus Gmelin. Handling times of C. novaezelandiae increased with prey size, larger crabs requiring less time to open and consume prey than small crabs. Bivalves were opened more quickly than gastropods, although at small shell size differences were negligible. Energy intake increased with crab size, despite crabs of all sizes consuming similar numbers of prey. Energy consumption was higher when feeding on mussels in comparison to whelks. Profitability curves were derived and predictions about optimum prey size and species were tested. When presented with a size range of prey, crabs selected sizes that minimized time spent foraging. However, when presented with different prey species, crabs maximized energy intake by selecting optimum prey species. Results suggest that C. novaezelandiae is capable of exhibiting a flexible foraging behaviour, which may allow crabs to maximize feeding efficiency.

Comparative Analysis of the Mating Strategies in Grapsid Crabs with Special References to the Intertidal Crabs Cyclograpsus Lavauxi and Helice Crassa (Decapoda: Grapsidae) from New Zealand

Abstract Field and laboratory observations were carried out on the reproductive behaviour of Cyclograpsus lavauxi and Helice crassa, and the results were compared with other Grapsidae, with emphasis on New Zealand species. Mating in all species typically occurs during the intermoult and often coincides with the time of oviposition. Females of several species have been reported to mate multiple times, often in the few days prior to oviposition, leading to sperm competition within the female spermatheca. Females were found to be sexually receptive only in a short period before oviposition (e.g., C. lavauxi, Hemigrapsus crenulatus, H. sexdentatus), although some species were also receptive for about two weeks after oviposition (e.g., H. crassa). The exact duration of female receptivity is unknown for most grapsid species. Female grapsid crabs exhibit a wide range of gonopore structures which either restrict female receptivity to certain times or allow them to mate at any time. In species with restricted female receptivity (e.g., C. lavauxi, H. crassa, H. crenulatus, H. sexdentatus), the operational sex ratio is typically highly male-biased. Male-male competition was found to be intense in the four New Zealand grapsids, with frequent male-male interactions during which larger males were typically more successful in fights over females, resulting in a greater number of matings. Postcopulatory guarding, which is assumed to reduce the risk of sperm competition and to ensure paternity, has been observed in a few grapsid species (H. sexdentatus and H. crenulatus) but not in others (e.g., C. lavauxi and H. crassa). Overall, grapsid crabs employ a variety of reproductive strategies including direct competition between males for females, with postcopulatory guarding; males securing resources as sites for mating; and males having only brief interceptions with receptive females. These differences in reproductive behaviour are discussed in the context of sexual selection and the ecological and environmental differences of the habitats that grapsid crabs occupy. Mating strategies of grapsid species can be very different even if they occupy the same habitat and the females have similar duration of sexual receptivity.

Responses to predation risk: alternative strategies in the crab Heterozius rotundifrons

The New Zealand endemic crab Heterozius rotundifrons displays an effective antipredator behaviour of fully extending all limbs for a number of seconds or even several minutes if presented with single sensory inputs (alarm odour of crushed conspecifics or an overhead shadow presentation) but fully extends all limbs for a shorter period when chemical and visual cues are presented simultaneously (Hazlett & McLay 2000, Animal Behaviour, 59, 965–974). Because the diversity of the sensory channels experienced and the total magnitude of cues indicating elevated risk were confounded in our earlier study, we separated the diversity and magnitude of cues to study the conditions that result in a switch in antipredator strategies. Crabs showed similar extended durations of the limb-extended state for all magnitudes of single modality treatments that were above threshold. Similarly, for all above-threshold combinations of modalities, the crabs showed similarly reduced durations of the limb-extended state compared with controls. It appears that the switch in strategies is induced just by the diversity of categories of risk cues, not their magnitude.

Tri-locus sequence data reject a “Gondwanan origin hypothesis” for the African/South Pacific crab genus Hymenosoma

Crabs of the family Hymenosomatidae are common in coastal and shelf regions throughout much of the southern hemisphere. One of the genera in the family, Hymenosoma, is represented in Africa and the South Pacific (Australia and New Zealand). This distribution can be explained either by vicariance (presence of the genus on the Gondwanan supercontinent and divergence following its break-up) or more recent transoceanic dispersal from one region to the other. We tested these hypotheses by reconstructing phylogenetic relationships among the seven presently-accepted species in the genus, as well as examining their placement among other hymenosomatid crabs, using sequence data from two nuclear markers (Adenine Nucleotide Transporter [ANT] exon 2 and 18S rDNA) and three mitochondrial markers (COI, 12S and 16S rDNA). The five southern African representatives of the genus were recovered as a monophyletic lineage, and another southern African species, Neorhynchoplax bovis, was identified as their sister taxon. The two species of Hymenosoma from the South Pacific neither clustered with their African congeners, nor with each other, and should therefore both be placed into different genera. Molecular dating supports a post-Gondwanan origin of the Hymenosomatidae. While long-distance dispersal cannot be ruled out to explain the presence of the family Hymenosomatidae on the former Gondwanan land-masses and beyond, the evolutionary history of the African species of Hymenosoma indicates that a third means of speciation may be important in this group: gradual along-coast dispersal from tropical towards temperate regions, with range expansions into formerly inhospitable habitat during warm climatic phases, followed by adaptation and speciation during subsequent cooler phases.

Use of camouflage materials as a food store by the spider crab Notomithrax ursus (Brachyura: Majidae)

Abstract In laboratory tests, the camouflage crab Notomithrax ursus utilised its algal mask as a source of food. Mask ingestion mainly involved the removal of algae from the carapace, with removal from legs occurring less commonly, implying different rates of algal turnover. When there was no other available source of food, crabs ate an average of 4.1 % of the mask per day but when an alternative source of food (mussel flesh) was available, only 2.8% of the mask was consumed per day. A variety of red, green, and brown algae were consumed. The dual role of the mask means that there is an interaction between camouflage and food storage. Our results suggest that for N. ursus, camouflage is more important because consumption constitutes only a minor part of mask turnover. The evolution of spider crab masking is discussed in relation to the published literature.

Responses of the Crab Heterozius rotundifrons to Heterospecific Chemical Alarm Cues: Phylogeny vs. Ecological Overlap

The big-handed brachyuran crab Heterozius rotundifronsextends the time spent in its anti-predator posture, limb extended posture, if exposed to chemical cues from crushed conspecifics. In this study, we tested whether crabs also respond to chemical cues from crushed heterospecific crabs, and if so, whether phylogenetic relations or ecological overlap is more important in influencing the duration of the anti-predator posture. Chemical cues from two other brachyuran crabs (Cyclograpsus lavauxi and Hemigrapsus sexdentatus), which do not overlap directly in ecological distribution with H. rotundifrons, elicited a duration of the anti-predator posture that was indistinguishable from that produced by conspecific chemical cues. In contrast, chemical cues from two anomuran crabs (Petrolisthes elongatus and Pagurus novizealandiae) that overlap in ecological distribution with H. rotundifrons, elicited durations of the antipredator posture that were significantly shorter than those of either conspecifics or more closely related crab species. Thus, phylogenetic relationship seems to be more important than ecological overlap in influencing anti-predator behavior in H. rotundifrons.

Use of the Sterile Male Technique to Investigate Sperm Competition, Storage, and Use in a Pill Box Crab, Halicarcinus cookii (Brachyura: Hymenosomatidae)

Abstract Halicarcinus cookii is a small intertidal New Zealand crab that has high levels of ovigerous females in all months. Continuous breeding requires continuous mating. Mating is not linked to moulting because the pubertal moult is terminal. Spermathecal fullness is modelled in terms of the difference between the rates of copulation and rates of brood production. It is estimated that females can fill their spermathecae with sperm after 3-4 copulations (depending on male size) and that approximately 15% of an ejaculate is used to fertilize each brood. Given the fact that females have multiple partners we asked the question: is it worthwhile for males to spend any time mate guarding? Using sterile males we showed that the majority of each brood is sired by the last male to mate. In females prevented from mating again, more of each successive brood is likely to be sired by earlier males who had mated with the female. This suggests that over periods exceeding one brood cycle (i.e., months) sperm slowly becomes mixed. Therefore, it is worthwhile for male H. cookii to invest time in guarding female partners.

An annotated key to the hermit crabs (Crustacea : Decapoda : Anomura) of the Otago region (southeastern New Zealand)

Abstract A key is given to the 17 species of hermit crabs of southeastern New Zealand (Otago region). The key is designed primarily for ecological work and emphasises characters readily seen in living animals still in their host shells. The depth range of each species is also included.