Paternity in horseshoe crabs when spawning in multiple-male groups

Unpaired or satellite male horseshoe crabs, Limulus polyphemus, are attracted to and often form a group around a pair (a female with an attached male) that is nesting in the high intertidal zone. These males are engaged in sperm competition. We observed nesting pairs and their associated satellites in the wild, collected and reared their eggs and used genetic markers to examine paternity. We found that the unpaired, satellite males are highly successful at fertilizing eggs; two satellites can leave the attached male with few fertilizations. Two satellites together are each as successful as one spawning with a pair. A satellite's location around the female greatly affects his success, and males compete for access to a position over the dorsal canal between the prosoma and opisthosoma of the female and under the front margin of the paired male where they are most likely to fertilize eggs. Although eggs and sperm retain their viability for some time after spawning, nearly all eggs are fertilized by the satellites that are around the nesting pair at the time of egg laying and by the attached male. A number of factors including beach current, female size and male behaviour affect the outcome of sperm competition in this externally fertilizing species. Copyright 2000 The Association for the Study of Animal Behaviour.     

A review on fisheries and conservation status of Asian horseshoe crabs

Horseshoe crabs are the only extant xiphosurans and are believed to be morphologically unchanged for more than 200 million years. Of the four extant species namely, Limulus polyphemus, Tachypleus tridentatus, Tapinauchenius gigas and Carcinoscorpius rotundicauda, the latter three are found in Asian waters. Recent evidences showed that Asian horseshoe crabs are facing serious threats such as degradation of their spawning grounds and habitat, environmental pollution, overexploitation as a culinary delicacy and biomedical bleeding practices. Baseline data on the distribution and existing population of the wild horseshoe crabs remain poorly known in several Asian regions. Several studies have clearly revealed that pressure due to over-fishing of wild stock has increased tremendously in the last decade. Due to an increase in demand for Tachypleus Amebocyte Lysate (TAL) analogous to Limulus Amebocyte Lysate (LAL) in the United States, there is an urgent need to comprehensively address their fishing and conservation measures in the Asian region. This review addresses the overall studies on three species of Asian horseshoe crabs in relation to their fishing practices, local exploitation of their wild stock either for human consumption (or) by biomedical industries. The authors have structured the discussion on an international scale to address the existing problems in fishing and conservation of horseshoe crabs. Since no specific regulatory force or legislative protection act or a policy to preserve their natural stock are available to this date, this paper strongly recommends representative countries to include horseshoe crabs under their wildlife protection act to avoid further unsustainable exploitation of their wild populations.     

Orientation of larval and juvenile horseshoe crabs Limulus polyphemus to visual cues:Effects of chemical odors

Adult horseshoe crabs Limulus polyphemus have long served as models for the study of vision in marine arthropods. Yet,little is known about the ability of early life history stages to detect and respond to visual cues.We examined the visually directed movements of larvae and first stage juveniles to horizons containing dark visual targets of different sizes.The study tested the hypotheses that (1) larval and juvenile crabs can detect and respond to visual targets and (2) the direction of orientation varies ...     

Orientation of larval and juvenile horseshoe crabs Limulus polyphemus to visual cues: Effects of chemical odors

<正> Adult horseshoe crabs Limulus polyphemus have long served as models for the study of vision in marine arthropods.Yet,little is known about the ability of early life history stages to detect and respond to visual cues.We examined the visuallydirected movements of larvae and first stage juveniles to horizons containing dark visual targets of different sizes.The studytested the hypotheses that (1) larval and juvenile crabs can detect and respond to visual targets and (2) the direction of orientationvaries with the presence of chemical cues associated with settlement habitats.Orientation of larval and juvenile crabs torectangles subtending angles from 30-330° was tested in a circular arena containing water that either lacked estuarine chemicalcues (offshore water) or contained odors from aquatic vegetation or known predators.In the absence of chemical odors,larvaeoriented toward and juveniles moved away from dark horizons subtending angles 60°.When placed in water containingchemical odors from potential nursery habitats,including the seagrasses Halodule wrightii and Syringodium filiforme,crabsreversed their direction of orientation relative to their responses in offshore water.Odors from two known predators,themummichug Fundulus grandis and blue crab Callinectes sapidus,had no affect on the orientation of larvae.Yet,juvenilesresponded to both odors by moving toward the visual target.Results support the hypothesis that the visual orientation of larvaland juvenile horseshoe crabs changes upon exposure to habitat and predator cues and that the direction of the response undergoesan ontogenetic shift following metamorphosis     

Starfish and horseshoe crab egg factors cause elevations of cyclic nucleotide concentrations in spermatozoa from starfish and horseshoe crabs.

Factors collected from the eggs of the starfish (Pisaster giganteus) and the horsehoe crab (Limulus polyphemus) caused significant increases in the sperm cyclic nucleotide concentrations of the respective species. Sea urchin egg factors, at concentrations that resulted in maximal cyclic nucleotide elevations in sea urchin spermatozoa, had no effect on those of starfish or horseshoe crab, suggesting a species specificity with respect to egg factor-induced changes in sperm cyclic nucleotide metabolism.     

Male horseshoe crabs Limulus polyphemus use multiple sensory cues to locate mates

<正> The use of multisensory cues to locate mates can increase an organism's success by acting as a back-up plan whenone system fails, by providing additional information to the receiver, and by increasing their ability to detect mates using sensesthat have different ranges in a variable aquatic environment. In this contribution we review the sensory cues that male horseshoecrabs Limulus polyphemus are known to use when locating mates and then provide new data that shed light on this subject. Duringthe breeding season, females migrate into shore during high tides to spawn. Males attach to females as they approach thebeach or are attracted to pairs already spawning. Vision is well established as an important cue in attracting males. Althoughchemoreception is well known in other marine arthropods, and horseshoe crabs have the anatomy available, there are few studieson chemical cues in this species. Experiments are presented here that provide evidence for chemical cue use. We show that the attraction,and retention, of attached and satellite males to actively spawning females and mating pairs involves multimodal cues     

An experimental approach to altering mating tactics in male horseshoe crabs (Limulus polyphemus)

Alternative reproductive tactics are often correlated with phenotype, density, environment, or social context. Male horseshoe crabs(Limulus polyphemus) have two mating tactics that are associatedwith phenotype. Males in good condition arrive at the nestingbeach and spawn while attached to females, whereas those inpoorer condition come ashore unattached and crowd around thenesting couples as satellites, fertilizing eggs through sperm competition. The correlation between mating tactic and phenotypemay be due to males choosing tactics based on condition, orit may be that males that have not found a female choose tocome ashore as satellites. To distinguish between these twopossibilities, I conducted an experiment on male horseshoe crabsin the field at Seahorse Key on the northern Gulf coast ofFlorida. I prevented males from attaching to females by placingsmall plastic bags over the claws they use to attach. The resultsshowed that males in poor condition came ashore as satellites,whereas males in good condition remained at sea. This meansthat mating tactics are cued by information about the male'scondition and not about whether he found a female. The evolutionof phenotype-correlated mating tactics can be represented bya model in which the fitness of each tactic changes with conditionand fitness curves cross. I hypothesize that male horseshoecrabs in good condition have higher fitness when attached and that males in poorer condition to better when unattached.     

Intraspecific shape variation in horseshoe crabs: The importance of sexual and natural selection for local adaptation

A morphometric analysis of the body shape of three species of horseshoe crabs was undertaken in order to infer the importance of natural and sexual selection. It was expected that natural selection would be most intense, leading to highest regional differentiation, in the American species Limulus polyphemus, which has the largest climatic differences between different populations. Local adaptation driven by sexual selection was expected in males but not females because horseshoe crab mating behaviour leads to competition between males, but not between females. Three hundred fifty-nine horseshoe crabs from nine populations, representing three species, were analyzed using a digitizer to position sixty morphometric landmarks in a three-dimensional space. Discriminant analysis revealed strong regional structuring within a species, which suggests strong philopatry, and showed the existence of geographically-based intraspecific variation. An admixture analysis showed regional intraspecific differentiation for males and females of L. polyphemus and males of the Asian horseshoe crab Carcinoscorpius rotundicauda, but not for females of C. rotundicauda and another Asian horseshoe crab, Tachypleus gigas. Differences in shape variation between sexes were tested with F-tests, which showed lower intrapopulation morphometric variation in males than females. These results indicate a lower degree of local adaptation on body shape in C. rotundicauda and T. gigas than in L. polyphemus and a lower degree of local adaptation in females than in males.     

Molecular phylogeny of extant horseshoe crabs (Xiphosura, Limulidae) indicates Paleogene diversification of Asian species

Horseshoe crabs are marine invertebrates well known for their exceptionally low rates of diversification during their entire evolutionary history. Despite the low species diversity in the group, the phylogenetic relationships among the extant species, especially among the three Asian species are still unresolved. Here we apply a new set of molecular genetic data in combination with a wide geographic sampling of the intra-specific diversity to reinvestigate the evolutionary history among the four living limulid xiphosurans. Our analysis of the intraspecific diversity reveals low levels of connectivity among Carcinoscorpius rotundicauda lineages, which can be explained by the estuarine-bound ecology of this species. Moreover, a clear genetic break across the Thai-Malay Peninsula suggests the presence of cryptic species in C. rotundicauda. The limulid phylogeny finds strong support for a monophyletic genus Tachypleus and a diversification of the three Asian species during the Paleogene period, with speciation events well separated in time by several million years. The tree topology suggests that the three Asian species originated in central South East Asia from a marine stem group that inhabited the shallow coastal waters between the Andaman Sea, Vietnam, and Borneo. In this region C. rotundicauda probably separated from the Tachypleus stem group by invading estuarine habitats, while Tachypleus tridentatus most likely migrated northeast along the Southern coast of China and towards Japan.     

Effects of chemical context on shell investigation behavior in hermit crabs

Specific chemicals in the environment evoke significant changes in the behavior of many aquatic organisms. We studied in the laboratory whether satiated individuals of the hermit crab, Pagurus longicarpus Say 1817, adjust their investigatory behavior towards an empty, optimal gastropod shell according to differences of chemical context. We also explored to what extent shell investigation by a crab in the same hunger state was affected by occupying an inadequately sized shell. Our results confirmed in part previous findings that crabs can discriminate the odor of freshly dead snails from the odor of freshly dead conspecifics. In the presence of the former odor, crabs inhabiting shells of inadequate size were more responsive and active than those in better-fitting shells. To the contrary, regardless of the quality of the inhabited shell, P. longicarpus remained practically motionless when presented with the odor of freshly dead conspecifics, possibly because the risks of incurring in predators would outweigh the benefits of acquiring a new shell. Unexpectedly, we found that crabs in both types of shell quality exhibited nearly the same behavior in control water, while crabs in adequate shells were more responsive in the presence of food odor. Individuals appeared insensitive to the odor of live snails; indeed, only one hermit crab species has been seen removing living snails from their shells. An intriguing result was that water conditioned by the odors of live conspecifics exerted a strong effect on all the individuals by inducing an intense shell investigation. Our study underlines the central role exerted by chemical detection in hermit crabs' behavior and demonstrates the existence of a complex interplay among chemical context, the physiological state of the animal, and the ecological pressures of the habitat.     

Seasonal movements of American horseshoe crabs Limulus polyphemus in the Great Bay Estuary,New Hampshire (USA)

<正> The goal of this study was to determine the year round movement patterns of American horseshoe crabs,Limuluspolyphemus,in the Great Bay Estuary,New Hampshire (USA) by using acoustic telemetry to track the movements of 37 adultLimulus,for periods ranging from 2 to 31 months.During the winter (December-March) horseshoe crabs moved very little.In thespring,when water temperatures exceeded 11℃,horseshoe crabs moved at least 1 km further up into the estuary to shallowersubtidal areas about a month prior to spawning.The mean distance traveled during spring migrations was 2.6 ± 0.5 (n=20) km upthe estuary.Mating occurred in May and June and during these months animals spent most of their time in shallow subtidal areasadjacent to mating beaches.In the summer (July-August),animals moved 1.5 ± 0.5 (n=26) km down the estuary,towards theocean,and ranged widely,using extensive portions of the estuary.In the fall (September-November) movement was more limited(0.5 ± 0.5 km;n = 24) while animals settled into wintering locations,where they remained until spring.The mean annual linearrange for all animals was 4.5 ± 0.3 km (n =35) and the maximum distance traveled by an individual horseshoe crab within oneyear was 9.2 km.There was no evidence that any of the horseshoe crabs tracked during this study left the estuary