Evidence for use of chemical cues by male horseshoe crabs when locating nesting females (Limulus polyphemus)

Chemically mediated rheotaxis of endangered tri-spine horseshoe crab: potential dispersing mechanism to vegetated nursery habitats along the coast

Background

An enhanced understanding of larval ecology is fundamental to improve the management of locally depleted horseshoe crab populations in Asia. Recent studies in the northern Beibu Gulf, China demonstrated that nesting sites of Asian horseshoe crabs are typically close to their nursery beaches with high-density juveniles distributed around mangrove, seagrass and other structured habitats.

Methods

A laboratory Y-maze chamber was used to test whether the dispersal of early-stage juvenile tri-spine horseshoe crab Tachypleus tridentatus is facilitated by chemical cues to approach suitable nursery habitats. The juvenile orientation to either side of the chamber containing controlled seawater or another with various vegetation cues, as well as their movement time, the largest distance and displacement were recorded.

Results

The juveniles preferred to orient toward seagrass Halophila beccarii cues when the concentration reached 0.5 g l^-1, but ceased at 2 g l^-1. The results can be interpreted as a shelter-seeking process to get closer to the preferred settlement habitats. However, the juveniles exhibited avoidance behaviors in the presence of mangrove Avicennia marina and invasive saltmarsh cordgrass Spartina alterniflora at 2 g l^-1. The juveniles also spent less time moving in the presence of the A. marina cue, as well as reduced displacement in water containing the S. alterniflora cue at 1 and 2 g l^-1. These results may explain the absence of juvenile T. tridentatus within densely vegetated areas, which have generally higher organic matter and hydrogen sulfide.

Conclusion

Early-stage juvenile T. tridentatus are capable of detecting and responding to habitat chemical cues, which can help guide them to high-quality settlement habitats. Preserving and restoring seagrass beds in the intertidal areas should be prioritized when formulating habitat conservation and management initiatives for the declining horseshoe crab populations.

Issues in the Classification of Multimodal Communication Signals

Communication involves complex behavior in multiple sensory channels, or "modalities." We provide an overview of multimodal communication and its costs and benefits, place examples of signals and displays from an array of taxa, sensory systems, and functions into our signal classification system, and consider issues surrounding the categorization of multimodal signals. The broadest level of classification is between signals with redundant and nonredundant components, with finer distinctions in each category. We recommend that researchers gather information on responses to each component of a multimodal signal as well as the response to the signal as a whole. We discuss the choice of categories, whether to categorize signals on the basis of the signal or the response, and how to classify signals if data are missing. The choice of behavioral assay may influence the outcome, as may the context of the communicative event. We also consider similarities and differences between multimodal and unimodal composite signals and signals that are sequentially, rather than simultaneously, multimodal.

Chemical and visual communication during mate searching in rock shrimp

Mate searching in crustaceans depends on different communicational cues, of which chemical and visual cues are most important. Herein we examined the role of chemical and visual communication during mate searching and assessment in the rock shrimp Rhynchocinetes typus. Adult male rock shrimp experience major ontogenetic changes. The terminal molt stages (named "robustus") are dominant and capable of monopolizing females during the mating process. Previous studies had shown that most females preferably mate with robustus males, but how these dominant males and receptive females find each other is uncertain, and is the question we examined herein. In a Y-maze designed to test for the importance of waterborne chemical cues, we observed that females approached the robustus male significantly more often than the typus male. Robustus males, however, were unable to locate receptive females via chemical signals. Using an experimental set-up that allowed testing for the importance of visual cues, we demonstrated that receptive females do not use visual cues to select robustus males, but robustus males use visual cues to find receptive females. Visual cues used by the robustus males were the tumults created by agitated aggregations of subordinate typus males around the receptive females. These results indicate a strong link between sexual communication and the mating system of rock shrimp in which dominant males monopolize receptive females. We found that females and males use different (sex-specific) communicational cues during mate searching and assessment, and that the sexual communication of rock shrimp is similar to that of the American lobster, where females are first attracted to the dominant males by chemical cues emitted by these males. A brief comparison between these two species shows that female behaviors during sexual communication contribute strongly to the outcome of mate searching and assessment.

Multimodal communication in crayfish: sex recognition during mate search by male Austropotamobius pallipes

The role of multiple signals in sex recognition in the crayfish Austropotamobius pallipes was investigated by analyzing the main channels (chemical or chemical plus visual) used by males during mate search. The behavioral responses of males were recorded when they were exposed to four different treatments: female odor only, male odor only, female odor plus visual cues, and male odor plus visual cues. Males did not respond to chemical cues alone emitted by a female, but required both chemical and visual stimuli. The high-flow conditions of the aquatic habitat occupied, the restricted mating period, and the low density of the population examined may explain the use of both cues in sex recognition by males.