About the Cover
Cover
Rock shrimp Rhynchocinetes typus inhabit shallow subtidal hard bottoms along the southeastern coast of the Pacific Ocean in Peru and Chile, where dense aggregations of shrimp can be found in cavities, cracks, and under boulder overhangs. Shrimp usually reproduce throughout the year, and adult females and males are equally abundant in natural populations. Females exist in only one morphological form, but males pass through distinct morphological stages as they molt and grow. Males first become sexually mature in a female-like "typus" stage, and then pass through several intermediate forms until the final molt, when they reach the "robustus" stage. Robustus males are characterized by their large, spear-like maxillipeds and strong chelipeds. Although relatively few robustus males occur in field populations, they gain most of the matings.
In this issue of The Biological Bulletin (p. 134) Eliecer R. Diaz and Martin Thiel report on the sensory cues that enable receptive females and dominant robustus males to find each other-a necessary preliminary to mating. Females prefer robustus males and they use chemical cues to find them. Nevertheless, they usually first encounter typus males because they are so much more abundant. The shrimp touch each other with their antennae, allowing the male to detect the receptive status of the female. Immediately thereafter he quickly and repeatedly transfers spermatophores to the female (shown in the small central inset). A receptive female surrounded by several subordinate typus males is shown in the upper left of the left-hand inset. Such agitated mating aggregations are visual cues, alerting nearby dominant robustus males that a receptive female is at hand. At the lower right of the left inset, a large robustus male-having seen the female-typus group-is approaching it and will quickly chase away the subordinate males. When the robustus male has secured the female, he will stand over her in the so-called cage state (right inset). The pair remains in this state throughout the mating, which may last up to three hours.
In the field, only a few female rock shrimp in a local aggregation become receptive at the same time. In contrast, most males are almost always ready to mate, so male-male competition for access to females is very strong. Because the dominant robustus males are physically stronger, they usually win these interactions and mate with most of the receptive females. In this paper, Diaz and Thiel show that females and robustus males use different cues to find their preferred mating partners, and that females play an active role in mate selection.
The photographs and cover design were prepared by Ivan Hinojosa with the aid of Erasmo Macaya; both are M.Sc. students at Universidad Catolica del Norte in Coquimbo, Chile. The final layout was by Beth Liles (Marine Biological Laboratory, Woods Hole, Massachusetts).
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Copyright © 2004 by the Marine Biological Laboratory.
