Endogenous rhythms of locomotion in the American horseshoe crab, Limulus polyphemus

Summary Discrete variation where one sex has more than one means of achieving the same functional end, such as mating or nesting (alternative strategies and tactics), requires an evolutionary explanation. Discrete differences in behavior, morphology, color, and life-history patterns are found in many contexts, such as alternative reproductive patterns, social insect castes, sex allocation, and mimicry polymorphisms. In this chapter I draw on the literature of these fields to identify similar patterns and common models that can be applied to our understanding of alternative phenotypes in general. I emphasize the invertebrate literature throughout because excellent reviews are available for vertebrates. Traditionally, the behavioral literature has separated alternative phenotypes into those due to genetic differences (e.g., polymorphisms) and those due to environmental or individual cues (e.g., conditional tactics). Such categories are impractical and they force us to dichotomize behavior as due to nature or nurture. Drawing on the well-developed sex-allocation literature, I suggest a different system for categorizing alternative phenotypes: irreversible or adult lifetime patterns (e.g., dioecy) and reversible or facultative patterns (e.g., hermaphroditism), which can be further subdivided into sequential and simultaneous. This new system for organizing alternative phenotypes allows us to see that condition dependence and frequency dependence may occur under any category. In general, alternative phenotypes can be maintained in a population when their fitness curves cross, that is, when each does better than the other under some conditions. A variety of factors can cause fitness curves to cross. Negative frequency dependence is one, which has the added advantage of explaining how patterns can be maintained at a stable frequency. Probably the most common explanation for the maintenance of discrete variants is switches in phenotype based on environmental or individual, condition-based cues. What are the circumstances that favor the evolution of irreversible phenotypes over switching? Lifetime patterns are favored when individuals that switch have lower fitness than those that do not. Lifetime patterns allow complex suites of adaptations to develop for each phenotype that are coordinated through physiological and developmental processes. When environmental conditions are predictable and correlate with fitness, selection favors the animal switching from one pattern to the other at the time that maximizes fitness. However, when the environmental conditions that favor two phenotypes are unpredictable and change frequently, selection favors animals that invest in what is necessary to perform in either role. Under most circumstances the animal will use decision rules based on information about what maximizes fitness, but if the individual does not have such information, yet must choose between exclusive alternatives, then selection will favor an arbitrary decision rule. A promising approach to studying alternative phenotypes is dynamic, state-variable modeling ( Houston and McNamara, 1999 ; Clark and Mangel, 2000 ), which is particularly good for evaluating the interacting effects of the organism's state, of environmental and social conditions, and of density and frequency dependence. When such multiple factors are evaluated together, surprising predictions about the conditions under which individuals should follow particular patterns sometimes result. An understanding of the evolution of alternative phenotypes requires a strong, integrative approach that seeks to understand the cues and mechanisms controlling behavioral decisions. When this approach is combined with dynamic, state-based models, it is possible to make predictions about the circumstances that favor the evolution of particular decision-making patterns over others.

The evolution of alternative strategies and tactics

The City of Quincy proposes the diversion of the existing Town Brook from a connecting point upstream of Quincy Center along the south side of the Concourse Roadway Improvement project, through 1,200 linear feet of closed and open channel culvert sections to the proposed connection point downstream of Quincy Center, where it will rejoin the existing Town Brook alignment. The Town Brook Enhancement Project is one of three Core Public Improvement projects the City of Quincy is undertaking to upgrade the aged infrastructure within Quincy Center and is the initial improvement project. Town Brook originates from freshwater wetlands in the Blue Hill Reservation and flows into the Old Quincy Reservoir in Braintree. It continues to flow through 1,700 linear feet of culvert section where it eventually meets the tidal Town River Bay. The Town Brook River system is a diadromous fish run supporting rainbow smelt (Osmerus mordax) and American eel (Anguilla rostrata). Town Brook was considered a relatively large rainbow smelt spawning run with high egg densities relative to the amount of available riffle habitat (approximately 800 m in length and 3,241 m 2 in area; Chase 2006). However, in the late 1980’s, the smelt run sustained acute impacts (egg mortality and spawning habitat loss). In addition, there are also chronic impacts to the smelt run (sedimentation, periphyton accumulation, reduced water depth and velocity) due to diminished base flow since the installation of flood control structures designed to reduce flooding in downtown Quincy in the late 1990s. The flood control projects caused habitat alteration and chronic decreases in water velocity and depth, reducing spawning attraction and egg survival. MarineFisheries, the City of Quincy and other interested parties have been coordinating efforts since this time to find ways to augment brook flows and reverse the degradation of this valuable fish run. The rainbow smelt are a pelagic, schooling species that spends most of its time in nearshore waters. The spawning season begins in early March and continues through May in MA. They deposit their eggs in small tributaries in swift, shallow riffles at night just upstream from the

/pdf/commonwealth-of-massachusetts-3qpv7flv16.pdf

Commonwealth of Massachusetts

Like a number of other species, horseshoe crabs lay their eggs on beaches in the intertidal zone The elevation of the beach on which they nest differs among populations We examined two factors that potentially affect egg survival at different beach elevations: erosion and rate of development We found no evidence that eggs buried at different elevations incur different risks of erosion by wave action However, the optimal beach elevation for egg development differed between our two study sites, Florida and Delaware, and the difference was related to beach morphology Rate of development increased with oxygen concentration, redox potential, and temperature, and all three of these factors changed with elevation The nests in the lower beach failed to develop, especially in Florida, where the beach was fine-grained and poorly drained The nests in the upper beach were prone to dessication, especially in Delaware, where the beach was course-grained and well-drained This means that differencces between sites in the optimal location for egg development coincided with horseshoe crab preferences in nestsite selection We suggest that horseshoe crabs synchronize their nesting with the tides that reach the aerobic sediments on the beach, resulting in nesting patterns that differ with differences in tidal regimes and beach morphology

Nest-Site Selection in the Horseshoe Crab, Limulus polyphemus.

Horseshoe crabs have an explosive breeding system not unlike that of some frogs and toads. They synchronize nesting to only a few hours each day at the time of the spring new and full-moon high tides. Males search for females as they come to the breeding beaches, grasp them with specially modified claws and cling to them, sometimes for weeks. Females lay several clutches of eggs in the sand and the male fertilizes them externally, the only extant arthropod with such a reproductive system. Unattached males cluster around the nesting couple, pushing on and occasionally displacing attached males. An experimental manipulation demonstrated that satellite males are capable of fertilizing eggs which suggests that sperm competition is the primary explanation for the presence of unattached males on the beach. Like other explosively breeding species, male Limulus search for females, often grabbing inappropriate objects, and satellite males compete for access to females. There is little assortative mating and attached and unattached males do not differ in size. In extreme explosively breeding species like Limulus, selection favors those males that are best able to locate and remain attached to females, and there is little opportunity for female choice or male-male competition.

Mating behavior of horseshoe crabs, limulus polyphemus

In the spring, horseshoe crabs (Limulus polyphemus) along the eastern coast of North America migrate toward shore to build nests close to the water's edge. In 1984 the mating season near Woods Hole, Massachusetts, extended from 14 May to 7 July. Mating activity during this period fluctuated with the phase of the moon, the height of the tide, and diurnal changes in daylight. As the moon approached new and full phases, large numbers of animals migrated into the intertidal zone to mate and build nests. They appeared 1-2 h before high tide, and returned to deep water about 2 h after high tide. No mating activity occurred during low tides. The two daily high tides in this region are unequal in height. The inequality is greatest during new and full moons. At these times most animals migrated toward shore on the higher tide, which occurred in the late afternoon and throughout the night. As the moon passed through quadrature the tidal inequality diminished and reversed. Mating activity changed accordingly: shore ...

Heidi Howard

Papers

Migration of limulus for mating: relation to lunar phase, tide height, and sunlight