Robert E. Loveland

Beach erosion and geochemical factors: Influence on spawning success of horseshoe crabs (Limulus polyphemus) in Delaware Bay

Horseshoe crab spawning activity is spatially patchy within the Delaware Estuary. This study investigated the importance of geochemical and erosional factors to the selection of breeding beaches. Two sandy beaches in Cape May county, New Jersey, USA, were studied; one beach had been subjected to considerable erosion, exposing underlying peat; the second beach, less than 1 km away, had only traces of peat. Reduced sediments with high levels of hydrogen sulfide were correlated with the presence of peat, and significantly fewer crabs utilized sediments in the proximity of peat beds for reproduction. The lower spawning activity on the beach in the vicinity of exposed peat, suggests that crabs may detect, at a distance, the nature of sediments and the quality of beach for spawning activity. Active salt marsh and peat-bank sediments dominate the upper bay shore; these sediments are unsuitable, or at best marginal, for horseshoe crab reproduction. Extensive bulkheading of eroding sandy beach along several New Jersey shore communities has further restricted the availability of suitable spawning habitat, making the remaining stretches of optimal sandy beach critical to the reproductive success of this species.

Reproductive risk: high mortality associated with spawning by horseshoe crabs (Limulus polyphemus) in Delaware Bay, USA

It has been presumed that intertidal spawning by Limulus polyphemus minimizes the loss of egges to subtidal predators; however, this strategy involves considerable risks. Massive beach strandings of adults accompany seasonal spawning migrations of crabs along Cape May in Delaware Bay, (USA). At least 190000 horseshoe crabs, approximating 10% of the adult population, died from beach stranding along the New Jersey shore of Delaware Bay during the 1986 (May to June) spawning season. Abnormalities of the telson (which is used in righting behavior) were significantly more common among stranded crabs than among individuals actively spawning on the intertidal beach. The number of stranded crabs per day was not correlated with tidal height or environmental variables (wind speed, wave height) which characterized the conditions at spawning. A complex suite of factors, including the size of the available spawning population, tidal and weather conditions, and beach slope, influence the number stranded during the breeding season. Horseshoe crab stranding results in a large loss of gravid females from the population, and may represent a major input of organic matter to intertidal sandy beaches in certain regions of Delaware Bay.

Abundance and dispersal potential of horseshoe crab (Limulus polyphemus) larvae in the Delaware estuary

The distribution, abundance, and dispersal patterns of horseshoe crab (Limulus polyphemus) trilobite larvae were determined from 671 plankton tows taken near a spawning beach in lower Delaware Bay, New Jersey, in 1998 and 1999. In both years, peaks in larval abundance occurred during periods of rough surf (>30 cm wave heights). Planktonic larvae were significantly more abundant nocturnally than during the day, but there was no evidence of a lunar component to larval abundance. Larvae were strongly concentrated inshore; trilobites were 10–100 times more abundant in the immediate vicinity of the shoreline than they were 100–200 m offshore. The strong tendency ofLimulus larvae to remain close to the beach suggests that their capability for long-range dispersal between estuaries is extremely limited. We suggest that limited larval dispersal potential may help explain previously observed patterns of genetic variation among the Mid-Atlantic horseshoe crab populations.

Size dimorphism and the mating system in horseshoe crabs Limulus polyphemis L.

Abstract This study asks whether male-male competition and female choice are important in the mating system of sexually dimorphic, adult horseshoe crabs (Chelicerata: Limulidae, Limulus polyphemus L.). Amplexed pairs approaching a beach for spawning are generally accompanied by a number of unattached, sexually mature suitor males. For the amplexed pair, the average pairwise ratio of male to female carapace width remains constant at 0·78 throughout the spawning season. Morphological characters that relate to mating in amplexed males were compared with unpaired males. Mated and suitor male crabs could not be distinguished on the basis of morphology or body size. Male clasper dimensions, relative to the point of amplexus on the female opisthosoma, suggest that there are no size-related constraints on the point of amplexus on the female opisthosoma, suggest that there are no size-related constraints on the ability of males to amplex with females. Although multiple males participate in the mating process, via the formation of spawning clusters of up to 15 males to one female, there is no evidence of successful male-male displacement. Size-assortative mating does not occur, and large females do not attract either larger males or larger numbers of suitor males in a cluster. A simulation model, using data from 1800 individual horseshoe crabs, generated mated pairs based on the assumption that amplexus was a completely random process with respect to prosoma width. The frequency distribution of actual mated pairs was indistinguishable from the output of the simulation model. The role of size dimorphism and the male-biased sex ratio in the mating system of horseshoe crabs are discussed.

Horseshoe crabs (Limulus polyphemus) in an urban estuary (Jamaica Bay, New York) and the potential for ecological restoration

We assessed the suitability of intertidal habitats for spawning by horseshoe crabs (Limulus polyphemus) at 12 proposed restoration sites identified by the United States Army Corps of Engineers along the shore of Jamaica Bay, a highly developed estuary in New York City. Based on beach geomorphology, we chose to quantify horseshoe crab activity at five of the sites during the May–July 2000 breeding season. Horseshoe crabs spawned intensively on small patches of suitable sand within larger areas of eroding shoreline with bulkheads and rubble fill. Small areas of sand behind grounded barges at Brant Point and Dubos Point had densities of over 100,000 eggs m−2, which was equal to or greater than the egg densities on longer, more natural appearing beaches at Spring Creek and Dead Horse Bay, or at a sand spit at Bayswater State Park. There were no significant differences in the percentage of Jamaica Bay horseshoe crab eggs that completed development when cultured using water from Jamaica Bay or lower Delaware Bay, a less polluted location. Only 1% of the embryos from Jamaica Bay exhibited developmental anomalies, a frequency comparable to a previously studied population from Delaware Bay. We suggest that the distribution and abundance of horseshoe crabs at our study areas in Jamaica Bay is presently limited by the availability of suitable shoreline for breeding, rather than by water quality. Restoration efforts that increase the amount of sandy beach in this urban estuary have a good likelihood of benefiting horseshoe crabs and providing additional value to migrating shorebirds that use horseshoe crab eggs as food.

Eye and clasper damage influence male mating tactics in the horseshoe crab, Limulus polyphemus

In the horseshoe crab mating system, mated pairs are frequently accompanied by unattached satellite males as they spawn on intertidal beaches. Previous studies have shown that males locate females visually using their lateral (compound) eyes, and that attached (mated) males generally have less heavily worn or damaged carapaces than unattached males. The purpose of this study was to investigate the influences of lateral eye condition and clasper abnormalities on male mating tactics. Sexually mature males had two kinds of eye damage: deterioration caused by disease, and overgrowth by sessile invertebrates, such as bryozoans, mussels, and tube-building polychaetes. The lateral eyes of attached males had significantly less decay than unattached males. On the other hand, coverage of the lateral eyes by encrusting invertebrates was more extensive among attached than unattached males. Although overgrowth did not appear to impair a male’s ability to pair with a female as severely as eye decay, it is conceivable that amplexus may have occurred before epibiont coverage was sufficient to obscure vision. Male crabs that were experimentally “blindfolded” by painting their lateral eyes with black nail polish were less likely to reattach to a female than controls. Appendage injuries were more frequent among unattached males than among attached males; in particular, 6.4% of unattached males but 0.0% of attached males had damaged claspers (the modified first legs required for amplexus). Unattached males in the population were “older,” as judged by the degree of carapace wear, than attached males. Severe visual impairment and/or clasper damage may explain the reduced pairing success of older male horseshoe crabs, and underlie their choice of the alternative satellite male mating tactics.

ORIENTATION OF THE HORSESHOE CRAB, LIMULUS POLYPHEMUS, ON A SANDY BEACH

Adult horseshoe crabs (Limulus polyphemus) spawn on sandy intertidal beaches and then return toward the water. Field experiments demonstrated that beach slope was more significant than vision in this orientation behavior. Both blinded and nor mally sighted crabs showed rapid seaward orientation on beaches with a seaward slope of approximately 6°. Orientation performance was poor on a flat beach, al though sighted crabs slightly out-performed blinded crabs. The observed orientation behavior was correlated with the large numbers of horseshoe crabs which failed to return to the water after spawning on sand bars or similar habitats lacking a slope gradient.

Body size, morphological constraints, and mated pair formation in four populations of horseshoe crabs (Limulus polyphemus) along a geographic cline

The relationship of body size to mating success was studied in four populations of horseshoe crabs, Limulus polyphemus L., along the east coast of the U.S.A. in the spring and summer from 1986 to 1989. Crabs of both sexes from Great Bay, New Hampshire, were significantly smaller than crabs from three middle Atlantic coast populations: Sandy Hook Bay and Delaware Bay, New Jersey, and Chincoteague Bay, Virginia. The formation of mated pairs was independent of body size in each population; there were no significant size differences between mated and single individuals, and size assortative mating did not occur. A comparision of male clasper dimensions with the corresponding point of attachment on the female indicates that there are no morphological constraints limiting amplexus between any male and any female within a population. The ratios of male to female prosoma width within amplexed mated pairs averaged from 0.78 to 0.80 in each population, despite the large difference in absolute size between southern and nothern populations. This may suggest a role for natural selection in regulating the relative sizes of each sex.

A calibration method to generate seasonal hatching profiles for the fiddler crabs Uca pugnax (Smith, 1870) and Uca minax (LeConte, 1855) (Decapoda: Brachyura)

Abstract A statistical method for generating seasonal egg hatching profiles is applied to the brachyuran crabs Uca pugnax (Smith) and U. minax (LeConte) in New Jersey. Calibration experiments quantified the time course of egg development, using serial egg sampling at 1- to 2- day intervals from marked ovigerous females maintained in cages in the field. Egg stage was ranked from 1 to 10, based on morphological changes during development. Equations for predicting the number of days remaining until hatching from egg developmental stage were obtained from the calibration experiments, using stepwise polynomial regression. To cover the reproductive season, three consecutive calibration experiments using 15 or more females were run for U. pugnax ; two for U. minax . Significant seasonal differences in the time course of egg development were detected. Weekly collections of females for each species were made; the date of larval release for each ovigerous female was predicted from the proximate calibration equation, yielding weekly hatching profiles. Weekly hatching profiles were summed to obtain seasonal hatching profiles. The average number of broods produced per female over the reproductive season was 1.9 for U. pugnax and 1.6 for U. minax . Hatching peaks for both species were associated with spring tides. The merits of this and other methods used to estimate daily variation in egg hatching of crabs are discussed.

Oxygen consumption and water movement in Mercenaria mercenaria

Abstract 1. 1. A method of relating oxygen consumption in Mercenaria mercenaria to pumping rates at 25°C and 22% is described. 2. 2. For specimens of Mercenaria from 5 to 285 g, the relationship between oxygen consumption and total wet weight was predicted by the regression equation: log Q O 2 = −0·344 log weight −1·023. 3. 3. The regression equation predicting the relationship between pumping rate and weight was: log pumping rate = 0·656 log weight−0·569. 4. 4. The efficiency of withdrawal of oxygen was constant for animals of all weights; this value was statistically 2·91 l. of water pumped for each 1·0 ml of oxygen removed.