Prosomal Width Research Articles

Fecundity and spawning of the Atlantic horseshoe crab,Limulus polyphemus, in Pleasant Bay, Cape Cod, Massachusetts, USA

AbstractThis study provided the first comprehensive analysis of Atlantic horseshoe crab (Limulus polyphemus) fecundity.Limulusappear to be a determinate spawner, maturing all eggs for the breeding season before spawning begins. On average, larger females held a larger number of eggs (63,500) than smaller females (14,500). By the end of the breeding season there was an average of 11,600 mature eggs per female left undeposited, regardless of female size. Larger females laid a higher percentage of the eggs they contained. Thus they not only contain more eggs, but are more effective at laying them as well. Size of spawning females ranged from about 185–300 mm prosomal width, with by far the highest concentration in the mid‐size ranges. Although on an individual basis large females carry and lay the greatest number of eggs, mid‐size crabs as a group contributed more to the horseshoe crab population in Pleasant Bay because they were more plentiful (net fecundity was highest for mid‐size crabs). These results have implications for the management of this important species, which is harvested for bait, scientific, and biomedical uses. Incorporation of these results into models and other management tools can help predict growth rates, effects of size‐selective harvest, reproductive value, and stable stage distribution of populations.

Estimation of total hemolymph volume in the horseshoe crab Limulus polyphemus

Biomedical companies extract blood from the horseshoe crab, Limulus polyphemus, for the production of Limulus Amebocyte Lysate, used worldwide for detecting endotoxins in injectable solutions and medical devices. Despite the extensive use of horseshoe crabs by the biomedical industry, total hemolymph volume for this species is not known. The hemolymph volume of 60 adult horseshoe crabs was estimated using an inulin dilution technique. Blood volume of the horseshoe crab represented as a percentage of wet body weight was 25 ± 2.2% for males and 25 ± 5.1% (mean ± SD) for females. Relationships between hemolymph volume and weight (p = 0.0026, r 2 = 0.8762), hemolymph volume and prosomal width (p < 0.0001), and hemolymph volume and inter-ocular width (p < 0.0001) were observed. No significant differences were observed between males and females. The relationship of animal size and hemolymph volume can be used to predict how much blood can be drawn from horseshoe crabs used by the biomedical industry, and can be of further use in future bleeding mortality studies.

Experimentally derived estimates of growth by juvenile Tachypleus tridentatus and Carcinoscorpius rotundicauda (Xiphosura) from nursery beaches in Hong Kong

Juvenile Tachypleus tridentatus and Carcinoscorpius rotundicauda with prosomal widths of between 17.1 and 91.1 mm were obtained from their nursery beaches in Hong Kong. They were kept in the laboratory and fed a mixture of squid, prawn and fish. Prosomal width and wet weight were measured weekly to obtain growth data. Over half of the individuals molted during a five and a half month captivity period. After every ecdysis, prosomal width and wet weight of T. tridentatus grew by averages of 24.2% and 71.5% over pre-molt measurements, respectively. Similar values were obtained for C. rotundicauda, i.e. 24.0% and 77.3%, respectively. Three T. tridentatus with prosomal widths of between 26.5 and 35.0 mm molted twice between 89 and 149 days, leading to an average growth rate of 0.1 mm·day −1 and 0.04 g·day −1 in terms of prosomal width and wet weight, respectively. A positive growth allometry ( b coefficient=2.97) was identified, indicating that weight gain for T. tridentatus, and possibly C. rotundicauda, was faster than prosomal width growth after each ecdysis. The effect of temperature on growth was also determined by comparing the percentages of T. tridentatus which molted at ∼28 °C and ∼18 °C. Fifty percent of individuals molted at the former, but only 10% at the latter. This study indicates that Hong Kong horseshoe crabs take a shorter time to reach sexual maturity, as compared with conspecifics in Japan, because they can molt more frequently at higher sediment/water temperatures (∼28 °C) if food is available.

The diets of juvenile horseshoe crabs, Tachypleus tridentatus and Carcinoscorpius rotundicauda (Xiphosura), from nursery beaches proposed for conservation in Hong Kong

Once common as a fishery resource, adult horseshoe crabs of two species occur in Hong Kong. A third, Tachypleus gigas, can no longer be found. Once common too were breeding and nursery beaches for horseshoe crabs in Hong Kong but overfishing of adults, pollution and coastal reclamation have reduced these to but three identified sandy mudflats and only one where juvenile Tachypleus tridentatus and Carcinoscorpius rotundicauda are sympatric. Nothing is known, however, of the conservation requirements of these two species locally and this study of the diets of these juvenile horseshoe crabs on two such nursery beaches aimed at providing information on this most elementary aspect of their biology. Gut contents of juvenile horseshoe crabs, i.e. nine Tachypleus tridentatus (8.5–67 mm prosoma width) and two Carcinoscorpius rotundicauda (50 and 52 mm prosoma width), were analysed and compared with the composition of the ambient assemblage of meiobenthos on a nursery beach at Pak Nai, Hong Kong. The obtained data suggest that juvenile horseshoe crabs of both species are selective benthic feeders and subsist mainly on insect larvae, polychaetes, oligochaetes, small crabs and thin-shelled bivalves. A strong, positive, preference for insect larvae (Chironomous sp.: Diptera: Chironomidae) was recorded, but with no preference for meiofauna over macrofauna.

Visual behavior of juvenile Limulus in their natural habitat and in captivity.

Limulus polyphemus, the common horseshoe crab, inhabits coastal estuaries of North America and the Yucatan peninsula. It has been an admirable model for vision research because its relatively simple visual system has marked functional similarities to those of many species, including humans (1). Adult male horseshoe crabs use their lateral eyes to find mates (2), and their ability to see underwater has been studied in the estuaries where mating occurs (3, 4, 5). However, studies of the visually guided behavior of adults in captivity have not been successful (R.B., pers. obs.). On the other hand, juvenile horseshoe crabs do exhibit visually guided behavior under laboratory conditions (6). Here we report a comprehensive investigation into the visually guided behavior of juveniles, both in their natural habitat and in captivity. Between May 15 and June 30, 2002, we studied the behavior of juvenile specimens of Limulus in Nantucket Sound, along the Western shore of the North Monomoy Island Refuge in Chatham, Massachusetts. The sand bars on the tidal flats of the island are a protected natural habitat for both juveniles and adults; the waters are relatively clear and calm. Juveniles were most active near shore at depths ranging from 30–120 cm, usually during the late afternoon and evening. In testing the visually guided behavior of a juvenile horseshoe crab, we followed the procedure of a previous study of such behavior in adults (4). First we determined its direction of travel along the bottom and then placed a small object directly in front of it at a distance of 20–40 cm. We used three cylindrical acrylic plastic objects (diameter 7.6 cm, height 5.2 cm): one was painted matte black, one matte gray, and one remained unpainted. Because these objects were designed for experiments with adults, their size was reduced proportionally for small juveniles (prosomal width range 2–14 cm). We filmed 713 trials, defined as an approach of an animal to an object, on a Sony camcorder (model DCR-PC110 NTSC; underwater housing by Gates Underwater Products) and then analyzed the behavior with a computer. All videotaped trials were analyzed with an eMac computer using iMovie software (copyright 2001–2002 Apple Corporation). We categorized each trial as representing one of three behaviors: “hit,” an animal moves directly toward an object and makes direct contact with it; “pass-by,” an animal passes by an object without changing direction; and “turn-away,” an animal turns away from an object. For turn-away behavior, we measured the distance from the center of the object to the middle of the animal at the point where it turned. For the pass-by behavior, we measured the distance of closest approach to the object. Figure 1 (top) shows percentages for all three behaviors. We performed a similar set of experiments in the laboratory, where we observed the behavior of 10 juveniles in a large trough (130 62 cm; the water 6.5 cm deep). The bottom of the trough was covered with a thin fiberglass mesh screen, giving the crabs traction for walking and righting themselves if overturned. The crabs were maintained in ambient diurnal lighting in a separate trough before being placed in the experimental trough, where we used diffuse overhead illumination to simulate, as closely as possible, an overcast day. The same three objects used in the ocean observations were now placed in the center of the trough and rotated every 15 min. The crab’s behavior was filmed and analyzed, as in the ocean trials. We defined a behavioral event as a crab entering the camera’s field of view (51 74 cm). We filmed 797 trials, mostly in the evening hours when the crabs were most active. Figure 1 (bottom) gives percentages for the laboratory trials.

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

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.

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.

Preliminary study of the shape of eurypterid prosomas using Fourier analysis

Shape identification of eurypterid prosomas currently uses the ratio of prosoma length to prosoma width and the lateral angle. Because many species of eurypterids have the same prosoma shape as determined by these criteria, the method is limited in its taxonomic applications. Thompson transformation grids are of little value due to the lack of sufficient topographic reference points, even though some limited success has been achieved in its application to allometry.Fourier analysis is a more rigorous mathematical descriptor of two-dimensional shape than is simple ratios. Being independent of topographic reference points and ratios, Fourier analysis is well suited, theoretically, to the identification of prosoma shape and potential taxonomic identifications. Problems arise, though, in that Fourier analysis is sensitive to distorted specimens.My study departs from previous methods of Fourier analysis in that the choice of the reference point from which to measure the radii is not the center of gravity, but instead is the midpoint between the eyes. My reasons for this unorthodox reference point are threefold: 1) this point is determinable on all specimens; 2) the position of the eyes is taxonomically significant (Kjellesvig-Waering, 1961) and 3) allows for the simultaneous identification of prosoma shape and the differentiation of taxa with similar outlines but different eye positions.As defined by current methods, the taxa used in this study (Eurypterus remipes, E. lacustris, E. pittsfordensis, and E. dekayi) all have similar prosoma shapes. When specimens of the same stage of development were compared statistically, Fourier analysis was successful in the differentiation between E. remipes, E. pittsfordensis and E. dekayi, and between E. lacustris, E. pittsfordensis and E. dekayi, but not between E. remipes and E. lacustris.The prosoma shapes as studied by Fourier analysis for both E. remipes and E. lacustris are also similar for all size classes examined. All size classes are dominated by the 4th harmonic. The contribution to the outline from other harmonics changes during ontogeny. The 3rd and 7th harmonics are the important components of the prosoma outline for the small size classes (classes 1-3 = maximum prosoma length of 17mm), while the 2nd and 7th harmonics are the important components for the larger size classes (classes 4-7). Further study involving other eurypterid taxa is in progress.

OSMOTIC AND IONIC REGULATION IN THE HORSESHOE CRABLIMULUS POLYPHEMUS(LINNAEUS)

1. Osmotic and ionic regulation of Limulus has been studied over a wide range of salinities. The blood is isosmotic in sea water of 100-200% (950-2084 milliosmoles) within 1.5%. It is hyperosmotic in dilute sea water of 10-66%. After a period of acclimatization in waters of 50-10%, some small horseshoe crabs (prosomal width about 7 cm) remained healthy in 5% sea water (48 milliosmoles. about 1.7‰ salinity) for 2-5 days, the blood in 4 specimens being kept at a level of 229-335 milliosmoles.2. Analyses of blood-serum as originally obtained and after dialysis against sea water give the following measure of ionic regulation: Na 101%, K 119%, Ca 91%, Mg 82%, Cl 100% and SO4 63%.3. Weight changes in waters of different salinity have been studied and the final water content of the muscles. The latter ranges from 67% in specimens from 200% sea water to 83% in specimens from 10%.4. From experiments with injected inulin solutions, the average extracellular volume in crabs of 100-634 g weight is 52% of the body weight.5. Estimates of extracellular water in the muscles (22.8% of total muscle water) have been used for calculations of intracellular concentrations of inorganic ions, phosphates and various non-protein nitrogenous compounds. Two-thirds of the osmotic concentration of the muscle cell is made up by the latter, of which free amino acids constitute only a small fraction.6. In whole muscle of specimens kept in 20% and 10% sea water the absolute amounts of ions and molecules fall in conformity with blood concentration, the ions increasing as a proportion of the serum concentration from 47% in 100% sea water to 58% and 63%, respectively. In 200% sea water ions in muscle as a proportion of those in serum also rise, to 62%.

Epibiont Infestation on Horseshoe Crab Tachypleus gigas (Müller) at Pantai Balok in Peninsular Malaysia

A one year study was conducted at Pantai Balok in the east coast of Peninsular Malaysia to investigate the epibiont infestation on horseshoe crab Tachypleus gigas. Number of horseshoe crabs landed was enumerated and each prosoma size (prosoma length and width) was measured. Prosoma width of the horseshoe crabs was used as a morphometric proxy to estimate the approximate age and instar stage of the individual crabs. All horseshoe crabs examined were adult individuals. The carapaces of horseshoe crabs were carefully examined for epibiont infestation. The epibiont was identified to genus level. In this study, acorn barnacle Balanus, pedunculate barnacle Octolasmis, conical slipper shells Calyptraea and flat slipper shells Crepidula were found on the carapaces of horseshoe crab Tachypleus gigas collected from Pantai Balok. DOI: http://dx.doi.org/10.3126/on.v9i1.5725