| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Boston University Marine Program, Marine Biological Laboratory, Woods Hole, Massachusetts 02543
1 Brown University, Providence, RI 02912.
During late spring and early summer, spawning horseshoe crabs (Limulus polyphemus) can be readily observed along the beaches of Cape Cod, Massachusetts. Males use specially modified claws to clasp onto the carapace of females (1). This pairing is called amplexus. Amplexed pairs attract unattached males that cluster around them during spawning. These satellite males are frequently successful in fertilizing a portion of the female’s eggs (2). What determines which males amplex with females, while others do not, has not been clearly defined. Numerous studies have investigated factors thought to influence pairing in L. polyphemus, such as visual (3, 4) and chemical cues (5, 6) and body size. Studies regarding how body size may affect pairing have generated conflicting results—some research suggests size-based pairing is random (7, 8), while other work has found a weak relationship between body size and mate selection (2, 9).
In this study we examined the relationship between male and female body size and its influence on mate selection for amplexus in L. polyphemus in field and laboratory studies. To describe the reproductive categories within a horseshoe crab population during the spawning season, we carried out field surveys within a known spawning area along the eastern shore of Pleasant Bay, Cape Cod. Field surveys were conducted 15 May–15 June in 2001 and 25 June–25 July in 2002. The 2001 survey spanned the peak of the spawning season; the 2002 survey spanned only the latter part of the spawning season. We collected data regarding sex, spawning state, and size (prosomal width) from 56 pairs along 125 m of beach in 2001, 84 pairs throughout the bay in 2002, and an additional 761 crabs, including single males and females and satellite males.
To determine whether pairing was mediated by horseshoe crab size, we first compared sizes of paired crabs in the field; and second, we collected animals from the field for laboratory studies on mate selection. Experimental studies were conducted in 140 cm x 90 cm x 40 cm recirculating aquaria, containing 4 cm of native beach sand. Trials were performed by pairing a single male (155–215 mm prosomal width) with 2, 3, 4, or 7 females (190–282 mm). Each male was used in three trials, and trials were allowed to run for 12 h or until amplexus was achieved. We examined the effect of number of individuals in these trials and found that density did not affect success of pairing. Similarly, there was no difference between the trials in which males were used more than once and those in which males were used once. Hence, all data were pooled in Fig. 1C.
|
To ascertain whether there was size-based mate selection in amplexed pairs in the field, we compared the prosomal widths of paired males and females (Fig. 1B) measured in the field surveys done in the two years. There was no significant difference in size of paired males and females sampled in 2001 and 2002. This finding implies that there were also no differences in pairs observed during the peak of spawning compared to those sampled post-peak (Males: t = 1.17, P = 0.24; Females: t = -0.67, P = 0.50). Hence, data were combined in Fig. 1B. The aggregated data indicated no significant size-based selective mating across the full range of sizes observed (R2 = 0.01, df = 83, P > 0.05). Pairings did not, however, occur between the smallest [Fig. 1A, B (area to the lower left of ---------)] or largest [Fig. 1A, B (area to the lower right of · · · · · ·)] males and females in the general field population. These results suggest that some of the smallest animals may not have reached sexual maturity and that some size selectivity may occur among larger males. In addition, size reversals among pairs (females having a smaller prosomal width than their male partner) were rarely observed (2%).
To further test the effect of size on mate selection, we compared the size of males to the size of the females chosen for amplexus during laboratory trials (Fig. 1C). In 35 trials, small males (154–170 mm) tended to select the smallest females offered (regardless of the females’ absolute size). Intermediate-sized males (171–190 mm) tended to prefer the smallest or intermediate-sized females, and the only large male (215 mm) to accomplish amplexus chose the largest female offered. Males did not amplex with the first female they encountered and took at a minimum 35 min to pair. Overall, small and medium-sized males were more successful in achieving amplexus than large males (Fig. 1C). These results suggest that some size-based mate selection may occur at the largest and smallest sizes only, as observed in the field, and that male size may also affect success of amplexus.
To further examine whether male size affects the likelihood of amplexus in the field, we compared the success of males of different sizes tested in the laboratory to those observed in the field. The percent of amplexus as a function of size was similar for both field and laboratory animals (Fig. 1D), where smaller male crabs (up to 180 mm) were generally more successful than larger crabs.
In sum, for the majority of male and female prosomal widths, size did not appear to affect mate selection. At the largest sizes, however, males tended to prefer larger-sized females and were generally less successful in achieving amplexus. Hence, our results are consistent with previous reports that selection is not size-based (7, 8) and also with reports that some size-based selection may occur (2, 9). Size selectivity and apparent random pairing can both occur, but do so within different size ranges.
This research was supported by NSF Research Experience for Undergraduates (OCE-0097498) and by the Friends of Pleasant Bay. Special thanks to Emily Gaines for her assistance in the field and to Jason Norberg for his technical assistance.
Literature Cited
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
