Mating Tactics of the American Horseshoe Crab (Limulus polyphemus)

Abstract

The reproductive behavior of horseshoe crabs (Limulus polyphemus) is easily observed, yet until recently, little was known about their unusual mating habits. Fertilization is external and occurs beneath the female as her eggs are being laid in the sand. Spawning is often synchronized to the highest spring tides of the year, so the time available for breeding is limited. Males have alternative mating tactics, some search and pair with a female offshore and migrate inshore to spawn, while others search onshore for nesting pairs and engage in sperm competition in group spawning. Females also have alternative tactics such that some spawn with a single attached male and others spawn with multiple males, which results in multiple male paternity of their offspring. Both male and female tactics are condition and context-dependent and are affected by breeding density, operational sex ratio, and the interplay between male and female tactics. To better understand horseshoe crab reproductive behavior, we review studies conducted during the past 25 years in one Gulf of Mexico population in northern Florida at Seahorse Key. We discuss the costs, benefits, and tradeoffs of alternative tactics for males and females. We synthesize the recent literature on mating tactics, resolve some conflicting results, and point to the future by identifying the questions that remain.