Costs of alternative mating tactics
We have uncovered a novel cost imposed by one of the two alternative reproductive tactics of male horseshoe crabs. Younger males in good condition find a female and attach to her out at sea, then come ashore to spawn. In contrast, older males in poor condition come ashore alone, join mating pairs, and engage in sperm competition with other males. When in the attached position males get more paternity, however we discovered that these attached males have a restricted ability to feed and that this causes a period of nutritional stress during the breeding season. Unattached males might father fewer of the offspring, but they can eat during the breeding season. So, it looks like younger males are willing and able to forego eating in favor of mating, yet older males may not be able to pay this cost. .pdf
Matthew Denman Smith, Ph.D. 
How long is the male attached to the female before the egg-laying and sperm release is finished? How long would he be deprived of food? Given the size of his mouth, I did not expect him to be able to eat seagrass. Is that seaweed? I thought he would ingest zooplankton and phylo or phytoplankton in water. Most invertebrates do well out of water if they are kept damp. I am puzzled by HSCs being nutritionally deprived during perhaps at most 12 hours.
This is a good question Jo. In Florida they attach to a female for at least the 5-7 day spawning cycle of a female, then detach and seek another female. So, in the course of a year they’re likely to be attached and not eating for half of the 3 month breeding season, and perhaps longer. In other populations they attach to females for longer periods, for example in New Hampshire they have been found overwintering in amplexus. – Matt