Importance of patch size variation for the population persistence of a decapod crustacean in seagrass beds

To understand the effects of patch attributes of seagrass beds on the persistence of an animal population, we examined shifts in patch utilization that occur with the life stages of a decapod crustacean, Hokkai shrimp Pandalus latirostris, inhabiting only eelgrass beds. The abundance of juveniles did not relate to the abundance of prey or patch size, but adult abundance decreased significantly when patches were smaller. Edge effects were suggested for adults. Since patch size and shoot density were inseparable structures at our study site, we conducted an experiment using artificial seagrass units (ASUs) to clarify structures that were effective as shrimp habitat. This experiment showed that while adults recruited mainly to the small patches regardless of shoot density, juveniles, though much smaller in number, recruited to all patch types. Adults frequently emerged from natural patches to bare spaces at night, while juveniles seldom used the bare spaces at all. When we experimentally released shrimps between ASUs at night, adults generally used the bare spaces while juveniles randomly moved to all habitat types. We tethered adults in the interior of seagrass patches and bare spaces during both daytime and nighttime, and found that the predation rate was high only in bare spaces during daytime. We demonstrated that the habitat functions of seagrass patches can vary with the life stages of this decapod crustacean, and suggest that spatial management of various sizes of patches is crucial for population persistence of Hokkai shrimp.