Factors affecting the occurrence of early maturing males in the protandrous pandalid shrimp Pandalus latirostris

Some Age-0 males of the protandrous pandalid shrimp Pandalus latirostris Ra thbun mature in their first year, and the proportion of these early maturin g males (EMMs) in a population varies both locally and annually. Two labora tory experiments and 2 field observations were conducted to clarify the fac tors affecting the occurrence of EMMs. The first experiment showed that EMM s needed to grow fast until the breeding season and began producing sperm a t about 14 mm carapace length. Individuals with an AMP (length of the endop od/length of the appendix masculina) value above 40 were defined as EMMs. I n the second experiment, the effects of food amount, hatching date and mate rnal size on growth at Age-0 were examined. Food amount strongly affected t he early growth. A difference in hatching date of >2 wk caused a large diff erence in body size of juveniles; this difference was maintained until the breeding season 3 mo later. Large females spawned larger larvae than small females, but the size difference between larvae from large females and thos e from small females decreased with time. Field observations showed that at one site in Saroma Lagoon, Hokkaido, Japan, hatching occurred over a 1 mo period. In 1996/1997, we found no obvious differences of size distribution of Age-0 individuals and the occurrence of EMM as a function of location in the lagoon or year. These results may have been caused by slow growth, bec ause water temperatures in the lagoon were much colder in 1996/1997 than du ring 1987 to 1995. We conclude that the occurrence of EMMs is closely relat ed to various environmental factors in shallow waters. Therefore, the propo rtion of EMMs may fluctuate both locally and annually. This study shows tha t a small difference in juvenile growth over a short period can alter the s ubsequent life history of P. latirostris.