Size and age at first reproduction of the ribbed mussel Geukensia demissa (Dillwyn) in relation to shore level in a New York salt marsh

The shell lengths and dry body weights of ribbed mussels ( Geukensia demissa) at the onset of sexual maturation were determined at two shore levels within the tall Spartina alterniflora zone in a New York City salt marsh. Mussels grow faster at the lower level (marsh edge) than at the higher shore level (15 m upshore of the edge), and have higher length-specific body weights. Most edge mussels mature in their second growing season at shell lengths between 15 and 20 mm and at dry body weights ≥0.04 g DW. At the higher site, mussels mature in their third or fourth growing season at shell lengths of 20–25 mm and at body weights ≥0.057 g DW. At the marsh edge, the timing of maturation of mussels in their second growing season is related to the size at the beginning of that season (which depends largely on when larvae settled the previous year). Larger mussels initiate maturation earlier in their second season than smaller mussels, which must grow for a period to attain critical body mass. Thus, sexual maturity in Geukensia demissa is determined primarily by body size rather than age. At the higher shore level, body growth rates are depressed due to food limitations, and mussels become sexually mature a year or more later than edge mussels of the same age. This pattern is different from some populations of Mytilus edulis, in which depressed growth rates induce rather than delay sexual maturity.