Kelp-sandhopper interactions on a sand beach in New Zealand. II. Population dynamics of Talorchestia quoyana (Milne-Edwards)

Abstract

The population structure and breeding biology of the supralittoral sandhopperTalorchestia quoyana (Milne-Edwards) were investigated over an annual cycle in relation to the availability of drift kelpMacrocystis pyrifera (L.) C.Ag. At neap tides the population was dispersed with amphipods associated with freshly deposited wrack and gravid females separated from other individuals. Size distributions were dominated by immature individuals. Recruitment occured throughout the year except during winter. Growth rates depended on sex and body size with a juvenile growth rate of 1.7 mm length·month⁻¹. Average life span was≈ 13months. There was no direct relationship between brood size and female length, the average brood size for a female length 16.2 mm was 20.5. Brood mortality during embryological development was low, egg volume was high, rapidly increasing during the later stages resulting in the release of large hatchlings. Amphipod biomass varied with shore level, habitat type and season. Biomass estimates were less variable than density and both showed little correlation with kelp biomass. Estimates of dry weight amphipod biomass · 5 m⁻¹ strip of beach ranged from 5.8 g (winter) to 1.2 kg (spring). The ratio of amphipod biomass to kelp standing stock varied with shore level and season due to irregular input ofM. pyrifera. The population characteristics and breeding biology, of this sandhopper allow exploitation of san-beach habitat which receives regular but low levels of kelp input.