Life-histories of four co-occurring amphipods from a shallow, sand bottom at Kaikoura, New Zealand

Abstract

Abstract The life histories of 4 co-occurring amphipods, Hippomedon whero Fenwick, Patuki roperi Fenwick, Diogodias littoralis (Cooper & Fincham), and Protophoxus australis Barnard, from a sand bottom at 6 m were examined by random quantitative sampling. Densities of all species increased from lowest (12–130 0.1 m−2) in winter (May–August) to summer (December) or autumn (March) maxima of 168–580 0.1 m−2; in Protophoxus some of this variation was attributable to winter immigration. Size-frequency analyses of monthly collections indicated that species populations consisted of 5–10 cohorts and that 5–12 cohorts were produced annually. Summer cohorts lived for up to 300 days, but over-wintering cohorts (spring breeders in Protophoxus) lived longer than their respective summer cohorts. Females of all species lived considerably longer than males. Some females of all species carried embryos in most months but there was appreciable recruitment only in spring-summer (September–December) in Patuki and Diogodias, continuing into autumn (to March) in Hippomedon and Protophoxus. Brood sizes were positively correlated with female sizes; mean brood sizes were 2.7–19.3 eggs. Egg losses from broods were 30–34.5% in Diogodias and Protophoxus, 14% in Hippomedon, and nil in Patuki. Egg sizes of all species varied irregularly with season, but in Patuki egg size was inversely related to sea temperature and perhaps to food availability. Embryos increased in length by 21.5–41% during development, which took about 15–21 days at 15°C. Females matured at a larger size than males and produced their first broods when 56–113 days old summer cohorts and 90–182 days old for over-wintering cohorts of Hippomedon, Patuki, and Diogodias. The larger females of Protophoxus matured when 191–242 days old. Females of all species produced multiple broods, theoretically up to 7–13 broods per life-time. With the exception of Diogodias, breeding was synchronous between cohorts of a species and in Patuki it had a lunar periodicity that was modified by storms. The overall population sex ratio was dominated by males in Hippomedon (2.1) and by females in Diogodias (0.8). In 3 species (Hippomedon, Patuki, and Protophoxus) seasonal data for sex ratio indicate that juvenile males moulted to maturity in time to coincide with hatchling release and availability of females for mating.