Population ecology and life-history tactics of shallow, sand-bottom crustaceans at Kaikoura
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Quantitative random sampling of a shallow, sand bottom exposed to unpredictable storm wave action at Kaikoura, New Zealand, between Oct. 1978 and Oct. 1980 provided detailed information on the population biologies of five crustaceans, the large myodocopid ostracod Cyaloleberis zealandiaa, and the gammaridean amphipods Hippomedon whero, Patuki roperi, Metaphoxus littoralis and Paraphoxus australis. Data for each species included seasonal changes in population density and structure, cohort composition, age at maturity, longevity, frequency and occurrence of breeding, egg size, duration of embryonic development, brood size and mortality, number of instars and broods per life-time and population sex ratios. The biology of myodocopid ostracods is reviewed and discussed with respect to Cyaloleberis and the question of instar size variations in ostracods generally is explored. Results for the amphipods are discussed in relation to current knowledge of amphipod biology. Utilization of the sand habitat was examined by comparing species population densities and compositions in ripples and in troughs at different times. Species sediment-depth distributions in ripples and in troughs were investigated in two successive months by sectioning cores into five layers. In some species juveniles, males and females inhabited different depths and other species were more abundant in either ripples or troughs. Sediment depths inhabited are discussed with respect to species burrowing rates, food, and an optimal sediment depth where niche overlap, and presumably competition also, is greatest. Recolonization of defaunated sand by each species was monitored over 1, 4, 8, 16 and 26 days initially and over 1 and 26 - 39 days during each of five subsequent months. Initial results showed remarkably fast recolonization by some species and changes in species population densities as recolonization proceeded. Within species the relative recolonization rates of juveniles, males and females may differ along with the sizes of recolonizing and control species subgroups. However, the experiments in subsequent months revealed that the pattern of species recolonization may change with time in response to no obvious factors. Results of the preceeding chapters are brought together in a final discussion of crustacean life-history tactics. Sediment depths inhabited provided a ranking of species habitat stabilities and mortality risks, both greatest at the sand surface. Population parameters, including net reproductive rates and innate capacities for increase, confirmed the ranking, but comparisons with species combinations of life-history traits strongly disagree with the predictions of two theories, r- and K-selection and bet-hedging. The failure of these theories is principally due to the existence of phylogenetic constraints, the lack of viable genetic alternatives for a trait. A new theory is required that encompasses phylogenetic constraints and recognizes the possibility of several equally successful combinations of traits in a given situation. Such a theory based on established ideas of costs of reproduction and of larger offspring from larger eggs being ecologically fitter must incorporate recent ideas on growth rates, body size and size-specific mortality.