An introduction to the ecology and behaviour of the prawn, Palaemon affinis Milne-Edwards, 1837 (Crustacea: Decapoda: Natantia).
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Palaemon affinis is the commonest prawn in intertidal habitats of the North and South Islands, New Zealand. The population dynamics, community composition, and natural food preferences of P. affinis were studied in the Avon-Heathcote Estuary, and rock pools at Taylors Mistake and Kaikoura, South Island, in 2000. Bimonthly samples were taken at all three sites at low tide using a suction sampler for the hyperbenthic communities and a corer or quadrat for the benthic communities. The distribution of P. affinis was patchy and its abundance fluctuated seasonally, peaking at the estuary and Kaikoura in winter (~580 individuals per m2) and at Taylors Mistake in autumn (294 individuals per m2). The carapace length (CL, mm), total body length (TBL, mm), and wet weight (W, g) of P. affinis were measured. Males were 15-19 mm CL, 19-50 mm TBL, 0.060-1.091 g W, and females were 5-21 mm CL, 13-54 mm TBL and 0.021-1.691 g W. The size ranges differed between the populations. Males from Kaikoura grew at a slower rate than females (CL against TBL or W), but not those from the Culvert or Taylors Mistake. There were no differences in growth rate between males and females for pooled data. For the pooled samples, males were significantly smallerllarger than females in TBL, but not in CL or W. The sex ratios were skewed: males dominated the medium size class (9-14 mm CL at the Culvert and Taylors Mistake; 9-12 mm CL at Kaikoura), and females the large size class (15-21 mm CL at the Culvert and Taylors Mistake, 13-15 mm CL at Kaikoura). The breeding season of P. affinis was in spring and summer, but the time when ovigerous femaLes first appeared differed between populations. Size at onset of maturity for females in the pooled data was 13 mm CL, but the size differed between sites. Ovigerous female were 13-20 mm CL, 33-53 mm TLB, and 0.370-1.729 g W and carried 110-690 eggs that weighed 0.033-0.260 g W. There was no relationship between femaLe body size and fecundity (number eggs), perhaps because of low sample size. The communities at the three sites shared 8 species in common: 2 molluscs (Melagraphia aethiops and Notoacmea sp.), 2 polychaetes (Haploscoloplos cylindrifer and Nereidae), a crab (Halicarcinus varius), an amphipod (Gammaridae), an isopod (Sphaeromatidae), and a brittle star (Amphiura hinemoa). Gammarid amphipods and P. affinis itself contributed most to the similarity index for withinpopulation analysis and to the dissimilarity between populations. Environmental variables (minimum/maximum water depth, water temperature, substratum, site size, salinity) were not well correlated with community composition. At Taylors Mistake, salinity was important (~ 89% similarity), but substratum type was responsible for most of the similarity (~51% similarity) between communities. Contrary to expectation, P. affinis was mainly carnivorous. Gammarid amphipods were the main food item which was found in ~ 71 % of P. affinis stomachs. Other items included unidentifiable matter, polychaetes, molluscs, plant material, ostracods, isopods, sand grains, sponge spicules and crab. No difference in diet between sexes, size classes or seasons was found. Low fullness values were found with only ~28% of 264 stomachs ~50% full. Periodicity in the locomotor and feeding activities of P. affinis from Kaikoura were examined in laboratory experiments in tanks provided with a continuous supply of fresh flowing seawater. Prawns were recorded using a 24 h time-lapse video camera under constant (red) light. To test for the effect of tide on locomotion, two experiments were conducted with artificial tide cycles simulating natural tide cycles at Kaikoura under constant light (water temperature, 12 ± 1°C), and under constant darkness (water temperature, 20 ± 2.5°C). The effects of day-night cycles on locomotion used a 12 h light: 12 h dark regime with a constant (low) tide (7 em water depth) at 14.5 ± 2°C. Feeding experiments were conducted using both a simulated 25 h tidal cycle and a 12 h light: 12 h dark regime (water temperature 19.5 ± 1.5°C). Diel and circatidal rhythms in locomotor activity were found, with maximum activity at night and at high tide. No rhythmicity was found in the feeding activity of P. affinis.