The retention mechanism of a population of the estuarine copepod Eurytemora affinis (Poppe) was determined within a strongly tidal, mixed estuary. Sampling in the water column at sites along the estuary was carried out to assess any tidal, die1 and semilunar patterns of change. The species showed distinct tidal differences in abundance with horizontal position, being maximally abundant on the flood tide at the most seaward site and on the ebb tide at the landward site, suggesting differential tidal swimming, leading to accumulation at an intermediate point. Monitoring over a springs-neapssprings cycle suggested that the position of this point changed over the semi-lunar cycle, with greatest numbers on flood tides over springs and on ebb tides over neaps. This was confirmed by repeated sampling along the length of the estuary, a specific region was identified where very high numbers of E affinis occurred. T h~s horizontal posit~on changed between springs and neaps, apparently maintaining the bulk of the population within a specific salinity zone. Sampling throughout the water column showed these horizontal movements to be facilitated by tidal vertical migrations.

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The distribution of Neomysis integer (Crustacea: Mysidacea) in a tidally mixed estuary was found to be concentrated in mid-estuary with animals extending, primarily in summer, further up and down estuary. Within this range there were distinct intra-specific differences in distribution, with males occurring more commonly down-estuary and ovigerous females and juveniles up-estuary. Examination of tidal abundances in mid-channel showed males to be more abundant on the ebb tide, corresponding with their more downstream distribution, and all other individuals to be present mainly on flood tides, which would maintain the species within the estuary. Endogenous circa-tidal swimming activity in this species was found to be phased after the time of expected high tide, which would suggest ebb tide swimming in the estuary. More detailed examination of the distribution of Neomysis across the width of the estuary showed greatest concentrations of animals in slow-flowing water at the edge of the channel during the ebb tide. The endogenous swimming rhythm may therefore act to prevent stranding of animals intertidally during the ebb tide, while maintenance of position along the estuary appears to be by flood tide swimming under exogenous control.

Intersexuality is described in Neomysis integer (Leach), apparently the first occurrence among the Mysidacea. Seven out of 13,700 specimens examined were masculinized females possessing elongated fourth pleopods, a male secondary sexual characteristic. Each masculinized female appeared to have reproduced as a normal female; there was no evidence of testicular tissue.

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