ESTUARINE ADAPTATION IN POPULATIONS OF PILAYELLA-LITTORALIS (L) KJELLM (PHAEOPHYTA, ECTOCARPALES)

The tolerances to five salinities (34‰, 22·7‰, 11·3‰, 5·7‰, 1·4‰) of Pilayella littoralis plants from populations systematically sampled from the head of the estuary of the Silver Burn, Castletown, Isle of Man, U.K., to the open sea have been ascertained by growth experiments. All isolates proved extremely euryhaline, growing as well in media of 11·3 ‰ salinity as in 34 ‰ (full seawater). Differences were observed in sensitivity to still lower salinities. Isolates from the inner estuary grew as well in media of 1·4‰ as in full seawater medium, whereas those from marine habitats showed severely reduced growth in a salinity of 5·7‰ and no growth in 1·4‰. A mid-estuarine transition zone between these two physiological populations is described, isolates varying widely between these two extremes. As the isolates had all been cultured in full seawater medium for a considerable period prior to experimentation, thus ruling out acclimatization effects, this variation must have a genetic basis. The demonstrated tolerances are shown to be closely related to salinity regimes in the sampling sites and it is concluded that the transition from moderately to markedly euryhaline populations is best described as an ecocline. © 1979.