Investigations on the euryhaline species Idotea chelipes (Pallas) [= viridis (Slabber)] (Naylor 1955) led to observations on other brackish water organisms in pools at Scarlett Point in the Isle of Man. These observations are worthy of discussion in relation to the recent literature on the ecology of Gammarus duebeni Lillj. (Segerstrale 1946, Forsman 1951, Hynes 1954) and furthermore, they illustrate the fact that the two British species of Jaera have fairly distinct habitats. There appears to be little published information on the ecological relationship between J. nordmanni (Rathke) and J. albifrons Leach. A series of three pools was studied in the igneous rock which fringes the tip of Scarlett Point; the approximate dimensions of the pools and their positions relative to each other are shown in Fig. 1. The lowest pool is situated at the head of a deep gully on an exposed part of the shore, and it opens to the gully by a narrow crevice 5 ft above MHWS. The middle pool is only about 6 in. above the level of the lower pool, while the upper pool is a further 3 ft higher. The depths of all three pools vary considerably, but each is between 2 and 3 ft at its deepest part. When there is any swell the funnelling action of the gully causes sea water to be forced into the lowest pool whenever the tide is above the level of about HWN. The amount of sea water entering depends upon the swell, and during storms so much sea water may be forced into the gully that the two lower pools merge completely. A small flow of fresh water from a spring farther inland trickles through the pools and out through the gully into the sea. After heavy rain its flow is considerable.
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Naylor et al. (1958) studied this question.