The Distribution of Nephtys cirrosa and N. hombergi on the South-Western Coasts of England and Wales

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Abstract
Around the coasts of south and southwestern England and South Wales, the nephytid polychaete, N. cirrosa, occurs on most clean, sandy beaches of the open coast, except in north Cornwall and Devon, where the beaches are exposed and the substratum is coarse. N. hombergi is found in estuarine or semi-estuarine conditions in the Bristol Channel and the Tamar and Exe estuaries, and in the sheltered but essentially marine beaches at Dale and in the Salcombe estuary. The limit of penetration of N. hombergi into estuaries is the point at which tidal salinity fluctuations, particularly in winter, are severe. Occurrence of one or other species on a beach can be correlated with the grade composition of the substratum. N. hombergi occurs in greatest numbers in a substratum containing 0.3% particles greater than 0.25 mm diameter and 5.8% particles smaller than 0.125 mm diameter. Optimum substratum for N. cirrosa contains between 4 and 7% particles larger than 0.25 mm and about 1.8% smaller than 0.125 mm. N. hombergi tolerates up to 3.8% particles greater than 0.25 mm and between 2.2 and 15.9% particles smaller than 0.125 mm. N. cirrosa tolerates up to 50% particles larger than 0.25 mm; coarser substrata may be colonized unless larger grades are composed of shell fragments; it will tolerate more than 1.8% fine particles, providing the substratum includes a proportion of coarse particles. Beaches composed of poorly sorted substrata containing relatively large proportions of coarse and fine material may be colonized by both species; they form a small minority of those investigated. Where they are numerous, N. cirrosa and N. hombergi generally occur below M T L, but are not restricted to a particular tidal zone. Their distribution on a beach is determined largely by substratum composition. N. hombergi is principally sublittoral and occurs intertidally only in the lower reaches of estuaries or on extremely sheltered beaches. Substrata suitable for N. cirrosa are rare in the sublittoral but usual on all but the most exposed marine sandy beaches; it is the predominant species around the south and west coasts of Britain.

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