Coexistence, size overlap and population regulation in tidal vs. non-tidal Palaemon prawns

The effect of increased habitat heterogeneity in tidal areas on coexistence between Palaemon prawns was studied at eight sites along the European Atlantic coast. Two species which are sympatric in non-tidal areas, Palaemon adspersus Rathke and P. squilla (L.) are largely allopatric in tidal areas, and the sympatric size difference decreases in allopatry. In tidal areas the smaller species, P. squilla, is restricted to brown algal belts and rockpools. A third species, P. serratus (Pennant), larger than the others, occurs under oceanic salinities in subtidal brown algal belts and there has forced P. squilla to restrict its habitat distribution to adjacent intertidal rockpools. At estuarine salinities, however, P. squilla also inhabits the brown algal belts. A larger diel variation in stomach fullness index in P. squilla than in P. adspersus persists in non-tidal areas.Abiotic factors probably restrict the two largest species, viz. P. adspersus and P. serratus, to subtidal environments; these species are sensitive to the extremes in salinity, temperature or O₂ levels characteristic of the intertidal zone. Competition and/or predation probably relegates P. squilla to the intertidal zone. In non-tidal areas, where this zone is reduced, P. squilla increases its niche width and coexists with P. adspersus, and the size differentiation associated with sympatry may reduce interspecific competition.