Some Factors Affecting the Distribution of Littorina saxatilis (Olivi)

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Abstract
L. saxatilis (Gastropoda) occurs in greater numbers at higher than at lower levels of the Whitstable (Kent), shore. Changes in ovaries and numbers of embryos in brood-pouches of females, testes and penis of males, and frequency of copulation all indicate an annual cycle of reproduction with a cessation in June-Aug. Specimens higher on the shore had larger ovaries and produced significantly more offspring than those downshore throughout the breeding period. Females transferred from one level to another came to produce numbers of young similar to the "natives" of their new zone within about 4 weeks. Greater egg production hgiher up the shore results directly in increased population density there because L. saxatilis is viviparous and its young emerge as complete crawling snails. Trematode infection was heaviest during summer months when reproduction was slowest, and it prevented recovery of gonads of many infected animals in autumn. Factors favoring the upshore population include greater tidal exposure allowing longer activity (expecially feeding), slight passive upshore movement of young by wave action, better shelter and security in deeper upshore shingle, and more abundant growth of algal food at higher levels. These factors appear to have a direct effect in controlling population distribution not seen in oviparous forms where there is a planktonic larva which falls randomly onto the shore.

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