The relationship between position on shore and shell ornamentation in two size-dependent morphotypes of Littorina striata, with an estimate of evaporative water loss in these morphotypes and in Melarhaphe neritoides
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Shells of small (< 7 mm) Littorina striata are frequently nodulose, but shells of larger individuals are striate. Nodulose L. striata dominated the littoral fringe of a black basalt Azorean shore where daytime rock temperatures rose significantly higher than nearby shores of different rock composition or colour. There was no evidence of intraspecific size-partitioning on the latter shores, where the numbers of striate and nodulose L. striata were approximately equal between high eulittoral (low-shore) and high littoral fringe (high-shore) localities. The prevalence of small L. striata in the littoral fringe is opposite to that usually characteristic of the Littorinidae, where the largest individuals usually occupy the higher positions on the shore. It is hypothesized that small L. striata attain a resting posture better able to minimize heat absorption from the substratum than attained by larger individuals. Smaller individuals also take advantage of both posture and a nodulose shell surface to re-radiate absorbed incident radiant thermal energy more effectively to the atmosphere by convection. Thus, small, nodulose L. striata are especially well adapted to occupy geologically young basaltic rocks commonly found fringing islands of the mid-Atlantic.
The rate of evaporative water loss was determined for Melarhaphe neritoides and striate and nodulose L. striata for approximately 11 days emersion. All three groups are exceptionally capable of controlling evaporative water loss. Total percent evaporative water loss by nodulose L. striata (17.9%) was significantly greater than that lost by either striate L. striata (14.1%) or M. neritoides (13.5%) but, among 15 species for which evaporative water loss has been determined by similar methodology, M. neritoides and striate L. striata are the most capable of conserving body water during 11 days of emersion.
Key wordsintertidal ecology size-partitioning spatial distribution evaporative water loss Littorina striata Melarhaphe neritoides
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