L Journal of Experimental Marine Biology and Ecology, 237 (1999) 255–270 Heat stability and activity levels of aspartate aminotransferase and alanine aminotransferase in British Littorinidae * S.L. Hull , J. Grahame, P.J. Mill School of Biology, University of Leeds, Leeds LS29JT, UK Received 24 August 1998; received in revised form 11 November 1998; accepted 15 December 1998 Abstract Aspartate aminotransferase and alanine aminotransferase activity levels were determined in crude homogenates of Littorina arcana, Littorina littorea, Littorina compressa, Littorina neglecta, Littorina obtusata, Littorina fabalis, Melarhaphe neritoides and three forms of Littorina saxatilis; high shore thin-shelled H, midshore thick-shelled M and barnacle dwelling B. In all the groups investigated, aspartate aminotransferase activity was higher than alanine aminotransferase, and high shore species appeared to have higher activity levels of aspartate aminotransferase relative to alanine aminotransferase than did low shore species. Aspartate aminotransferase was more heat stable than alanine aminotransferase at 458C, and it was more heat stable in high shore than in low shore groups. Heat stability also varied within a species; the enzymes of L. saxatilis H were more heat stable than those of L. saxatilis M from the same site. The enzymes of a small-barnacle dwelling form of L. saxatilis showed greater heat stability than sympatric samples of L. neglecta. The possible factors involved in these differences are discussed.  1999 Elsevier Science B.V. All rights reserved. Keywords: Heat stability; Enzyme activity; Aspartate aminotransferase; Alanine aminotransferase; Littorina 1. Introduction Sibling species complexes are of enormous potential value for the study of evolution (Knowlton, 1993). Adaptations arise as a result of evolution, these will include morphological, physiological and biochemical adaptations. Where groups of sibling *Corresponding author. Present address: University College Scarborough, Filey Road, Scarborough, YO11 3AZ, UK. 0022-0981 / 99 / $ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S0022-0981(99)00006-4