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Adaptive significance of variation in vertebral number in fishes : evidence in Gasterosteus aculeatus and Mylocheilus cautines Swain, Douglas Paul


Variation in vertebral number is widespread in fishes, and is partly genetic in origin. Its adaptive significance was tested in young threespine sticklebacks Gasterosteus aculeatus by predation experiments and by direct measurements of swimming performance, and in young peamouth chub Mylocheilus caurinus by predation experiments. Counts of wild fry grouped by length or age were examined for evidence of selection for vertebral number in the wild. Survival of sticklebacks exposed to predation by sunfish Lepomis gibbosus was greater for fish with 31 TV (total vertebrae) than for those with 32 TV at lengths of 8.1-8.3 mm (mean at end of experiments), but not at smaller (7.6 mm) or larger (8.9-11.2 mm) lengths, using fish from either Holden or Kennedy Lake, B.C. Among Holden Lake sticklebacks, greater survival of fish with 31 TV at small lengths (8.1-8.3 mm) was due to fish with a high ratio of 14AV/17CV (abdominal/caudal vertebrae). At intermediate (9.8-10.0 mm) or large (11.2 mm) lengths, survival was greatest for fish with intermediate (14/18, 13/17) or low (13/18) ratios, respectively. Among Kennedy Lake sticklebacks, the ratios at advantage during predation also decreased as prey size increased, but higher ratios were favoured at a given size. Selective predation for vertebral number or ratio was not attributable to size selection or selection for body proportions (precaudal/caudal or abdominal/caudal lengths), and did not vary between cover (presence/absence) or temperature (15/ 20 or 25C) treatments. Burst swimming performance of Holden Lake sticklebacks at 15C was superior among fry with a high ratio of 14AV/17CV at small lengths (7.4-7.8 mm), among those with an intermediate ratio (14/18) at intermediate lengths (7.8-8.3 mm), and among those with a low ratio (13/18) at large lengths (8.3-9.0 mm). At even larger lengths (9.0-11.5 mm), performance was unrelated to AV/CV. Effects of AV/CV were similar at 25C, except that fry with high or intermediate ratios were superior at slightly shorter lengths at this higher temperature. The effect of AV/CV on performance differed greatly between water and an 0.1% solution of methylcellulose at 15C. Effects of AV/CV could not be attributed to effects of precaudal/caudal length ratio. Differences between predation and swimming performance experiments in the lengths at which particular ratios were optimal probably reflected growth and the concentration of mortality at smaller lengths during predation experiments. Vertebral counts of wild stickleback fry in Holden Lake indicated selection favouring a high ratio (14AV/17CV) and a low total count (31) at small lengths (7.3-7.8 mm), and intermediate ratios (14/18, 13/17) and a high total count (32) at slightly larger lengths (7.8-8.3 mm). Survival of peamouth chub during predation by sunfish or smallmouth bass Micropterus dolomieui was greater for fish with 44 TV than for those with 45 TV at lengths of 10.8-11.1 mm (mean at end of experiments), but not at smaller lengths (when vertebrae of most fry were undeveloped). Selection for vertebral number could not be attributed to size selection by predators. Selection in the wild favoured fry with 44 TV at small lengths (9.3-9.9 mm), those with 45 TV at slightly larger lengths (9.9-10.6 mm), and neither number at even larger lengths (10.6-14.0 mm). Effects of vertebral number on performance may involve optimal anteroposterior gradients in flexibility. Explanations are suggested for pleomerism and Jordan's rule, in terms of selection operating on the larvae or fry soon after hatching. Results are related to the maintenance of variation in vertebral number within populations, and to the possible adaptive significance of environmentally induced variation in vertebral number.

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