Biomechanical assessment of adaptive radiation in threespine sticklebacks: (gasterosteus spp.)
Creators: name: Law, Tara
Publisher: name: University of British Columbia
Year: 2009

description: This study compares the morphological characteristics and swimming performances of a sympatric species pair of threespine stickleback in order to gain an understanding of the processes involved in their divergence. The fishes are young (13,000 years old), morphologically distinct, and inhabit different niches in the lake (Paxton Lake, Texada Island, British Columbia). Experiments were conducted to compare the steady swimming and escape fast-start performances of these fishes. I tested predictions of two hypotheses: 1) Selection has acted on the steady swimming ability of limnetics, and therefore I predicted that limnetics have greater steady swimming performance than benthics. 2) The specialization for steady swimming has compromised the fast-start performance of limnetics; consequently I predicted that benthics have greater fast-start performance than limnetics. As predicted, limnetics had greater steady swimming performance than benthics. The mean regression of the logarithm of fatigue time (F.T.) on swimming speed (U, in body lengths/second) for limnetics [Log (ET.) = 5.24 - 0.46U] was significantly higher (p &lt;0.05) than that for benthics [Log (F.T.) = 3.77 - 0.43U]. The streamlined shape (to reduce drag), larger pectoral muscles (to provide power), and larger pectoral fin areas (to provide thrust) all contribute to the higher performance of limnetics. However, the fast-start performance of the two fishes was not significantly different for any linear parameter (pooled mean values for total fast-start: duration 0.049 s, distance 0.034 m, maximum velocity 1.10 m/s, and maximum acceleration 133.9 mIs2). With large, caudally placed fins limnetics achieved equal fast-start performance to the deeper bodied benthics. The prevailing view of functional morphology in fishes is that adaptations for high performance in one swimming mode compromise those for high performance in the other swimming modes. For example, optimal characteristics for fast-starts are thought to trade-off against optimal traits for manoeuvering and steady swimming. However, both of the sticklebacks studied here were capable of good manoeuverability and fast-start performance. The propulsive systems of these fish aie decoupled; consequently there is no trade-off between adaptations for these swimming modes. Limnetics achieved high performance in all three swimming categories with no compromise between good steady swimming and fast-start ability. The differences in prolonged swimming performance are linked to morphological and habitat differences. I suggest that selection due to differential resource use has lead to divergence of body form and therefore of steady swimming performance. However, the results of this study also suggest that selection from predators has lead to maintenance of high performance in fast-start swimming. Predation pressure is probably similar on these fishes; consequently selection would favour traits which enhance high escape fast-start performance in both species.