REPRODUCTIVE CHARACTER DISPLACEMENT OF MALE STICKLEBACK MATE PREFERENCE: REINFORCEMENT OR DIRECT SELECTION?

Reproductive isolation can evolve between species as a byproduct of adaptation to different niches, through reinforcement, and by direct selection on mating preferences. We investigated the potential role of direct selection in the reproductive isolation between sympatric species of threespine sticklebacks. Each sympatric pair consists of a small-bodied limnetic species and large-bodied benthic species. We compared the mate preferences and courtship behavior of males from one sympatric limnetic population and two allopatric populations. We used limnetic-like allopatric populations to control for the effects of ecological character displacement and adaptation to different niches on mate preferences. The sympatric limnetic males preferred the small limnetic females, whereas the allopatric limnetic-like males preferred the large benthic females, suggesting that adaptation to the limnetic niche does not automatically confer a preference for small limnetic females. This reproductive character displacement of male preference is consistent with the predictions of both reinforcement and direct selection on mate preferences. To test for direct selection, we assessed a prediction of one proposed mechanism: predation by benthic females on eggs guarded by limnetic males. The allopatric males come from populations in which there is no egg predation. Sympatric limnetic males were more aggressive toward benthic females than toward limnetic females, whereas the allopatric limnetic-like males did not treat the two types of females differently. The contrast in male behavior suggests that egg predation has shaped male preferences. Direct selection is potentially more effective than indirect selection via reinforcement, and it is likely that it has been important in building up reproductive isolation between limnetic and benthic sticklebacks.     

Neomysis integer in a shallow hypertrophic brackish lake: distribution and predation by three-spined stickleback (Gasterosteus aculeatus)

The mysid shrimp Neomysis integer is a common invertebrate predator in brackish waters of Western Europe and is thought to play a central role in the food web owing to its predation on zooplankton. Neomysis distribution and abundance were investigated for 3 years in brackish, shallow and hypertrophic Lake Ferring (surface area: 3.2 km², mean depth: 1.4 m, salinity: 3–6, total P: 0.29–0.78 mg P l^-1, Secchi depth: 0.14–0.22 m). Mean summer abundance of Neomysis varied from 53 to 882 ind. m^-2. Neomysis density within the lake was relatively uniform and not related to sediment type, but increased with increasing depth. The high abundance of Neomysis is considered to reflect a fish stock almost completely dominated by small-sized fish species (mainly three-spined stickleback, Gasterosteus aculeatus). Three-spined stickleback density was high and catch per unit effort ranged between 30 and 80 per gill net. Stomach analyses showed that the sticklebacks preyed on Neomysis, but preferred specimens smaller than 3–4 mm, and only occasionally consumed those larger than 5–7 mm. In summer, between 33 and 67% of the Neomysis ingested by sticklebacks were smaller than 3 mm, while in the lake as a whole, only 5–14% were smaller than 3 mm. The periods when Neomysis is vulnerable to stickleback predation are restricted to a few weeks in late May and late July, when the new generations emerge. Sticklebacks therefore have a limited capacity for controlling large Neomysis, including gravid females.     

No sibling odor preference in juvenile three-spined sticklebacks

Laboratory-bred juvenile three-spined sticklebacks from 11 sibshipsdid not prefer to shoal with their siblings when they were offeredthe choice between odor from unfamiliar siblings and non-kinin a fluviarium, although the power for finding a significantpreference was very high (0.99). The test fish preferred theside where odor from the heavier shoal was supplied; this shows thatthey could appreciate odor cues from conspecifics in our apparatusand should have preferred their siblings if such a preferenceexists. Our results are compatible with theoretical predictionsbut are at variance with previous findings by other authors.We used independent replicates in a blind protocol with strictrandomization of fish and procedures.     

How to quantify embryo survival in nest-building fishes, exemplified with three-spined sticklebacks

Egg survival in manipulated nests of three-spined sticklebacks Gasterosteus aculeatus in the field was not significantly different from that in unmanipulated nests.     

A non-invasive morphometric technique for estimating cestode plerocercoid burden in small freshwater fish

A non-invasive morphometric technique is presented which can be used to predict the infection status and the proportion of infected fish weight contributed by parasite tissue in three-spined sticklebacks Gasterosteus aculeatus infected with plerocercoids of Schistocephalus solidus (Cestoda: Pseudophyllidea)