An examination of the co-existence of the threespine (Gasterosteus aculeatus) and the ninespine (Pungitius pungitius) sticklebacks and its consequences for morphology and behaviour

In Britain two species of stickleback, the threespine and the ninespine are often found living side-by-side in freshwater streams. The two stickleback species share a close life-history and in particular similar dietary habits. The situation of two such ecologically similar species being able to co-exist without high levels of competition removing one or the other from the habitat begs us to question exactly what ecological process is allowing this situation to occur. Three separate populations of stickleback were observed in the study; a population of allopatric threespine sticklebacks, a ninespine population and a population of threespines that live sympatrically with ninespine fish. Morphological analysis of the three populations of sticklebacks revealed divergence amongst the sympatric threespine fish. The sympatric threespine fish were shown to have a deep mid-body depth and deeper caudal peduncle, which possibly serve to increase the fish's body acceleratory skills. In addition the sympatric threespines have a small conical shaped mouth and meristic analysis indicates an increase in the number of the fish's gill rakers. Combination of these adaptations indicates that the sympatric threespine fish are effective zooplanktivores. It was questioned whether character displacement initiated by competition from the ninespine fish in the sympatric threespines environment is the process responsible for driving the observed divergence. In turn each of the criteria of the character displacement hypothesis was tested. Investigation of the allopatric and sympatric environment indicated little difference between the sites other than the presence of the ninespine fish in the sympatric sites. Stomach contents analysis showed that the sympatric fish have increased their consumption of zooplankton. Behavioural analysis showed how the morphological adaptations have improved the sympatric fish's foraging performance in the limnetic habitat.