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Source: https://theses.gla.ac.uk/4839/ Parent: https://theses.gla.ac.uk/view/subjects/QH.html

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The population biology of Crenobia alpina (Dana)

Baird, Donald John (1983) The population biology of Crenobia alpina (Dana). PhD thesis, University of Glasgow.

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Printed Thesis Information: https://eleanor.lib.gla.ac.uk/record=b1632572

Abstract

An investigation was carried out into the\ relationship between environment and reproduction in the\ freshwater triclad species. By studying two\ populations occupying adjacent habitats, under varying\ conditions of temperature, flow rate and resource\ availability, the effects of habitat variability on the\ di~ribution~ density, size structure and the levels of\ se>:ual and ase>:ual reproduction within each population\ were assessed. The population occupying a habitat which\ was characterised by its eurythermic temperature regime,\ high flow rates and complex macroinvertebrate community,\ occurred at low densities. Within this population,\ individuals were larger, on average, than in the adjacent\ population, and se>:ual reproduction occurred at high\ levels throughout the year, with asexual reproduction (by\ binary and multiple fission> also occurring throughout the\ yea~, but at lower levels. The seasonal nature of this\ habitat was reflected within the popUlation of ,\ which exhibited spring peaks in density, followed by\ summer peaks in the level of sexual reproduction.\ In contrast, the adjacent population occupied a\ habitat which was characterised by its stenothermic\ temperature regime, low flow rates and a less comple>\ macroinvertebrate community, and occurred at much higher\ densities. Within this population, individuals were\ smaller, on average, than in the adjacent population, and\ sexual reproduction was virtually absent, wi th asexual

reproduction (by binary. and multiple fission) occurring\ throughout the year at appreciable levels. The lack of\ seasonality within this habitat was similarly reflected in\ the lack of any seasonal fluctuations in density, or level\ of (in this case ase>:ual) reproduction within the\ population. The level of food availability varied\ seasonally in both habitats, however, and was generally\ similar, in terms of biomass, in bath areas.\ A hypothesis was presented which\ of triclads within each habitat\ related the density\ to the total food\ availability, measured as stream drift. It was suggested\ that at low population densities, the relatively higher\ levels of net resource availability per i ndi vi dual\ favoured the occurrence of sexual reproduction, in\ contrast with situations of high population density, where\ the relatively lower levels of net resource availability\ per individual inhibited the occurrence of sexual\ reproduction, thus favouring asexual reproduction.\ This hypothesis was supported by the results of\ laboratory investigations in other studies, together with\ the evidence gained from field observations from this\ study, particularly the observation that in the low\ dem::.i ty population, seasonal cycles in the level of sexual\ reproduction were out of phase with seasonal cycles of a\ similar nature in the level of popUlation denSity.\ Further evidence, from the results of .a field\ manipulation experiment, was presented which supported the\ hypothesis. In an area of high popUlation denSity, density\ was reduced artificially, resulting in a significant\ increase in the level of sexual

reproduction within the\ population.\ population,\ It was concluded that in the high density\ intense intraspecific competition for food\ resulted in a low net level of food availability per\ individual, which in turn inhibited the process of\ sexualisation in triclads from that area.\ The results from these two populations of ~~~!e~~~,\ indicating that net food availability controls the\ occurrence of sexual reproduction in this species, are in\ marked contrast to the findings of previous studies, in\ which habitat temperature is implicated as the dominant\ environmental influence on this process.

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