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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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
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.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Scanned by EthOs
Subjects: Q Science > QH Natural history
Q Science > QL Zoology
Colleges/Schools: College of Medical Veterinary and Life Sciences
Supervisor's Name: Vickerman, Prof. K. and Tippett, Dr. R. and Calow, Dr. P.
Date of Award: 1983
Depositing User: Miss Louise Annan
Unique ID: glathesis:1983-4839
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 16 Jan 2014 12:26
Last Modified: 24 Jan 2014 15:24

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