Three gastropods occupy a range of habitats along gradients in hydrothermal flux at Juan de Fuca Ridge vents. I examined how these species co-exist and identified mechanisms driving their abundances. First, I measured temperatures and spatial patterns in adult densities of the three species at three distances from vents to test if thermal regime relates to their habitat selection. Lepetodrilus fucensis and Depressigyra globulus were most dense in-vent (0-25 cm) at variable temperatures (10+5°C): 2100 and 240 incl. dm -2 (respectively). Provanna variabilis was most abundant far-vent (51-75 cm: 60 ind. dm 2) at stable temperatures (3±0.5°C). Thermal conditions are key in their habitat selection: behavioural experiments showed that these gastropods select fluid temperatures<18°C. L.fucensis and D. globulus preferred 5-15°C, while P. variabilis preferred 4-12°C.
The next studies sought to explain how Lepetodrilus fucensis reaches order of magnitude higher densities in comparison to other gastropods. First. I quantified L. fucensis recruitment and sex ratio patterns to identify innovative life history traits. I meaured size structure and density at in- and far-vent locations. Early postlarval juveniles occupied far-vent at remarkable densities (2419 ind. dm-2). To test for sex ratio biases, I sexed animals from different habitats and sizes. Populations nearest vents hosted the largest females (>6.0 mm), while peripheral habitats were male-biased. A transplant experiment showed that female survivorship and gonad fullness were significantly lower than males in far-vent locations. Sex ratio biases are driven by two mechanisms: females maximize their reproductive output by selecting optimal habitats and suffer relatively higher mortality in low flux.
Next, I hypothesized that the Lepetodrilus fucensis gill symbiosis is a key adaptation. I used multiple approaches to determine if the prevalence of the association and relationship to the limpets condition support this hypothesis. FISH probes specific to the 16S rRNA molecule of a gamma-Proteobacteria hybridized where bacteria were present. Direct sequencing using symbiont-specific primers gave a single unambiguous sequence. indicating high specificity. Light and TEM micrographs of gill tissue from a range of species also showed that the symbiosis is ubiquitous. In addition, the gills of in-vent animals had high surface area. dense symbiont populations and healthy tissues, while far-vent animals showed the reverse trend, suggesting that the symbiosis benefits L. fucensis. Carbon fixation by gill tissues was stimulated by inorganic sulfide and related to the abundance of bacteria on the gill. These data indicate a persistent and specific symbiosis that is dependent on access to sulphide.
I further examined feeding by Lepetodrilus fucensis to determine if the bacteria contribute to their host's nutrition. The morphology of feeding structures were compared among Lepetodrilus species. L. fucensis exhibited specialized features: the gill is enlarged. the lamellae are free of the mantle. do not narrow and are stabilized by ciliary junctions. The radula and stomach of L. fucensis are also reduced. Shipboard observations confirmed suspension feeding by L. fucensis. In addition, the symbiont may be ingested because its phylotype was well-represented in food material on the gill. The limpet's morphological specializations are consistent with dependence on suspension feeding and/or symbiont farming; however, L. fucensis also grazes. a mechanism likely important in peripheral locations.
Lepetodrilus fucensis populations are partitioned by size and sex along environmental gradients near vents. Peripheral populations are dominated by recruits and adults tend to be male: grazing is likely their primary feeding mode. Larger animals form stacks in venting fluids and are female-biased. These populations access suspended particles for food and sulphide, which generates dense symbiont populations for ingestion. Multiple feeding modes sustain high L. fucensis densities in a space-limited environment and may be an innovative strategy that drives its remarkable abundances.
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/1859 |
| Date | 17 November 2009 |
| Creators | Bates, Amanda Elizabeth |
| Contributors | Tunnicliffe, Verena |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
| Rights | Available to the World Wide Web |
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