Phenological, physiological, and ecological factors affecting the epiphyte Notheia anomala and its obligate host Hormosira banksii
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Notheia anomala is an obligate epiphyte commonly found on the abundant habitat-forming alga Hormosira banksii in intertidal areas throughout temperate Australasia. The tight co-evolved relationship between these species is unique because: (i) Notheia is a true obligate epiphyte, which is uncommon in the marine environment, (ii) the order Fucales is over 70 million years old and includes over 10 families, but Notheia is one of few fucoid epiphytes, and (iii) phylogenetically close species are rarely so closely linked (Hormosira, the obligate host of Notheia, is also a fucoid). This project is the first to address the phenological, physiological, and ecological factors affecting the Notheia-Hormosira relationship through a combination of field surveys and manipulative experiments.
Phenological observations indicated that the two species may have asynchronous life cycles. I found that Notheia reproduction peaked in April (Austral autumn) when seawater temperatures were mild, whereas previous studies have shown peak reproduction in Hormosira during the period July to October (Austral winter/spring). There were differences in the development of Notheia conceptacles across different habitats (high shore areas, low shore areas and tide pools). Conceptacles developed faster, and were at full maturity for longer in the tide pool habitat. It is likely that lower levels of desiccation stress in tide pools allow faster conceptacle development and longer periods of reproductive maturity.
From an evolutionary and ecological perspective, it is expected that the distribution of Notheia should closely resemble that of Hormosira across spatial and temporal scales. To test this, I compared distribution patterns of Hormosira and Notheia from the large continental scale to the small individual host plant scale. While Notheia biogeographical distribution is intricately linked to its host Hormosira, I found contrasting ecological habitat preferences, with tide pools hosting the lowest abundance of Hormosira and the highest abundance of Notheia respectively. At the host plant scale, I found that Hormosira plants from the high shore had the greatest number of Notheia clumps attached near the low-holdfast region. In the low shore and tide pools the pattern was opposite, with most Notheia clumps attached to the mid and high regions of the host. Notheia was equally likely to be found attached to male and female host plants, and more epiphytes were found attached to older than younger host plants.
Using field tagging and translocation experiments, I also quantified the survival and growth of Notheia at different densities exposed to various stressful environmental conditions. Tagged Notheia clumps, with different plant densities and sizes, from the low shore and tide pools all experienced high mortality over a five-month period associated with high dislodgement rates of the host Hormosira. In translocation experiments of Notheia fronds (without its host), I found that individuals translocated to the high shore experienced close to 100% mortality, suggesting that desiccation and possibly photo inhibition are the main factors limiting the upward distribution of Notheia. Translocations to the low shore and tide pools demonstrated that Notheia fronds can survive and grow detached from its obligate host and suggest that the obligate dependency is most likely an early life stage requirement.
Finally, I tested whether the abundance of invertebrate inhabitants associated with Hormosira varies in the presence of Notheia across spatio-temporal scales. Field surveys showed that, as predicted, there were strong positive density-dependent effects of Notheia on both richness and abundance of invertebrates, regardless of the spatio-temporal context and resident invertebrate taxa, providing one of the first examples of a habitat cascade occurring in rocky intertidal systems.
Through a recolonization experiment, I tested whether invertebrate abundance was driven by (1) Notheia or Hormosira, (2) high or low amounts of Notheia and (3) living Notheia fronds or abiotic mimics. Hypotheses 1 and 2 were strongly supported, with more biomass of Notheia (as opposed to Hormosira) supporting more invertebrates, but not Hypothesis 3, as richness and abundances of inhabitants were similar between living Notheia fronds and artificial mimics. This suggests that Notheia is primarily providing habitat rather than food to the invertebrate inhabitants. Based on these results I hypothesized that invertebrates exert little or no grazing pressure on Hormosira and Notheia. This was tested in a laboratory food choice experiment focusing on potential grazing effects from herbivorous gastropods. In contrast to this hypothesis, I found negative effects of gastropods on both Hormosira and Notheia, with stronger grazing on Notheia. However, grazing rates were low overall and are likely to play only a minor role in regulating the abundance and distribution of the two species under natural field conditions. In support of the spatio-temporal surveys and colonization experiment, the grazing experiment also suggests that Notheia provide a better habitat for small invertebrates than Hormosira.
Seaweeds are key components of coastal ecosystems, providing habitat and food for a wide range of marine organisms. Therefore, understanding their life history patterns and reproduction dynamics is essential for managing coastal areas and assessing ecosystem health. This study is the first to explore the long-term phenology and periodicity of reproduction in Notheia. Furthermore, my results support a growing number of habitat cascade studies from different ecosystems, and suggest that these processes are common in marine benthic systems.