Variation in corallite morphology and Symbiodiniaceae communities of corals at the Rangitāhua archipelago : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Conservation Biology at Massey University, Albany, New Zealand
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2022
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Massey University
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Abstract
Anthropogenic activity is causing climate change, exposing corals to intolerable conditions. The depletion of corals can severely degrade reef-associated species. Therefore, a greater understanding of corals’ responsiveness to environmental conditions is needed, especially near their tolerance limits. Here, we examined the corallite morphology and Symbiodiniaceae community of corals at Rangitāhua (the Kermadec Islands), New Zealand. Due to the high latitude of Rangitāhua, corals and their symbionts experience light conditions near their lower tolerance limit, therefore we expected to find morphological signatures of adaptation to low-light conditions indicative of a trophic shift, Symbiodiniaceae depth zonation (5-30m), and a high abundance of the physiologically diverse coral symbiont Cladocopium. We also expected that environments might differ among islands, and therefore corallite morphology and symbiont communities would vary among locations. We found a high abundance of Cladocopium, and a much lower abundance of Symbiodinium, Breviolum, and Fugacium. At Napier Island, Symbiodiniaceae communities of Astrea curta, Hydnophora pilosa, and Montipora spongodes had the fewest unique Symbiodiniaceae types and were most compositionally dissimilar to the communities at the more sheltered Meyer Islands. Furthermore, corals at Napier Island had small, densely arranged corallites, with few, short septa, suggesting low water flow. Over depth, shallow corals (5m) had high tissue coverage (i.e., large, close corallites of M. spongodes and greater corallite coverage of H. pilosa) presumably to optimise autotrophic feeding in energy-rich waters. The greater corallite spacing seen over depth could be a mechanism to alleviate self-shading but could also be indicative of poor health. The Symbiodiniaceae communities at intermediate depths (10-20m) were taxonomically richer and compositionally dissimilar from communities at the edges of the depth gradient (5-10m and 20-25m). Although there were general trends, the corallite morphology and Symbiodiniaceae community of Goniastrea favulus, Montipora capricornis, and Turbinaria frondens were relatively consistent across the depth and locations sampled. Nearly half of the Symbiodiniaceae types identified (22/42) were undocumented, highlighting the significance of marginal populations to our understanding of how environmental conditions can influence corals. Our examination of the coral population at Rangitāhua further supports the well-documented trends of a high abundance of Cladocopium symbionts at high latitudes, specialised corallite morphology to increase the light-gathering abilities of shallow corals, and increased corallite spacing over depth, as well as Symbiodiniaceae depth zonation and diversity among nearby locations. Overall, we provide insights into the morphological and symbiont diversity of marginal coral populations in low light conditions, helping us to understand how corals may persist, and perhaps refuge, in high latitudes.