Reproduction and larval ecology of broadcast spawning corals at the Solitary Islands, Eastern Australia

Joanne Ruth Wilson

This study examined the reproduction and early life history of broadcast spawning coral species at the Solitary Islands, a subtropical reef community on the east coast of Australia. Studies of reproduction are central to understanding both the ecological processes which maintain these high latitude communities, and the biogeography of corals generally. Field observations, experimental studies and remote sensing techniques were employed to examine the extent and pattern of sexual reproduction in broadcast spawning coral species, the effect of temperature and biological conditioning on settlement success, and the likely source and fate of coral larvae dispersing to and from the Solitary Islands. Corals from a range of taxonomic groups and biogeographic origins were studied. Evidence of sexual reproduction was observed in 24 of 27 coral species studied, and broadcast spawning was observed or inferred in 20 of these species. Corals had a single annual cycle of reproduction. Oocytes developed over 8-10 months in acroporids and over 3-5 months in massive species, and testes appeared between 2-7 months prior to spawning in acroporids and between 2-3 months in massive species. Overlapping gametogenic cycles were detected in some colonies of Acropora solitaryensis and A. glauca. This is the first report of overlapping gametogenic cycles in acroporids with an annual cycle of gametogenesis. Gametogenic studies indicated that acroporid corals at the Solitary Islands had a higher fecundity and produced larger eggs than acroporid species in tropical latitudes. All corals studied released mature gametes annually between December to April each year. Spawning was staggered among species and among colonies within some species resulting in a relatively asynchronous pattern of reproduction compared to the "mass coral spawning" events reported from the Great Barrier Reef. The main period of spawning occurred in January and February and in some cases, colonies of multiple species released gametes on the same night. Spawning in massive species occurred in the lunar months of January, February and March between 8-12 nights after the full moon, and was generally more synchronous and predictable than for acroporid species. The pattern of spawning in acroporids varied among species but was not strongly linked to a particular lunar phase. Colonies of A. solitaryensis released gametes during an extended period from December to March whereas A. glauca and A. hyacinthus released gametes over a much shorter period between late in December to mid January. Water temperatures in the region of the Solitary Islands are highly variable over periods of days due to the sporadic influence of the East Australian Current on this area of the continental shelf. Individual responses of corals to this highly variable temperature regime may explain the extended, asynchronous and sometime unpredictable nature of the spawning pattern observed at the Solitary Islands. Larvae were reared successfully from gametes collected from four species of massive coral which spawned in aquaria (Cyphastrea serailia, Montastrea curta, Acanthastrea lordhowensis and Goniastrea australensis). Competency periods ranged from 26 days for C. serailia, up to 56 days for G. austra/ensis and 78 days for A. lordhowensis. These data greatly extend the known competency periods for larvae of broadcast spawning corals and indicate the potential for transport of broadcast spawned coral larvae over large distances and for larval exchange among subtropical reefs. Oceanographic studies at the Solitary Islands showed that current speed and direction was highly variable around the time of spawning. At times when the East Australian Current reached inshore, water currents moved rapidly southwards and probably swept larvae away from the region. At other times, nearshore currents oscillated slowly northward and southward potentially transporting larvae to more northern subtropical reefs, or returning larvae to the region of the Solitary Islands after spending some time in the water column. The settlement responses of larvae to different levels of biological conditioning and temperature were studied experimentally. Settlement of M. curta and G. australensis larvae was significantly higher on tiles with a diverse fouling community which included some cover of crustose coralline algae. The effect of temperature (22 to 30°C) on larval settlement varied among species and between years for the same species. For M. curta, G. australensis and A. lordhowensis in 1995, significantly higher settlement occurred at 26°C. In contrast, settlement of A. lordhowensis in 1996 and C. serailia, was not significantly affected by temperature although settlement was highest at 30°C. Post-settlement mortality of M. curta, A. lordhowensis and G. australensis on terracotta tiles placed on reefs was high with less than 1 % of spat surviving after 7-9 months. This study has contributed to the growing body of evidence that, despite predictions to the contary, corals at high latitude locations are sexually reproductive and that larvae of broadcast spawning corals can survive for extended periods. These findings suggest that subtropical coral populations are important sources of larvae and play an important role in the biogeography of scleractinian corals. Further research is needed to determine the factors which contribute to the asynchronous nature of reproduction at the Soitary Islands, and the larval ecology of other ecologically important coral taxa such as acroporids.

Reproduction and larval ecology of broadcast spawning corals at the Solitary Islands, Eastern Australia

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Reproduction and larval ecology of broadcast spawning corals at the Solitary Islands, Eastern Australia

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