During my PhD, I wanted to determine whether thermal tolerance of early life stages (i.e. embryos and larvae) varies among corals from thermally distinct regions in eastern Australia. To do this, I visited island research stations across an 17º latitudinal gradient from reefs on the Great Barrier Reef to Lord Howe Island, a high latitude reef (31.5ºS).
I would time my fieldwork to coincide with coral spawning season, after the November full moon on the Great Barrier Reef and after the January full moon at Lord Howe Island. See below for a video of Hard Coral Porn, showing colonies of Acropora spathulata and Goniastrea favulus spawning in aquaria. The music has a lunar theme to reference the influence of the moon on spawning time.
After spawning, I would spend days observing the early life stages in the lab, watching the fertilized eggs develop into larvae at different temperature treatments, and recording survivorship, abnormality, and development rate. Here is some surveillance footage of experiments conducted at Lizard Island Research Station in the northern Great Barrier Reef:
Here is an animation of coral embryos developing (species: Goniastrea favulus). Photographs are real (taken with a microscope), motion and speed are embellished:
My findings are reported in the following journal articles. Click on the title to visit to the journal's site. If you would like a pdf, please contact me at firstname.lastname@example.org.
Abstract: Organisms living in habitats characterized by a marked seasonal temperature variation often have a greater thermal tolerance than those living in more stable habitats. To determine the extent to which this hypothesis applies to reef corals, we compared thermal tolerance of the early life stages of five scleractinian species from three locations spanning 17° of latitude along the east coast of Australia. Embryos were exposed to an 8 °C temperature range around the local ambient temperature at the time of spawning. Upper thermal thresholds, defined as the temperature treatment at which the proportion of abnormal embryos or median life span was significantly different to ambient controls, varied predictably among locations. At Lizard Island, the northern-most site with the least annual variation in temperature, the proportion of abnormal embryos increased and life span decreased 2 °C above ambient in the two species tested. At two southern sites, One Tree Island and Lord Howe Island, where annual temperature variation was greater, upper temperature thresholds were generally 4 °C or greater above ambient for both variables in the four species tested. The absolute upper thermal threshold temperature also varied among locations: 30 °C at Lizard Island; 28 °C at One Tree Island; 26 °C at Lord Howe Island. These results support previous work on adult corals demonstrating predictable differences in upper thermal thresholds with latitude. With projected ocean warming, these temperature thresholds will be exceeded in northern locations in the near future, adding to a growing body of evidence indicating that climate change is likely to be more detrimental to low latitude than high latitude corals.
Abstract: Increased temperatures are deleterious to early life stages in many organisms; however, the biological effects of decreased temperatures are rarely explored. For example, the tolerance of marine invertebrate larvae to temperatures lower than ambient might affect the capacity of species to disperse from tropical to subtropical locations. In addition, reduced rates of development are likely to affect the proportion of larvae retained on natal reefs. Here, we explore the relationship between temperature, embryonic development and larval survival over an 8°C temperature range (-4 to +4°C around the ambient temperature at the time of spawning of 24°C) in 2 reef-building corals, Goniastrea favulus and Acropora spathulata from One Tree Island in the southern Great Barrier Reef. Rates of development were generally slower at lower temperatures: embryos of both species took longer to complete gastrulation and to become motile at temperatures below ambient. In contrast, temperatures below ambient did not affect larval survivorship in either species. A. spathulata larvae were more sensitive to increased temperatures than G. favulus, which also had higher survivorship than A. spathulata at all temperatures except 20°C. These results suggest that fluctuations in temperature at the time of spawning will influence patterns of coral larval dispersal. Furthermore, cold water is unlikely to prevent the dispersal of tropical corals to subtropical locations.
Abstract: Self-fertilization is unusual in most animals but common in some scleractinian corals, in particular, the family Faviidae. High levels of self-fertilization in many faviids may contribute to their large latitudinal range size and their dominance of some isolated, high latitude coral assemblages in eastern Australia. In this study, conducted at One Tree Island in the southern Great Barrier Reef, the thermal tolerance of self-fertilized Goniastrea favulus embryos was compared to outcrossed embryos across five temperature treatments (20°C, 22°C, 24°C, 26°C, and 28°C). Response variables were fertilization success, development rate, and larval survivorship. Fertilization was high (85-100%) across all treatments in cross-fertilized embryos. In self-fertilized G. favulus fertilization was low in treatments below ambient (27 and 60 % at -4 and -2 °C, respectively), though the effects on fertilization were not significant among the treatments. Development rates (i.e. mean times to the planula stage) were similar in selfed and outcrossed embryos. Development occurred more rapidly at raised temperatures for both groups. High mortality occurred at raised temperatures, at +4°C especially, for both self and crossed-fertilized G. favulus. Survivorship curves, median lifespans, and overlaps in 95% confidence intervals suggest reduced survivorship in selfed embryos. These results suggest self-fertilization has a negative effect on dispersal potential in reef-building corals.