THE EFFECTS OF ENVIRONMENTAL PH ON SCLERACTINIAN MICROMORPHOLOGY

Coral reefs are diverse environments and home to thousands of species of fish and invertebrates. The products of reefs and tourism associated with tropical reefs are important parts of the economy of over a hundred nations. Reefs are also rapidly declining in coral cover. Coral reefs are being faced with many challenges. Anthropogenic disturbances and climate change are putting increasing pressure on hermatypic species of scleractinian corals. While temperature related bleaching is a commonly cited problem, the excess carbon dioxide in the atmosphere causes the less heralded problem of oceanic acidification. As the ocean becomes more acidic, reef building corals will have an increasingly difficult time calcifying their skeleton. In order to test the effects of declining environmental pH on the morphology and micromorphology of scleractinian corals, experimental conditions were created in laboratory aquariums with environmental pH as the sole independent variable. The levels of acidification studied in this study are pH 8.2 (control), pH 7.7, and pH 7.4. Three species of corals were used for the experiments to predict the common skeletal response throughout Scleractinia: Stylophora pistillata, Galaxea astreata, and Euphyllia paradivisa. Colonies were grown from a single mother colony for each species used in the experiment to limit genetic variability. After growing the specimens under experimental conditions for three months, the specimens were collected and cleaned for imagining using scanning electron microscopy (SEM). Specimens were examined for skeletal differences on the whole corallite to micron scale. Micromorphological characters (skeletal features best viewed using scanning electron microscopy) were examined for additional variability. Micromorphological characters are starting to be used in morphology-based systematics, but their plasticity and stability is poorly understood. We found distinct morphological changes within Stylophora and Galaxea in response to depressed pH, but little change in Euphyllia. Micromorphology did show minor differences between the various pH levels examined, but the micromorphological characters were still recognizable even at severe acidification levels.