It’s not all about temperature for corals

31 05 2017

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Three of the coral species studied by Muir (2): (a) Acropora pichoni: Pohnpei Island, Pacific Ocean — deep-water species/IUCN ‘Near threatened’; (b) Acropora divaricate: Maldives, Indian ocean — mid-water species/IUCN ‘Near threatened’; and (c) Acropora gemmifera: Orpheus Island, Australia — shallow-water species/IUCN ‘Least Concern’. The IUCN states that the 3 species are vulnerable to climate change (acidification, temperature extremes) and demographic booms of the invading predator, the crown-of-thorns starfish Acanthaster planci. Photos courtesy of Paul Muir.

Global warming of the atmosphere and the oceans is modifying the distribution of many plants and animals. However, marine species are bound to face non-thermal barriers that might preclude their dispersal over wide stretches of the sea. Sunlight is one of those invisible obstacles for corals from the Indian and Pacific Oceans.

If we were offered a sumptuous job overseas, our professional success in an unknown place could be limited by factors like cultural or linguistic differences that have nothing to do with our work experience or expertise. If we translate this situation into biodiversity terms, one of the best-documented effects of global warming is the gradual dispersal of species tracking their native temperatures from the tropics to the poles (1). However, as dispersal progresses, many species encounter environmental barriers that are not physical (e.g., a high mountain or a wide river), and whose magnitude could be unrelated to ambient temperatures. Such invisible obstacles can prevent the establishment of pioneer populations away from the source.

Corals are ideal organisms to study this phenomenon because their life cycle is tightly geared to multiple environmental drivers (see ReefBase: Global Information System for Coral Reefs). Indeed, the growth of a coral’s exoskeleton relies on symbiotic zooxanthellae (see video and presentation), a kind of microscopic algae (Dinoflagellata) whose photosynthetic activity is regulated by sea temperature, photoperiod and dissolved calcium in the form of aragonite, among other factors.

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