Abstract
Hydra is emerging as a model organism for studies of ageing in early metazoan animals, but reef corals offer an equally ancient evolutionary perspective as well as several advantages, not least being the hard exoskeleton which provides a rich fossil record as well as a record of growth and means of ageing of individual coral polyps. Reef corals are also widely regarded as potentially immortal at the level of the asexual lineage and are assumed not to undergo an intrinsic ageing process. However, putative molecular indicators of ageing have recently been detected in reef corals. While many of the large massive coral species attain considerable ages (>600years) there are other much shorter-lived species where older members of some populations show catastrophic mortality, compared to juveniles, under environmental stress. Other studies suggestive of ageing include those demonstrating decreased reproduction, increased susceptibility to oxidative stress and disease, reduced regeneration potential and declining growth rate in mature colonies. This review aims to promote interest and research in reef coral ageing, both as a useful model for the early evolution of ageing and as a factor in studies of ecological impacts on reef systems in light of the enhanced effects of environmental stress on ageing in other organisms.
Highlights
Hydra (Martinez, 1998; Schaible et al, 2015), similar demographic data have been reported for other species (Finch, 2009; Jones et al, 2014)
This review aims to promote interest and research in reef coral ageing, both as a useful model for the early evolution of ageing and as a factor in studies of ecological impacts on reef systems in light of the enhanced effects of environmental stress on ageing in other organisms
This is because the force of selection at any age is proportional to the fraction of an individual’s expected lifetime reproductive output that still remains in its future, which will decline as a consequence of prevailing mortality if nothing else
Summary
It is generally accepted that ageing is driven by progressive accumulation of molecular and cellular defects, leading eventually to functional impairments, chronic diseases and death. Given the scope for asexually generated coral colonies to achieve considerable age, it is unsurprising that significant somatic mutation can accumulate across the colony. This was regarded by van Oppen et al (2011) as a potentially important contributor to adaptation and evolution of reef corals, since germ-cell differentiation was believed to occur continuously from somatic stem cells (Buss, 1983). For the short-lived branching coral Acropora digitifera telomere length was significantly longer in sperm than in the planulae which was in turn longer than in adult polyps (Tsuta et al, 2014) In another species, Galaxea fascicularis, no significant differences were found (Tsuta & Hidaka, 2013). Bleaching is a response to the combined stresses of temperature and irradiance, where the coral pales as a result of loss of symbiotic zooxanthellae and/or their pigments (Brown, 1997), and its increasing occurrence all around the world is placing corals under pressure (Rinkevich, Avishai & Rabinowitz, 2005; Lesser, 2011)
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