Trends in the density of zooxanthellae in Acropora millepora (Ehrenberg, 1834) at the Palm Island Group, Great Barrier Reef, Australia

It is important to understand the natural variation of coral zooxanthellae considering the implications of loss of these micro-algal symbionts for coral survival during bleaching. This study explores the seasonal trend of zooxanthellae densities in the common coral, Acropora millepora over a period of seven months at a relatively undisturbed environment, the inshore reefs of the Palm Island Group in the Central Section of the Great Barrier Reef. Zooxanthellae densities increased twofold at both sites with the onset of colder temperatures and lower solar flux. This strongly suggests that a seasonal component is responsible for the observed changes in algal densities in A. millepora. Zooxanthellae densities also varied between colonies (F=4.858, p=0.000) and branches (F=1.565, p=0.001). The results of this study stress the importance of accounting for seasonal variation and for variation within and among colonies when designing studies to determine zooxanthellae densities and assessing the intensity of bleaching during mass bleaching events

Natural spatial variability of algal endosymbiont density in the coral Acropora globiceps: a small-scale approach along environmental gradients around Moorea (French Polynesia)

This study provides a baseline describing natural small scale variability of Symbiodinium density in the sentinel coral Acropora globiceps during the summer, under non-bleaching conditions. Spatial scales investigated range from the colony scale (1–10 cm, i.e. among branches of the same colony) to the reef scale (1–10 km, i.e. among stations distributed over several locations and depths), at Moorea Island, French Polynesia. The coral–Symbiodinium symbiosis is a key process in scleractinian coral physiology, and Symbiodinium density provides an easy-to-measure and inexpensive biomarker of this symbiosis health. Spatial variability of three major environmental factors: light intensity, sedimentation and water motion was also assessed to evaluate their potential link with Symbiodinium density. Density of Symbiodinium did not significantly differ within colonies or among colonies within a station. However, a marked depth gradient was observed, showing increasing density with increasing depth and decreasing light intensity. These observations provide an interesting reference for forthcoming comparisons with disturbed conditions, such as bleaching events

Ecological barcoding of corallivory by second internal transcribed spacer sequences: hosts of coralliophiline gastropods detected by the cnidarian DNA in their stomach

The second internal transcribed spacer (ITS2) of the nuclear ribosomal RNA cluster (rDNA) is significantly smaller in the Cnidaria (120–260 bp) than in the rest of the Metazoa. ITS2 is one of the fastest evolving DNA regions among those commonly used in molecular systematics and has been proposed as a possible barcoding gene for Cnidaria to replace the currently problematic mitochondrial sequences used. We have reviewed the intraspecific and interspecific variation of ITS2 rRNA sequences in the Anthozoa. We have observed that the lower limits of the interspecific DNA divergence ranges very often overlap with intraspecific ranges, and identical sequences from individuals of different species are not rare. This finding can result in problems similar to those encountered with the mitochondrial COI, and we conclude that ITS2 does not prove significantly better than COI for standard taxonomic DNA barcoding in Anthozoa. However, ITS2 appears to be a promising gene in the ecological DNA barcoding of corallivory, where taxonomic accuracy at genus or even family level may represent a significant improvement of current knowledge. We have successfully amplified and sequenced ITS2 from template DNA extracted from foot muscle and from stomach contents of corallivorous gastropods, and from their anthozoan hosts. The small size of cnidarian ITS2 makes it a very easy and efficient tool for ecological barcoding of associations. Ecological barcoding of corallivory is an indispensable approach to the study of the associations in deep water, where direct observation is severely limited by logistics and cost

Decadal changes in common reef coral populations and their associations with algal symbionts ( Symbiodinium

Decadal populations changes in four coral taxa and their patterns of association with algal symbionts (Symbiodinium spp. – 10 years of sampling) were examined on Kenyan back reefs over a period of climatic disturbances (1991−2009). Some of the better surviving taxa, Pavona and Pocillopora, were associated with variable temperature regimes and >50% of sampled colonies in these taxa had some of the more thermally tolerant Symbiodinium in clade D. In contrast, only around 35% of Acropora and no branching forms of Porites contained detectable levels of clade D, and both taxa experienced high levels of thermally‐induced mortality and poor recovery. Overall, however the relationship between Symbiodinium clade and population‐level success of coral hosts was not strong, and differential success inside and outside fisheries closures suggests that other factors, such as predation on corals, were also influential. Consequently, while Symbiodinium in clade D may contribute to the success of coral hosts across thermal disturbances, multiple ecological factors and additional biological traits also influence their long‐term survival