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SUMMARY Tentacle expansion and contraction were investigated in four zooxanthellate coral species and one azooxanthellate coral (Cladopsammia gracilis). Favia favus, Plerogyra sinuosa and Cladopsammia gracilisexpand their tentacles at night, while tentacles in Goniopora lobataand Stylophora pistillata are expanded continuously. Light at wavelengths in the range 400-520 nm was most effective in eliciting full tentacle contraction in F. favus and in P. sinuosa. Higher light intensities in the range 660-700 nm also caused tentacle contractions in F. favus. Tentacles in C. gracilis did not respond to light. Zooxanthellar densities in tentacles were significantly higher in G. lobata, which has continuously expanded tentacles, than in F. favus and P. sinousa, where tentacles are expanded at night. Photosynthetic efficiency in F. favus and P. sinuosa was lower in specimens with contracted tentacles. However, in the dark, no differences were found in the maximum quantum yield of photochemistry in PSII(Fv/Fm) of the expanded versus the contracted tentacles of any of the four species. This work suggests that species whose tentacles remain continuously expanded have either dense algal populations in their tentacles,as in G. lobata, or minute tentacles, like S. pistillata. Dense algal populations in tentacles allow harvesting of light while small tentacles do not scatter light or shade zooxanthellae in the underlying body of the polyp.

The cycling of ammonium between the cnidarian animal host and intracellular symbiotic algae was investigated in the zooxanthellate coral Stylophora pistillata , obtained from the Gulf of Eilat. Excretion of ammonium into the surrounding water by freshly collected corals was not detectable because of efficient recycling of inorganic nitrogen between the symbionts. Excretion into surrounding water was induced by incubation in the dark, by treatment with the photosynthetic electron transport inhibitor DCMU, and by treatment with azaserine, an inhibitor of glutamine 2-oxoglutarate amido transferase (GOGAT). Methionine sulphoxamine, an inhibitor of glutamine synthetase (GS), killed the animals. The three non-lethal treatments gave similar excretion rates of 0.0112 ± 0.0011 μmol NH + 4 cm -2 h -1 . Whole coral respiration, and animal glutamate dehydrogenase (GDH) activity, which reversibly catalyses the reductive deamination of glutamate, were also measured. The ratios of GDH activity: excretion and respiration: excretion were remarkably constant, averaging 6.2 ± 1.2 mol NH + 4 mol -1 NH + 4 and 57 ± 7 mol O 2 mol -1 NH + 4 respectively. Our results suggest that GDH activity and dark respiration rates may be used to estimate quantitatively the rate of ammonium excretion in S. pistillata . From knowledge of the nitrogen content per unit area of zooxanthellae and animal tissue, the mitotic index of the algae, and the calculated excretion rate of the animal, we constructed a nitrogen budget for the coral. Our calculations suggest that recycled nitrogen accounts for 90% of the zooxanthellae nitrogen demand, and that nitrogen turns over at a rate of 0.13 per day in the algae and 0.013 per day within the animal tissue.

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In oxygenic photosynthesizing organisms, it has been noticed on a number of occasions that photosynthetic performance was lower in the afternoon than in the morning, at the same light intensities. This hysteresis phenomenon is called the 'afternoon depression' and has been observed in phytoplankton, macroalgae, and higher plants. Here we characterize, with high temporal resolution, in situ diel courses of oxygen evolution and chlorophyll fluorescence yields in 3 Indo-Pacific corals (Favia favus, Goniopora lobata, Plerogyra sinuosa) and 2 Caribbean symbiotic corals (Montastrae faveolata and Porites astreoides) using a 3-chamber submersible respirometer and a SCUBAbased fast repetition rate fluorometer. In contrast to all previously published cases, the oxygen measurements revealed an unexpected hysteresis, with higher photosynthetic rates occurring in the afternoon than in the morning. The oxygen diel patterns were highly consistent in all organisms examined, including the 3 corals and 2 macroalgae (Ulva sp. and Gracilaria sp.) from the Red Sea. Surprisingly, the diurnal patterns of the quantum yields of photochemistry in Photosystem II (PSII), assessed from variable fluorescence (quantum yield of photochemistry in PSII measured under ambient light, ∆F '/F m ') exhibited much higher variability and often showed a hysteresis pattern opposite to that of oxygen. In most organisms the values of ∆F '/F m ' and the deduced rates of photosynthetic electron transport were higher in the morning than in the afternoon; however, the opposite trend with lower values of ∆F '/F m ' in the morning was also observed. Lower values of ∆F '/F m ' were always accompanied by higher quantum yields of non-photochemical quenching, consistent with the energetic balance within the primary photosynthetic reactions. The direction of the diurnal hysteresis in variable fluorescence appears to be species-specific and may vary even within the same species, reflecting microscale variability in bio-optical properties and gross photosynthesis of the corals.

Intertidal inhabitants are exposed to the 24-hour solar day, and the 12.4 hour rising and falling of the tides. One or both of these cycles govern intertidal organisms’ behaviour and physiology, yet little is known about the molecular clockworks of tidal rhythmicity. Here, we show that the limpet Cellana rota exhibits robust tidally rhythmic behaviour and gene expression. We assembled a de-novo transcriptome, identifying novel tidal, along with known circadian clock genes. Surprisingly, most of the putative circadian clock genes, lack a typical rhythmicity. We identified numerous tidally rhythmic genes and pathways commonly associated with the circadian clock. We show that not only is the behaviour of an intertidal organism in tune with the tides, but so too are many of its genes and pathways. These findings highlight the plasticity of biological timekeeping in nature, strengthening the growing notion that the role of ‘canonical’ circadian clock genes may be more fluid than previously thought, as exhibited in an organism which has evolved in an environment where tidal oscillations are the dominant driving force.

BackgroundWe present a multi-locus phylogenetic analysis of the shallow water (high intertidal) barnacle genus Chthamalus, focusing on member species in the western hemisphere. Understanding the phylogeny of this group improves interpretation of classical ecological work on competition, distributional changes associated with climate change, and the morphological evolution of complex cirripede phenotypes.Methodology and FindingsWe use traditional and Bayesian phylogenetic and ‘deep coalescent’ approaches to identify a phylogeny that supports the monophyly of the mostly American ‘fissus group’ of Chthamalus, but that also supports a need for taxonomic revision of Chthamalus and Microeuraphia. Two deep phylogeographic breaks were also found within the range of two tropical American taxa (C. angustitergum and C. southwardorum) as well.ConclusionsOur data, which include two novel gene regions for phylogenetic analysis of cirripedes, suggest that much more evaluation of the morphological evolutionary history and taxonomy of Chthamalid barnacles is necessary. These data and associated analyses also indicate that the radiation of species in the late Pliocene and Pleistocene was very rapid, and may provide new insights toward speciation via transient allopatry or ecological barriers.

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