Influence of dead coral substrate morphology on patterns of juvenile coral distribution

This study examines the abundances of three morphological categories of juvenile corals (massive, branching and encrusting) on two different types of natural substratum, dead massive and dead branching corals. The overall results show that the morphological characteristics of dead coral substratum have a significant influence on the coral recruitment patterns with respect to the morphology of the recruits: juvenile corals of massive and branching types were more abundant on substrates of corresponding morphology. The results obtained from this study suggest that dead coral might attract coral larvae that are morphologically similar. On the other hand, it may be the result of post-settlement mortality. Whatever the mechanism shaping the patterns is, it seems that the physical morphology of the dead coral substrate has a significant influence on the coral recruit assemblage. Hence, we suggest that substrate morphology can be an important qualitative factor for coral settlement and a possible determinant of community structure.     

The methane mussel: roles of symbiont and host in the metabolic utilization of methane

Methane mussels (Bathymodiolus sp., undescribed; personal communication by R. Turner to CRF) were collected in September 1989 and April 1990 from offshore Louisiana in the Gulf of Mexico. These mussels contain endosymbiotic methane-oxidizing bacteria and are capable of utilizing environmental methane as a source of energy and carbon. Oxygen consumption, methane consumption, and carbon dioxide production were measured in mussels with intact symbionts, functionally aposymbiotic mussels, and separated symbiont preparations under controlled oxygen and methane conditions, in order to study the roles of the symbionts and the hosts in methane utilization. The association was found to be very efficient in fixing methane carbon (only 30% of CH₄ consumed is released as CO₂), and to be capable of maximal rates of net carbon uptake of nearly 5 mol g^-1 h^-1. Rates of oxygen and methane consumption were dependent upon oxygen and methane concentrations. Maximal consumption rates were measured at 250 to 300 M O₂ and 200 to 300 M CH₄, under which conditions, oxygen consumption by the gill tissues (containing symbionts) had increased more than 50-fold over rates measured in the absence of methane. A model is proposed for the functioning of the intact association in situ, which shows the symbiosis to be capable of achieving growth rates (net carbon assimilation) in the range of 0.003 to 0.50% per day depending upon oxygen and methane concentrations. Under the conditions measured in the seep environment (200 M O₂, 60 M CH₄), a mussel consuming methane at rates found to be typical (4 to 5 mol g^-1 h^-1) should have a net carbon assimilation rate of about 0.1% per day. We suggest that the effectiveness of this symbiosis arises through integration of the morphological and physiological characteristics inherent to each of the symbiotic partners, rather than from extensive specialization exhibited by other deep-sea chemotrophic associations.     

Light harvesting in the green alga Ostreobium sp., a coral symbiont adapted to extreme shade

Ostreobium sp. (Chlorophyta: Siphonales) can be found as green bands within the skeletal material of a number of stony corals in the Indo-Pacific and Caribbean regions. Many of these corals also contain symbiotic dinoflagellates in the overlaying coral polyps that effectively screen out all the typical photosynthetically active radiation from the algae in the green bands below. Ostreobium sp., nevertheless, grows photosynthetically. Its action spectrum and absorption spectrum have been shown to extend much further into the near infra-red compared to other green algae. In the present study, carried out in 1987, fluorescence excitation and emission spectra were measured in Ostreobium sp. and compared to spectra obtained from the green alga Ulva sp. and the brown alga Endarachne sp. Xanthophylls, probably siphonein and an unidentified xanthophyll probably related to siphonaxanthin, are photosynthetically active in Ostreobium sp., and can sensitize Photosystem II fluorescence at 688 nm and Photosystem I (PS I) fluorescence at 718 nm. The fluorescence emission spectra of Ostreobium sp. measured at 25° C and 77 K were not remarkably different from those of the green alga Ulva sp. Absorbance changes induced by light were measured in Ostreobium sp. from 670 to 750 nm and were like those normally seen in green plants except that, in addition to the minimum expected for the reaction-center chlorophyll of PS I (P700) at 703 nm, another minimum was seen at 730 nm. It is possible that this spectrumreflects the functioning of a reaction center of Photosystem I that has adapted to function in light highly enriched in far-red wavelengths.CIW-DPB Publication No. 1021     

Movements between coral colonies in Trapezia ferruginea (Crustacea: Brachyura), an obligate symbiont of scleractinian corals

Trapezia ferruginea appears to depend entirely on its coral host (Pocillopora spp.) for shelter and food. Field and laboratory experiments have shown that the brightly colored crabs move between colonies at night. Analysis of movements in crabs randomly distributed among randomized Pocillopora damicornis colonies under laboratory conditions showed that after 6 days there was a significant increase in the number of heterosexual pairs (P<0.001) and a significant correlation between the number of crabs and the size of colonies (P=0.003), as well as between the size of crabs and the size of colonies (P=0.020) — conditions which are also observed in the field. No such correlations were obtained (P>0.05) in a similar experiment where very intense predation by carnivorous fishes was present. This host selection by adults appears to be elicited by the lack of a suitable partner and the inadequate size of their established territory in the colony.     

Impairment of symbiont photosynthesis increases host cell proliferation in the epidermis of the sea anemone Aiptasia pallida

Corals exposed to environmental stresses need to engage appropriate physiological strategies to survive. Here, we examined tissue modifications following algal dysfunction. Aiptasia pallida was exposed during 1 week to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), the herbicide called diuron. DCMU treatment produced a drastic loss in photosynthetic efficiency and a subsequent diminution of algae density over the following days. Cell proliferation evaluated by measuring the number of cells labeled with a thymidine analogue (EdU) revealed a significant increase in EdU+ cells in the epidermis after 1 week of DCMU incubation and in the gastrodermis at 4 weeks. TUNEL histology showed that the extent of cell death was, however, similar in the epidermis of control and treated specimens. In addition, we noticed a significant effect of DCMU treatment on the density of epidermal mucocytes after 1, 2 and 4 weeks. These results show that inhibition of Symbiodinium photosynthesis in the absence of any known direct effect of DCMU on host cells can induce an increase in epidermal host cell proliferation in both the epidermis and the gastrodermis. While new host gastrodermal cells are likely to promote tissue regeneration in order to recruit new algae, the new host epidermal cells may contribute to tissue adaptation following a decrease in energy income. Some of these new epidermal cells, such as mucocytes, may contribute to an eventual increase in the host heterotrophic ability until restoration of algal autotrophic contribution.     

Unravelling host and symbiont phylogenies of halichondrid sponges (Demospongiae, Porifera) using a mitochondrial marker

We present the first comparative phylogenetic analysis of a selected set of marine sponges and their bacterial associates. The Halichondrida form an important order in demosponge systematics and are of a particular interest due to the production of secondary metabolites. We sequenced a fragment of the cytochrome oxidase subunit 1 (CO1) gene of the sponges and their bacterial associates, compared the reconstructed phylogenies and found evidence for radiation in coevolution. The tree of six host-species associations showed four supported cospeciation events between the sponges and the bacteria. In addition, we present the first gene tree of sponges based on a mitochondrial marker. The tree shows major congruences with previous morphological studies and suggests the applicability of a mitochondrial marker in sponge molecular systematics.     

Plasticity in size and age at maturity in a monogamous fish: effect of host coral size and frequency dependence

The number and maximum body size of the gobioid fish, Paragobiodon echinocephalus, increase with the size of its obligate host coral, Stylophora. Only the largest two individuals breed monogamously in each coral head, and the reproductive success of each spawning is positively correlated with body size. In this study, the plasticity in size and age at maturity in P. echinocephalus was examined. We analyzed life history data from gobies 15–20 mm TL (total length) at their initial marking. Gobies found in larger corals were of lower rank in size order and began to breed later at a larger size, usually upon moving to other corals. The size at maturity ranged widely from 17.2 to 36.0 mm TL. After maturation, growth rates decreased. Mortality, however, was not affected by the timing of maturation. The host coral size did not affect the growth and mortality of marked fish, but the mortality rate of juveniles prior to marking appeared to be higher in smaller corals. The estimated lifetime reproductive success did not differ between the gobies inhabiting corals of different size. Thus the plasticity in size and age at maturity in this species may be maintained by frequency-dependent selection in choosing a host coral size that affects an individual’s social status. Received: 5 April 1995/Accepted after revision: 18 February 1996     

The microhabitat of Symbion pandora (Cycliophora) on the mouthparts of its host Nephrops norvegicus (Decapoda: Nephropidae)

The microhabitat of Symbion pandora (Cycliophora) was described by recording the prevalence and distribution of two life cycle stages, namely feeding individuals and chordoid cysts on the mouth appendages of 65 Norway lobsters. The commensals infested lobsters with a carapace length >35 mm, while the intensity of the commensals increased with host size up to more than 1,100 feeding individuals and 173 chordoid cysts. Feeding individuals and chordoid cysts were found on all six pairs of mouth appendages, but were rare on Mxp3. A Kruskal–Wallis analysis of variance showed that the distribution of the two stages over individual segments deviated significantly from random expectations (P<0.001). Feeding individuals densely aggregated on a few medial segments, generally those that are pervaded by suspended food particles during host feeding. Chordoid cysts, which arise from non-feeding females occurred in low numbers and were more evenly distributed over the segments, often aggregating on the lateral parts and in the articulations of the segments. Based on these findings we suggest that the feeding activity of S. pandora is synchronized with that of the host.     

Effects of dissolved ammonium addition and host feeding with Artemia salina on photoacclimation of the hermatypic coral Stylophora pistillata

 Effects of nutrient treatments on photoacclimation of the hermatypic coral Stylophora pistillata (Esper) were studied. Studies on photoacclimation of colonies from different light regimes in the field were evaluated and used to design laboratory experiments. Coral colonies were collected in the Gulf of Eilat (Israel) from January to March 1993. Exterior branches of colonies from different depths (1 to 40 m) displayed different trends in production characteristics at reduced and very low levels of illumination. From 24 ± 3% to 12 ± 2% of incident surface photosynthetic active radiation (PAR_o), zooxanthella population density and chlorophyll a+c per 10⁶ zooxanthellae increased, a trend seen in the range of light levels optimal for coral growth (90 to 30% PAR_o). The P _max of CO₂ per 10⁶ zooxanthellae decreased, while P _max of CO₂ per 10³ polyps increased, indicating an increase in zooxanthella population density at low light levels. Proliferous zooxanthella frequency (PZF, a measure of zooxanthella division) declined significantly at light levels <18 ± 3% PAR_o. At the lowest levels of illumination (<5% PAR_o), zooxanthella population density decreased, as did the PZF; chl a+c per 10⁶ zooxanthellae was unchanged. In 28-d experiments, exterior coral branches from the upper surfaces of colonies from 3 m depth (65 ± 4% PAR_o) were incubated in aquaria under bright (80 to 90% PAR_o), reduced (20 to 30% PAR_o), and extremely low (2 to 4% PAR_o) light intensities. At each light intensity, the corals were maintained in three feeding treatments: sea water (SW); ammonium enriched SW (SW + N); SW with Artemia salina nauplii (SW + A). An increase in P _max of CO₂ per 10³ polyps was found in corals acclimated to reduced light (20 to 30% PAR_o) in nutrient-enriched SW, while in SW, where the increase in zooxanthella population density was smaller, it did not occur. Nutrient enrichments (SW + N at 2 to 4% PAR_o and SW + A at 20 to 30% PAR_o) increased zooxanthella population density, but had no effect on chl a+c per 10⁶ zooxanthellae. Acclimation for 14 d to reduced (10 to 20% PAR_o) and extremely low (1 to 3% PAR_o) light intensities shifted ¹⁴C photoassimilation into glycerol and other compounds (probably glycerides), rather than sugars. Both ammonium addition and feeding with Artemia salina nauplii resulted in an increase in photosynthetic assimilation of ¹⁴C into amino acids. We conclude that acclimation to reduced light consists of two processes: an increase in photosynthetic pigments and in zooxanthella population density. Both processes require nitrogen, the increase in zooxanthella population density needing more; this adaptation is therefore limited in nitrogen-poor sea water. Received: 19 June 1998 / Accepted: 13 June 2000     

Jellyfish aggregations and leatherback turtle foraging patterns in a temperate coastal environment

Leatherback turtles (Dermochelys coriacea) are obligate predators of gelatinous zooplankton. However, the spatial relationship between predator and prey remains poorly understood beyond sporadic and localized reports. To examine how jellyfish (Phylum Cnidaria: Orders Semaeostomeae and Rhizostomeae) might drive the broad-scale distribution of this wide ranging species, we employed aerial surveys to map jellyfish throughout a temperate coastal shelf area bordering the northeast Atlantic. Previously unknown, consistent aggregations of Rhizostoma octopus extending over tens of square kilometers were identified in distinct coastal "hotspots" during consecutive years (2003-2005). Examination of retrospective sightings data (>50 yr) suggested that 22.5% of leatherback distribution could be explained by these hotspots, with the inference that these coastal features may be sufficiently consistent in space and time to drive long-term foraging associations.     

Growth,reproduction and nutrition of some Barents Sea hydromedusae in natural aggregations

Mass aggregations of the hydromedusae Tiaropsis multicirrata and Rathkea octopunctata have been studied in the Jarnyshnaya Fjord on the Kola Penninsula during the period of domination of the summer neritic plankton complex. The mass aggregations persist at least over periods of 2 months. No hyrometereological parameters could be established that would conceivably favour such aggregations. The factor most likely responsible for creating the aggregations is assumed to be related to the stable level of the summer zooplankton, production in the study area. The vast individual numbers of hydromedusae as well as their polyphageous feeding behaviour and high growth rates suggest that the medusae represent the most substantial consumers of plankton in inshore waters. The low and highly variable percent of medusae individuals which reveal food in their intestinal system lead the authors to assume additional food sources: nannoplankton, bacteria, and dissolved organic matter.     

Experimental ecology of the temperate scleractinian coral Astrangia danae I. Partition of respiration,photosynthesis and calcification between host and symbionts

Calcification, photosynthesis and respiration of the scleractinian coral Astrangia danae were calculated from the changes in total alkalinity, pH, calculated total CO₂, and oxygen concentration produced by colonies incubated in glass jars. A correction for changes in ammonia, nitrate and nitrite was taken into account and the method evaluated. The fluxes of oxygen and CO₂ were highly correlated (r=0.99). The statistical error of alkalinity determinations was less than 10% of the changes observed in the slowest calcifying samples. Metabolism of polyparium alone was estimated by difference after removal of tissue and reincubation of bare corallum. Zooxanthellae concentration in the polyps was obtained from cell counts made on homogenates of polyp tissue. The calculated photosynthetic rate of the zooxanthellae in vivo was 25 mol O₂ (10⁸ cell)^-1 h^-1 at a light intensity of 120 Ein m^-2 s^-1. In corals having 0.5x10⁹ zooxanthellae/dm² of colony area up to 8% of the total photosynthesis was attributed to the corallum microcosm. Polyp respiration, photosynthesis, and CaCO₃ uptake rates were all much higher than rates previously reported from A. danae, apparently because in these experiments the organisms were better fed. This increased photosynthesis in turn enhanced calcification still further. The symbiosis therefore appears to provide a growth advantage even to fed corals, under the conditions of these experiments.     

Reproductive biology of a small isopod symbiont living on a large isopod host: from the maternal marsupium to the protective grip of guarding males

Mating systems of many symbiotic crustaceans are characterised by a high degree of mate guarding. A peculiar case of mate guarding has been reported for small symbiotic janirid isopods where males mate with immature females. Field samples of individual hosts and laboratory experiments were conducted to reveal the mating behaviour of the symbiont in a natural environment, that is, on their hosts. Along the coast of the Magellan Strait, Chile, the janirid isopod Iais pubescens was frequently found on the shore-living isopod Exosphaeroma gigas. Symbiont prevalence (percent hosts occupied) was high at eight of the nine sampling sites. Mean symbiont intensity was very low at one site (<<1 individual host^-1), intermediate at two sites (1-10 individuals host^-1) and high at the other sites (10-40 individuals host^-1). The mean sex ratio (males:females) was male biased at most sampling sites (n=7). Females of I. pubescens reached substantially larger sizes (1.5-3.0 mm body length, BL) than males (1.1-1.9 mm BL). The majority of males were carrying small juveniles (66.15%), and males with juveniles were significantly larger than males without juveniles - this suggests that males prefer virgin juveniles to adult females and that they compete for small juveniles. In laboratory observations, males were seen to manipulate the marsupium of adult females that were about to release small juveniles. Males obtained virgin juveniles in this manner. Juveniles were carried for ~7 days, and they moulted shortly before being fertilised and released by males. The high proportion of juveniles carried by males in the field (68.2%) supports previous observations that males initially are not able to distinguish male and female juveniles. It is suggested that the mating system of symbiotic janirid isopods with long-term sperm storage and continuous receptivity in females and male mating with virgin females has evolved in response to highly unpredictable encounter probabilities between the sexes. Mate guarding and manipulation of small virgin juveniles may be favoured on the highly mobile hosts of symbiotic janirid isopods. Furthermore, adult females may gain by leaving their emerging offspring in the protective grip of guarding males, thereby reinforcing the maintenance of this peculiar mating system.     

Dimethylsulfoniopropionate and dimethylsulfide in the marine flatworm Convoluta roscoffensis and its algal symbiont

 A distinct smell of dimethylsulfide (DMS) was noted at the edge of the intertidal mudflat of Marennes-Oléron Bay, at the French Atlantic coast, where dense populations of the marine flatworm Convoluta roscoffensis Graff (Platyhelminthes: Turbellaria) were present. DMS is the cleavage product of dimethylsulfoniopropionate (DMSP). DMSP was shown to be present in high amounts in sediment containing the flatworm as well as in axenic cultures of the symbiotic alga Tetraselmis sp. that was isolated from the flatworm. In untreated sediment samples containing C. roscoffensis the concentration of DMS was as high as ∼55 μmol l⁻¹ sediment, and in samples that were fixed with glutaraldehyde the concentration of DMS was even three orders of magnitude higher (∼66 mmol l⁻¹ sediment). This rapid cleavage of DMSP to DMS in fixed samples was unexpected. Pure DMSP was stable in glutaraldehyde, and it was therefore concluded that a DMSP-lyase was responsible for cleavage in the field samples. The isolated symbiotic alga, Tetraselmis sp., did not show DMSP-lyase activity, indicating that DMSP-lyase may have been present in the flatworm, although the role of bacteria could not be excluded. The Chl a-specific DMSP content of C. roscoffensis (∼200 mmol g⁻¹) was much higher than that of Tetraselmis sp. (∼30 mmol g⁻¹). Possibly, DMSP was not only present in the symbiotic alga, but was also incorporated in the body tissue of the flatworm. It remains unclear what the function of DMSP is in C. roscoffensis. In Tetraselmis sp., but not in C. roscoffensis, DMSP increased with increasing salinity. It was concluded that salinity probably does not play an important role in the dynamics of DMSP and DMS in sediment containing C. roscoffensis. Received: 21 January 2000 / Accepted: 29 August 2000     

Communication and coexistence in a coral community

A large number of biotic relationships among dissimilar organisms is characteristic of tropical coral reefs. Such relationships involve at least four of the 16 species associated with the coral Pocillopora damicornis (L.) at Heron Island on the Great Barrier Reef. Behavioural interactions among the most conspicuous resident species (2 fish, 1 crab and 1 shrimp) were studied in the field under natural and experimental conditions. The inhabitants have the potential to harass, capture and eat, or exclude one another from the coral habitat but a system of signals among the residents facilitates coexistence and maintains the integrity of the community. The signals include contact and shivering movements by the fish which apparently inform the crustaceans of the fish's resident status.     

Seasonal breeding aggregations in low-density populations of the bathyal echinoid Stylocidaris lineata

Reproduction of bathyal cidaroid sea urchins was studied between 1985 and 1991 with the aid of a manned submersible in the northern Bahamas. During May 1988 and February 1990, discrete, single-species aggregations of Stylocidaris lineata containing 2 to 6 individuals were observed between 510 and 640 m depths. Reproductive conditions of isolated and aggregated individuals were determined histologically. In February, all individuals, regardless of their spatial distribution, contained developing gametes. During the May spawning season, most isolated individuals contained spent gonads with only relict gametes, whereas individuals in discrete aggregations mostly contained ripe gonads with mature or nearly-mature gametes. Aggregations were not observed during the autumn months, when gonads were spent or immature. These data suggest that bathyal cidaroids aggregate for reproduction. An in situ survey during May indicated that pairs of S. lineata are more common than larger clumps, but no more so than predicted by chance. Homosexual and heterosexual pairs occur at the relative frequencies predicted on the basis of sex ratio. We evaluated the need for reproductive aggregation by applying Denny's (1988) model that predicts downstream sperm concentration in a turbulent boundary layer. Flow parameters were measured with dye injected from the submersible, sperm-release rate was estimated in the laboratory, and the relationship between fertilization success and sperm concentration was determined in a laboratory dilution-experiment. The model predicted that, on smooth sandy bottoms at bathyal depths, sperm concentration should remain high enough to fertilize at least some eggs several meters directly downstream from a spawning male. Aggregation may facilitate spawning synchrony, increase gamete encounter probabilities, or cause gametes to be retained at high concentrations near the adults long enough for fertilization to occur.