Modes of regeneration and adaptation to soft‐bottom substrates of the free‐living solitary scleractinian Deltocyathoides orientalis

Scleractinian corals adapt to various substrate conditions with a variety of growth morphologies and modes of life. The azooxanthellate solitary scleractinian Deltocyathoides orientalis exhibits slightly flattened, bowl‐shaped corallites. This study describes in detail the modes of skeletal regeneration after fragmentation in association with exquisitely adaptive strategies of the corals for life on soft substrates. Larger fragments of individuals retaining almost two‐thirds to five‐sixths of the original skeletal area inherit the densely dilated, lower central skeleton, so as to keep a stable life position on soft substrates and regenerate the lost parts promptly. Even highly fragmented individuals preserving less than 10% of the original skeleton still regenerate and repair. Fragmented individuals with almost one‐sixth to one‐third original skeleton actively maintain a posture with the oral disc upward using movements of remaining tentacles. Damaged and missing soft tissues are then efficiently regenerated to form a mouth and gastrovascular cavity near the new centre of the corallum. Every regenerated individual reuses skeleton and soft tissues, and is capable of burrowing before the completion of growth morphology. The mode of regeneration characteristic of D. orientalis is thus effective and adaptive for maintenance of a stable life position on soft substrates for this solitary scleractinian. As fragmentation in deeper‐water, soft‐bottom settings is likely due to predation rather than turbulence, the rapid corallum regeneration and burrowing strategy may both represent adaptive strategies for life on soft substrates and exploitation of new niches, such as an infaunal mode of life, in a predator‐rich environment.     

Comprehensive characterization of skeletal tissue growth anomalies of the finger coral Porites compressa

The scleractinian finger coral Porites compressa has been documented to develop raised growth anomalies of unknown origin, commonly referred to as “tumors”. These skeletal tissue anomalies (STAs) are circumscribed nodule-like areas of enlarged skeleton and tissue with fewer polyps and zooxanthellae than adjacent tissue. A field survey of the STA prevalence in Oahu, Kaneohe Bay, Hawaii, was complemented by laboratory analysis to reveal biochemical, histological and skeletal differences between anomalous and reference tissue. MutY, Hsp90a1, GRP75 and metallothionein, proteins known to be up-regulated in hyperplastic tissues, were over expressed in the STAs compared to adjacent normal-appearing and reference tissues. Histological analysis was further accompanied by elemental and micro-structural analyses of skeleton. Anomalous skeleton was of similar aragonite composition to adjacent skeleton but more porous as evidenced by an increased rate of vertical extension without thickening. Polyp structure was retained throughout the lesion, but abnormal polyps were hypertrophied, with increased mass of aboral tissue lining the skeleton, and thickened areas of skeletogenic calicoblastic epithelium along the basal floor. The latter were highly metabolically active and infiltrated with chromophore cells. These observations qualify the STAs as hyperplasia and are the first report in poritid corals of chromophore infiltration processes in active calicoblastic epithelium areas.     

Skeletal growth, ultrastructure and composition of the azooxanthellate scleractinian coral Balanophyllia regia

The biomineralization process and skeletal growth dynamics of azooxanthellate corals are poorly known. Here, the growth rate of the shallow-water dendrophyllid scleractinian coral Balanophyllia regia was evaluated with calcein-labeling experiments that showed higher lateral than vertical extension. The structure, mineralogy and trace element composition of the skeleton were characterized at high spatial resolution. The epitheca and basal floor had the same ultrastructural organization as septa, indicating a common biological control over their formation. In all of these aragonitic skeletal structures, two main ultrastructural components were present: “centers of calcification” (COC) also called rapid accretion deposits (RAD) and “fibers” (thickening deposits, TD). Heterogeneity in the trace element composition, i.e., the Sr/Ca and Mg/Ca ratios, was correlated with the ultrastructural organization: magnesium was enriched by a factor three in the rapid accretion deposits compared with the thickening deposits. At the interface with the skeleton, the skeletogenic tissue (calicoblastic epithelium) was characterized by heterogeneity of cell types, with chromophile cells distributed in clusters regularly spaced between calicoblasts. Cytoplasmic extensions at the apical surface of the calicoblastic epithelium created a three-dimensional organization that could be related to the skeletal surface microarchitecture. Combined measurements of growth rate and skeletal ultrastructural increments suggest that azooxanthellate shallow-water corals produce well-defined daily growth steps.     

Perspectives in coral reef hydrodynamics

Knowledge of skeletogenesis in scleractinian corals is central to reconstructing past ocean and climate histories, assessing and counteracting future climate and ocean acidification impacts upon coral reefs, and determining the taxonomy and evolutionary path of the Scleractinia. To better understand skeletogenesis and mineralogy in extant scleractinian corals, we have investigated the nature of the initial calcium carbonate skeleton deposited by newly settling coral recruits. Settling Acropora millepora larvae were sampled daily for 10 days from initial attachment, and the carbonate mineralogy of their newly deposited skeletons was investigated. Bulk analyses using Raman and infrared spectroscopic methods revealed that the skeletons were predominantly comprised of aragonite, with no evidence of calcite or an amorphous precursor phase, although presence of the latter cannot be discounted. Sensitive selected area electron diffraction analyses of sub-micron areas of skeletal regions further consolidated these data. These findings help to address the uncertainty surrounding reported differences in carbonate mineralogy between larval and adult extant coral skeletons by indicating that skeletons of new coral recruits share the same aragonitic mineralogy as those of their mature counterparts. In this respect, we can expect that skeletogenesis in both larval and mature growth stages of scleractinian corals will be similarly affected by ocean acidification and predicted environmental changes.     

寒武纪早期帽状动物壳体Lathamellids类的微细构造及其系统分类

帽状动物壳体 lathamellids 类化石包括 Lathamella caeca, Lathamella sp. nov. 以及 Lathamella symmetrica sp. nov. 三种,目前仅发现于四川峨嵋麦地坪下寒武统麦地坪组上段地层,它们皆以磷质内核方式保存.通过内核化石表面所复制的原始壳体微细构造印痕的研究表明,易漏螺类壳体为双层结构:内层——纤晶层,外层——棱柱层;其壳体原始成分可能为文石质.据上述现象推断,lathamellids 并非为磷质无绞纲腕足类,相反,而与软体动物更为接近,但其在壳腔内具—纵向中突,主要位于壳顶附近,很难与已知的任何一类软体动物直接对比,可能为一类独特的、已经绝灭了的软体动物的1个早期分支.     

Skeletal development inAcropora cervicornis

Scanning electron microscopy, field studies using dyes which become incorporated into the skeleton of living corals as time markers, and petrographic and mineralogic techniques were used to describe the diel pattern of calcium carbonate accretion in the extending axial corallite ofAcropora cervicornis. The axial corallite extends by the formation of randomly oriented fusiform crystals at the distal tip of the branch. Morphological and mineralogical characteristics suggest that these might be calcite crystals. They form a framework upon which needle-like aragonite crystals (initially small tufts) begin to grow. As the needles elongate, groups of them form well defined bundles, fasciculi, which compose the primary skeletal elements. There is a diel pattern in the deposition of the skeleton. At night (1800–0600 hours) the distal spines are pointed and composed primarily of fusiform crystals. During the day (0600–1800 hours) mineral accretion occurs on all surfaces of the skeleton, apparently by epitaxial growth on the aragonite needles of the fasciculi.     

Molecular and mineral responses of corals grown under artificial Calcite Sea conditions

The formation of skeletal structures composed of different calcium carbonate polymorphs (e.g. aragonite and calcite) appears to be both biologically and environmentally regulated. Among environmental factors influencing aragonite and calcite precipitation, changes in seawater conditions—primarily in the molar ratio of magnesium and calcium during so-called ‘Calcite’ (mMg:mCa below 2) or ‘Aragonite’ seas (mMg:mCa above 2)—have had profound impacts on the distribution and performance of marine calcifiers throughout Earth's history. Nonetheless, the fossil record shows that some species appear to have counteracted such changes and kept their skeleton polymorph unaltered. Here, the aragonitic octocoral Heliopora coerulea and the aragonitic scleractinian Montipora digitata were exposed to Calcite Sea-like mMg:mCa with various levels of magnesium and calcium concentration, and changes in both the mineralogy (i.e. CaCO₃ polymorph) and gene expression were monitored. Both species maintained aragonite deposition at lower mMg:mCa ratios, while concurrent calcite presence was only detected in M. digitata. Despite a strong variability between independent experimental replicates for both species, the expression for a set of putative calcification-related genes, including known components of the M. digitata skeleton organic matrix (SkOM), was found to consistently change at lower mMg:mCa. These results support the previously proposed involvements of the SkOM in counteracting decreases in seawater mMg:mCa. Although no consistent expression changes in calcium and magnesium transporters were observed, down-regulation calcium channels in H. coerulea in one experimental replicate and at an mMg:mCa of 2.5, pointing to a possible active calcium uptake regulation by the corals under altered mMg:mCa.     

Attachment structures in Rhizotrochus (Scleractinia): macro‐ to microscopic traits and their evolutionary significance

Marine sessile benthic organisms living on hard substrates have evolved a variety of attachment strategies. Rhizotrochus (Scleractinia, Flabellidae) is a representative azooxanthellate solitary scleractinian coral with a wide geographical distribution and unique attachment structures; it firmly attaches to hard substrates using numerous tube‐like rootlets, which are extended from a corallum wall, whereas most sessile corals are attached by stereome‐reinforced structures at their corallite bases. Detailed morphological and constructional traits of the rootlets themselves, along with their evolutionary significance, have not yet been fully resolved. Growth and developmental processes of spines in Truncatoflabellum and rootlets in Rhizotrochus suggest that these structures are homologous, as they both develop from the growth edges of walls and are formed by transformation of wall structures and their skeletal microstructures possess similar characteristics, such as patterns of rapid accretion and thickening deposits. Taking molecular phylogeny and fossil records of flabellids into consideration, Rhizotrochus evolved from a common free‐living ancestor and invaded hard‐substrate habitats by exploiting rootlets of spines origin, which were adaptive for soft‐substrate environments.     

Isolation of Bacteria from the Corallum of Porites lobata (Vaughn) and Its Possible Significance

Bacteria were recovered from loci within skeletal regions ofthe glomerate coral,Porites lobata. The origin of these bacteriais unknown, although areas of discoloration suggest invasionfrom the substratum in the region of basal attachment. Weakenedareas of internal corallum contained from 10⁴ to 10⁵ bacteriaper gram dry weight as determined by plate counts on a peptone-agarmedium. Some of the isolated bacteria were capable of digestingchitin in vitro. This rinding suggested that the mechanism forskeletal weakening might be bacterial breakdown of the organicmatrix. Absence of change from aragonitic to calcitic crystalsfrom a discolored region supported the contention that skeletalweakening was due to the breakdown of organic matrix ratherthan dissolution of carbonate. Results were obtained as part of a survey to determine the numberand distribution of bacteria in some coral reef environments.     

Morphology,microstructure, crystallography,and chemistry of distinct CaCO3 deposits formed by early recruits of the scleractinian coral Pocillopora damicornis

Scleractinian corals begin their biomineralization process shortly after larval settlement with the formation of calcium carbonate (CaCO₃) structures at the interface between the larval tissues and the substrate. The newly settled larvae exert variable degrees of control over this skeleton formation, providing an opportunity to study a range of biocarbonate structures, some of which are transient and not observed in adult coral skeletons. Here we present a morphological, structural, crystallographic, and chemical comparison between two types of aragonite deposits observed during the skeletal development of 2‐days old recruits of Pocillopora damicornis: (1) Primary septum and (2) Abundant, dumbbell‐like structures, quasi‐randomly distributed between initial deposits of the basal plate and not present in adult corals—At the mesoscale level, initial septa structures are formed by superimposed fan‐shaped fasciculi consisting of bundles of fibers, as also observed in adult corals. This organization is not observed in the dumbbell‐like structures. However, at the ultrastructural level there is great similarity between septa and dumbbell components. Both are composed of <100 nm granular units arranged into larger single‐crystal domains.Chemically, a small difference is observed between the septae with an average Mg/Ca ratio around 11 mmol/mol and the dumbbell‐like structures with ca. 7 mmol/mol; Sr/Ca ratios are similar in the two structures at around 8 mmol/mol—Overall, the observed differences in distribution, morphology, and chemistry between septa, which are highly conserved structures fundamental to the architecture of the skeleton, and the transient, dumbbell‐like structures, suggest that the latter might be formed through less controlled biomineralization processes. Our observations emphasize the inherent difficulties involved in distinguishing different biomineralization pathways based on ultrastructural and crystallographical observations. J. Morphol. 276:1146–1156, 2015. © 2015 Wiley Periodicals, Inc.     

Clypeotheca,a new skeletal structure in scleractinian corals: a potential stress indicator

Physiological responses to environmental stress are increasingly well studied in scleractinian corals. This work reports a new stress-related skeletal structure we term clypeotheca. Clypeotheca was observed in several live-collected common reef-building coral genera and a two to three kya subfossil specimen from Heron Reef, Great Barrier Reef and consists of an epitheca-like skeletal wall that seals over the surface of parts of the corallum in areas of stress or damage. It appears to form from a coordinated process wherein neighboring polyps and adjoining coenosarc seal themselves off from the surrounding environment as they contract and die. Clypeotheca forms from inward skeletal centripetal growth at the edges of corallites and by the merging of flange-like outgrowths that surround individual spines over the surface of the coenosteum. Microstructurally, the merged flanges are similar to upside-down dissepiments and true epitheca. Clypeotheca is interpreted primarily as a response to stress that may help protect the colony from invasion of unhealthy tissues by parasites or disease by retracting tissues in areas that have become unhealthy for the polyps. Identification of skeletal responses of corals to environmental stress may enable the frequency of certain types of environmental stress to be documented in past environments. Such data may be important for understanding the nature of reef dynamics through intervals of climate change and for monitoring the effects of possible anthropogenic stress in modern coral reef habitats. Communicated by Geology Editor Dr Bernhard Riegl     

Fine-scale mapping of physicochemical and microbial landscapes of the coral skeleton

The coral skeleton harbours a diverse community of bacteria and microeukaryotes exposed to light, O₂ and pH gradients, but how such physicochemical gradients affect the coral skeleton microbiome remains unclear. In this study, we employed chemical imaging of O₂ and pH, hyperspectral reflectance imaging and spatially resolved taxonomic and inferred functional microbiome characterization to explore links between the skeleton microenvironment and microbiome in the reef-building corals Porites lutea and Paragoniastrea benhami. The physicochemical environment was more stable in the deep skeleton, and the diversity and evenness of the bacterial community increased with skeletal depth, suggesting that the microbiome was stratified along the physicochemical gradients. The bulk of the coral skeleton was in a low O₂ habitat, whereas pH varied from pH 6–9 with depth. Physicochemical gradients of O₂ and pH of the coral skeleton explained the β-diversity of the bacterial communities, and skeletal layers that showed O₂ peaks had a higher relative abundance of endolithic algae, reflecting a link between the abiotic environment and the microbiome composition. Our study links the physicochemical, microbial and functional landscapes of the coral skeleton and provides new insights into the involvement of skeletal microbes in the coral holobiont metabolism.     

Electron backscatter diffraction (EBSD) as a tool for detection of coral diagenesis

Fine-scale structures of intact modern and fossil coralline skeletons were analysed to determine alteration to secondary cements and phases using electron backscatter diffraction (EBSD). EBSD analysis revealed secondary aragonite cements in endolithic borings in the modern skeleton and whole dissepiments of the fossil skeleton replaced by calcite, despite X-ray diffraction (XRD) bulk analysis of the general area suggesting only aragonite was present. Non-destructive, in situ screening of coral samples by EBSD analysis provides a valuable tool for assessing the extent of alteration and can determine which areas may produce more reliable climate proxy data. Communicated by Geology Editor Dr. Bernhard Riegl     

Stratigraphic analysis of a fossil Neogoniolithon-capped patch reef and associated facies,San Salvador,Bahamas

An unusual Pleistocene patch reef is exposed in a coastal cliff at Grotto Beach, San Salvador, Bahamas. The reef is a coralline framestone constructed mainly by Porites astreoides together with a few large heads of Diploria strigosa and Montastrea annularis, and is capped by a dense thicket of Neogoniolithon strictum that is interpreted as marking the subtidal/intertidal boundary. The reef is flanked to the northeast by laminated to low-angle cross-laminated intraclastic grainstones and to the southwest by skeletal rudstone of reefal and interreefal derivation. Uranium-series dating of pure aragonite from a Diploria corallum yielded an age of 123 000±9000 years. Reef growth began on an erosional surface underlain by steeply crossbedded eolian grainstone. As the reef grew upward, it also grew laterally over adjacent penecontemporaneous subtidal sediments. The reef was eventually buried by 2.3 m of shallow subtidal and beach sediments that apparently prograded seaward during a highstand, or possibly while sea level was still rising. The shallow subtidal sediments are mainly peloidal, ooidal and skeletal grainstones that are pervasively bioturbated. The overlying beach facies comprises predominantly laminated, sparsely burrowed grainstone. The beach and shallow subtidal facies contain boulders of fine-grained laminated grainstone that are interpreted as storm-tossed blocks of beachrock. Living analogs of the Grotto Beach fossil reef lie off East Beach, San Salvador. Several of these have a flourishing cap of Neogoniolithon that extends above low-tide level and we believe that the Neogoniolithon cap of Grotto Beach reef did likewise. Wherever found in the stratigraphic record this facies should serve to identify the subtidal/intertidal boundary. The uppermost Pleistocene beach sediments associated with Grotto Beach fossil reef lie 5.8 m above present-day mean sea level, which ist strong evidence that this portion of San Salvador has undergone little subsidence since the Grotto Beach section was deposited.     

Appendicular skeletal morphology in minute salamanders,genus Thorius (amphibia: Plethodontidae): Growth regulation,adult size determination,and natural variation

Patterns of growth and variation of the appendicular skeleton were examined in Thorius, a speciose genus of minute terrestrial plethodontid salamanders from southern Mexico. Observations were based primarily on ontogenetic series of each of five species that collectively span the range of adult body size in the genus; samples of adults of each of seven additional species provided supplemental estimates of the full range of variation of limb skeletal morphology. Limbs are generally reduced, i.e., pedomorphic, in both overall size and development, and they are characterized by a pattern of extreme variation in the composition of the limb skeleton, especially mesopodial elements, both within and between species. Fifteen different combinations of fused carpal or tarsal elements are variably present in the genus, producing at least 18 different overall carpal or tarsal arrangements, many of which occur in no other plethodontid genus. As many as four carpal or tarsal arrangements were observed in single population samples of each of several; five tarsal arrangements were observed in one population of T. minutissimus. Left-right asymmetry of mesopodial arrangement in a given specimen is also common. In contrast, several unique, nonpedomorphic features of the limb skeleton, including ossification of the typically cartilaginous adult mesopodial elements and ontogenetic increase in the degree of ossification of long bones, are characteristic of all species and distinguish Thorius from most related genera. They form part of a mechanism of determinate skeletal growth that restricts skeletal growth after sexual maturity. Interspecific differences in the timing of the processes of appendicular skeletal maturation relative to body size are well correlated with interspecific differences in mean adult size and size at sexual maturity, suggesting that shifts in the timing of skeletal maturation provide a mechanism of achieving adult size differentiation among species. Processes of skeletal maturation that confer determinate skeletal growth in Thorius are analogous to those typical of most amniotes – both groups exhibit ontogenetic reduction and eventual disappearance of the complex of stratified layers of proliferating and maturing cartilage in long bone epiphyses – but, unlike most amniotes, Thorius lacks secondary ossification centers. Thus, the presence of secondary ossification centers cannot be used as a criterion for establishing determinate skeletal growth in all vertebrates.     

Fluorescent and skeletal density banding in Porites lutea from Papua New Guinea and Indonesia

Shallow water Porites lutea corals were collected along two transects normal to mainland shorelines, parallel to gradients in water quality: one, 7 km long, near Motupore Island in South Papua New Guinea, the other, 70 km long, from Jakarta Bay along the Pulau Seribu chain in the Java Sea. The corals were slabbed and studies were made of skeletal density bands as revealed by X-ray photography and fluorescent bands as revealed by ultraviolet light. Water quality measurements and rain-fall data were assembled for the two areas and related to skeletal banding patterns. For both areas, with increasing distance form mainland there is a decrease in overall brightness of fluorescence in corals and an increase in the contrast between bright and dull fluorescent bands. Fluorescence is bright, but seasonal banding is obscure in corals within about 2 km of stream mouths at Motopure and about 5 km of the coast in Jakarta Bay; this suggests that, despite low freshwater run-off during dry seasons, there are sufficient organic compounds which cause fluorescence in coral skeletons, to swamp seasonal effects. During the wet seasons, deluges of freshwater consequent on mainland rainfall of greater than about 150 mm/ month extend at least 7 km offshore in the Motupore area and perhaps tens of kilometres into Java Sea, giving distinctive bright and dull fluorescent banding in off-shore corals. The fluorescent banding pattern within corals on the Motupore reefs is similar in most corals along the transect and it correlates well with the Port Moresby monthly rainfall data. This relationship suggests that the same body (or bodies) of freshwater affect all reefs of the area during the wet season. The fluorescent banding in Java Sea corals does not show a precise correlation with either mainland or island monthly rainfall data; indeed the pattern of fluorescent banding on Pulau Seribu can only be matched in corals from reefs less than about 25 km apart. This suggests that in this area discrete water bodies carrying the relevant organic acids for coral fluorescence affect the fringing reefs on the chain of islands. Comparisons of fluorescent and density banding have revealed that for these areas, in general, periods of high freshwater run-off are times of deposition of less dense skeleton in Porites lutea corals.     

Calcite Formation in Soft Coral Sclerites Is Determined by a Single Reactive Extracellular Protein

Calcium carbonate exists in two main forms, calcite and aragonite, in the skeletons of marine organisms. The primary mineralogy of marine carbonates has changed over the history of the earth depending on the magnesium/calcium ratio in seawater during the periods of the so-called “calcite and aragonite seas.” Organisms that prefer certain mineralogy appear to flourish when their preferred mineralogy is favored by seawater chemistry. However, this rule is not without exceptions. For example, some octocorals produce calcite despite living in an aragonite sea. Here, we address the unresolved question of how organisms such as soft corals are able to form calcitic skeletal elements in an aragonite sea. We show that an extracellular protein called ECMP-67 isolated from soft coral sclerites induces calcite formation in vitro even when the composition of the calcifying solution favors aragonite precipitation. Structural details of both the surface and the interior of single crystals generated upon interaction with ECMP-67 were analyzed with an apertureless-type near-field IR microscope with high spatial resolution. The results show that this protein is the main determining factor for driving the production of calcite instead of aragonite in the biocalcification process and that –OH, secondary structures (e.g. α-helices and amides), and other necessary chemical groups are distributed over the center of the calcite crystals. Using an atomic force microscope, we also explored how this extracellular protein significantly affects the molecular-scale kinetics of crystal formation. We anticipate that a more thorough investigation of the proteinaceous skeleton content of different calcite-producing marine organisms will reveal similar components that determine the mineralogy of the organisms. These findings have significant implications for future models of the crystal structure of calcite in nature.     

Simultaneous extension of both basic microstructural components in scleractinian coral skeleton during night and daytime,visualized by in situ 86Sr pulse labeling

Using in situ (12 h) pulse-labeling of scleractinian coral aragonitic skeleton with stable ⁸⁶Sr isotope, the diel pattern of skeletal extension was investigated in the massive Porites lobata species, grown at 5 m depth in the Gulf of Eilat. Several microstructural aspects of coral biomineralization were elucidated, among which the most significant is simultaneous extension of the two basic microstructural components Rapid Accretion Deposits (RAD; also called Centers of Calcification) and Thickening Deposits (TD; also called fibers), both at night and during daytime. Increased thickness of the ⁸⁶Sr-labeled growth-front in the RADs compared to the adjacent TDs revealed that in this species RADs extend on average twice as fast as TDs. At the level of the individual corallite, skeletal extension is spatially highly heterogeneous, with sporadic slowing or cessation depending on growth directions and skeletal structure morphology. Daytime photosynthesis by symbiotic dinoflagellates is widely acknowledged to substantially increase calcification rates at the colony and the corallite level in reef-building corals. However, in our study, the average night-time extension rate (visualized in three successive 12 h pulses) was similar to the average daytime extension (visualized in the initial 12 h pulse), in all growth directions and skeletal structures. This research provides a platform for further investigations into the temporal calibration of coral skeletal extension via cyclic growth increment deposition, which is a hallmark of coral biomineralization.     

Occurrence and Distribution of Stony Corals in the Gulf of Cariaco,Venezuela

Occurrence and distribution of stony corals (Milleporina and Scleractinia) in the Gulf of Cariaco were investigated quantitatively using skin-diving methods. Unusual temperature conditions due to cold upwelling limit coral growth to a depth of 15 m. A clear vertical zonation of species was observed, with five zones, in order of increasing depth, dominated by Porites porites, Millepora complanata, Siderastrea siderea, Dichocoenia stokesi, and Solenastrea hyades. Twenty-one species of scleractinians and three species of hydrocorals have so far been found in the gulf. Of the total area covered by corals, over 50% is inhabited by Siderastrea siderea, Millepora spp., and Porites porites. A horizontal zonation of the littoral was established based on species composition and density of the coral cover.