Corals, including some major extinct groups Rugosa and Tabulata, have been important reef builders through much of the Phanerozoic since the Ordovician Period. However, other organism groups, such as calcifying algae, especially members of the red algae Rhodophyta, and molluscs (especially the rudist bivalves during the Cretaceous Period) have created massive structures at various times. During the Cambrian Period, the conical or tubular skeletons of Archaeocyatha, an extinct group of uncertain affinities (possibly sponges), built reefs. Other groups, such as the Bryozoa have been important interstitial organisms, living between the framework builders. The corals which build reefs today, the Scleractinia, arose after the Permian–Triassic extinction event that wiped out the earlier rugose corals (as well as many other groups), and became increasingly important reef builders throughout the Mesozoic Era. They may have arisen from a rugose coral ancestor. Rugose corals built their skeletons of calcite and have a different symmetry from that of the scleractinian corals, whose skeletons are aragonite. However, there are some unusual examples of well-preserved aragonitic rugose corals in the late Permian. In addition, calcite has been reported in the initial post-larval calcification in a few scleractinian corals. Nevertheless, scleractinian corals (which arose in the middle Triassic) may have arisen from a non-calcifying ancestor independent of the rugosan corals (which disappeared in the late Permian).
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Blowhole Cliffed coast Coastal biogeomorphology Coastal erosion Concordant coastline Current Cuspate foreland Discordant coastline Emergent coastline Feeder bluff Fetch Flat coast Graded shoreline Headlands and bays Ingression coast Large-scale coastal behaviour Longshore drift Marine regression Marine transgression Raised shoreline Rip current Rocky shore Sea cave Sea foam Shoal Steep coast Submergent coastline Surf break Surf zone Surge channel Swash Undertow Volcanic arc Wave-cut platform Wave shoaling Wind wave Wrack zone
Along with the blacktip reef shark (C. melanopterus) and the whitetip reef shark (Triaenodon obesus), the grey reef shark is one of the three most common sharks inhabiting Indo-Pacific reefs. They actively expel most other shark species from favored habitats, even species larger in size.[3] In areas where this species co-exists with the blacktip reef shark, the latter species occupies the shallow flats, while the former stays in deeper water.[4] Areas with a high abundance of grey reef sharks tend to contain few sandbar sharks (C. plumbeus), and vice versa; this may be due to their similar diets causing competitive exclusion.[11]
Juvenile Caribbean reef sharks are preyed upon by larger sharks such as the tiger shark (Galeocerdo cuvier) and the bull shark (C. leucas). Few parasites are known for this species; one is a dark variegated leech often seen trailing from its first dorsal fin.[4] Off northern Brazil, juveniles seek out cleaning stations occupied by yellownose gobies (Elacatinus randalli), which clean the sharks of parasites while they lie still on the bottom.[10] Horse-eye jacks (Caranx latus) and bar jacks (Carangoides ruber) routinely school around Caribbean reef sharks.[11]
Another danger posed to humans by the Caribbean reef shark involves the accumulation of toxins in the flesh of the shark. Since sharks are apex marine predators, they may contain toxic levels of mercury and other heavy metals due to bioaccumulation (increasing concentrations at higher levels in the food web). It was found that methylmercury levels (MeHg) in sharks off the coast of Florida were higher than the FDA guidelines.
The snout is rather short, broad, and rounded, without prominent flaps of skin beside the nostrils. The eyes are large and circular, with nictitating membranes (protective third eyelids). There are 11–13 tooth rows in either half of both jaws. The teeth have broad bases, serrated edges, and narrow cusps; the front 2–4 teeth on each side are erect and the others increasingly oblique. The five pairs of gill slits are moderately long, with the third gill slit over the origin of the pectoral fins.[4] The first dorsal fin is high and falcate (sickle-shaped). There is a low interdorsal ridge running behind it to the second dorsal fin, which is relatively large with a short free rear tip. The origin of the first dorsal fin lies over or slightly forward of the free rear tips of the pectoral fins, and that of the second dorsal fin lies over or slightly forward of the anal fin. The pectoral fins are long and narrow, tapering to a point.[2] The dermal denticles are closely spaced and overlapping, each with five (sometimes seven in large individuals) horizontal low ridges leading to marginal teeth.[4]
Blowhole Cliffed coast Coastal biogeomorphology Coastal erosion Concordant coastline Current Cuspate foreland Discordant coastline Emergent coastline Feeder bluff Fetch Flat coast Graded shoreline Headlands and bays Ingression coast Large-scale coastal behaviour Longshore drift Marine regression Marine transgression Raised shoreline Rip current Rocky shore Sea cave Sea foam Shoal Steep coast Submergent coastline Surf break Surf zone Surge channel Swash Undertow Volcanic arc Wave-cut platform Wave shoaling Wind wave Wrack zone
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