The coloration is grey above, sometimes with a bronze sheen, and white below. The entire rear margin of the caudal fin has a distinctive, broad, black band. There are dusky to black tips on the pectoral, pelvic, second dorsal, and anal fins.[9] Individuals from the western Indian Ocean have a narrow, white margin at the tip of the first dorsal fin; this trait is usually absent from Pacific populations.[5] Grey reef sharks that spend time in shallow water eventually darken in color, due to tanning.[10] Most grey reef sharks are less than 1.9 m (6.2 ft) long.[4] The maximum reported length is 2.6 m (8.5 ft) and the maximum reported weight is 33.7 kg (74 lb).[9]
The Caribbean Reef Shark is known to become aggressive in the presence of food, but they are mostly only considered dangerous to humans because of its size. This shark was fished in Belize for almost the entire 20th century. They were used to make local delicacies in addition to liver oil (mostly used in cosmetics). Their low reproduction rate combined with a high level of hunting and fishing have caused the numbers to dwindle. The shark is now considered to be near threatened. Many countries and organizations have banned the commercial fishing of this species.
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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Despite its abundance in certain areas, the Caribbean reef shark is one of the least-studied large requiem sharks. They are believed to play a major role in shaping Caribbean reef communities. These sharks are more active at night, with no evidence of seasonal changes in activity or migration. Juveniles tend to remain in a localized area throughout the year, while adults range over a wider area.[7]
Blacktip reef sharks, Carcharhinus melanopterus (Quoy and Gaimard, 1824), are small sharks measuring up to 1.8 m with short, bluntly-rounded snouts, oval eyes, and narrow-cusped teeth. They have 2 dorsal fins and no interdorsal ridges. Juveniles (< 70 cm) are yellow-brown on their dorsal (upper) sides, white on their ventral (under) sides; adults are brownish-gray and white, respectively. All their fins have conspicuous black or dark brown tips, and posterior (rear) dark edges on their pectoral fins and their upper lobe of their caudal (tail) fins. The prominent black tips of their first dorsal fin contrasts with a light band below it; a conspicuous dark band on their flanks which extends to their pelvic fins. Maximum weight: 24 kg; frequents depth ranges from the surface to 75 m.
Off Enewetak, grey reef sharks exhibit different social behaviors on different parts of the reef. Sharks tend to be solitary on shallower reefs and pinnacles. Near reef drop-offs, loose aggregations of five to 20 sharks form in the morning and grow in number throughout the day before dispersing at night. In level areas, sharks form polarized schools (all swimming in the same direction) of around 30 individuals near the sea bottom, arranging themselves parallel to each other or slowly swimming in circles. Most individuals within polarized schools are females, and the formation of these schools has been theorized to relate to mating or pupping.[25][26]
There is a variety of biotic reef types, including oyster reefs and sponge reefs, but the most massive and widely distributed are tropical coral reefs. Although corals are major contributors to the framework and bulk material comprising a coral reef; the organisms most responsible for reef growth against the constant assault from ocean waves are calcareous algae, especially, although not entirely, coralline algae.
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