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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Coral reef | 6/13 | https://en.wikipedia.org/wiki/Coral_reef | reference | science, encyclopedia | 2026-05-05T07:34:41.179437+00:00 | kb-cron |
Corals reproduce both sexually and asexually. An individual polyp uses both reproductive modes within its lifetime. Corals reproduce sexually by either internal or external fertilization. The reproductive cells are found on the mesenteries, membranes that radiate inward from the layer of tissue that lines the stomach cavity. Some mature adult corals are hermaphroditic; others are exclusively male or female. A few species change sex as they grow. Internally fertilized eggs develop in the polyp for a period ranging from days to weeks. Subsequent development produces a tiny larva, known as a planula. Externally fertilized eggs develop during synchronized spawning. Polyps across a reef simultaneously release eggs and sperm into the water en masse. Spawn disperse over a large area. The timing of spawning depends on the time of year, water temperature, and tidal and lunar cycles. Spawning is most successful when there is little variation between high and low tide. The less water movement, the better the chance for fertilization. The release of eggs or planula usually occurs at night and is sometimes in phase with the lunar cycle (three to six days after a full moon).
The period from release to settlement lasts only a few days, but some planulae can survive afloat for several weeks. During this process, the larvae may use several cues to find a suitable settlement site. At long distances sounds from existing reefs are likely important, while at short distances chemical compounds become important. The larvae are vulnerable to predation and environmental conditions. The lucky few planulae that successfully attach to the substrate then compete for food and space.
== Gallery of reef-building corals ==
== Other reef builders == Corals are the most prodigious reef-builders. However, many other organisms living in the reef community contribute skeletal calcium carbonate in the same manner as corals. These include coralline algae, some sponges and bivalves. Reefs are always built by the combined efforts of these different phyla, with other organisms leading reef-building in other geological periods.
=== Coralline algae ===
Coralline algae are essential contributors to reef structure. Although their mineral deposition rates are much slower than corals, they are more tolerant of rough wave-action, and so help to create a protective crust over those parts of the reef subjected to the most significant forces by waves, such as the reef front facing the open ocean. They also strengthen the reef structure by depositing limestone in sheets over the reef surface. Furthermore, in locations unfavorable to the growth of corals, coralline algae can be the primary builders of an algal reef.
=== Sponges ===
Sponge reefs are reefs produced by sea sponges. Hexactinellid sponges are known to form reefs off the coast of British Columbia, southeast Alaska, and Washington state. Reefs discovered in Hecate Strait, British Columbia, have grown to up to 7 kilometres long and 20 metres high. Hexactinellid sponge reefs were first identified in the Middle Triassic (245–208 million years ago). The sponges reached their full extent in the late Jurassic (201–145 million years ago) when a discontinuous reef system 7,000 km long stretched across the northern Tethys and North Atlantic basins, but have since declined and were thought to be extinct until existing reefs were discovered in 1987–1988. Archaeocyatha, an extinct clade of sponges, were the planet's first reef-building animals and are an index fossil for the Lower Cambrian worldwide. Similarly, Stromatoporoidea was another extinct clade of reef-building sponges. Unlike corals, stromatoporoids usually settled on soft substrates, so their 'reefs' occupied only a single level rather than a multi-tiered vertical framework of built-up skeletons.
=== Bivalves ===
Oyster reefs are dense aggregations of oysters living in colonial communities. Other regionally specific names for these structures include oyster beds and oyster banks. Oyster larvae require a hard substrate or surface to attach to, which includes the shells of old or dead oysters. Thus, reefs can build up over time as new larvae settle on older individuals. Crassostrea virginica were once abundant in Chesapeake Bay and shorelines bordering the Atlantic coastal plain until the late nineteenth century. Ostrea angasi is a species of flat oyster that has also formed large reefs in South Australia. Hippuritida, an extinct order of bivalves known as rudists, were major reef-building organisms during the Cretaceous. By the mid-Cretaceous, rudists became the dominant tropical reef-builders, becoming more numerous than scleractinian corals. During this period, ocean temperatures and saline levels—which corals are sensitive to—were higher than they are today, which may have contributed to the success of rudist reefs.
=== Gastropods === Some gastropods, like family Vermetidae, are sessile and cement themselves to the substrate, contributing to the reef building.
== Darwin's paradox ==