Jump to reproduction.
Polyps feed on a variety of small organisms, from microscopic zooplankton to small fish. The polyp's tentacles immobilize or kill prey using stinging cells called nematocysts. These cells carry venom which they rapidly release in response to contact with another organism. A dormant nematocyst discharges in response to nearby prey touching the trigger (Cnidocil). A flap (operculum) opens and its stinging apparatus fires the barb into the prey. The venom is injected through the hollow filament to immobilise the prey; the tentacles then manoeuvre the prey into the stomach. Once the prey is digested the stomach reopens allowing the elimination of waste products and the beginning of the next hunting cycle.[31]: 24 Intracellular symbionts Many corals, as well as other cnidarian groups such as sea anemones form a symbiotic relationship with a class of dinoflagellate algae, zooxanthellae of the genus Symbiodinium, which can form as much as 30% of the tissue of a polyp.[31]: 23–24 Typically, each polyp harbors one species of alga, and coral species show a preference for Symbiodinium.[32] Young corals are not born with zooxanthellae, but acquire the algae from the surrounding environment, including the water column and local sediment.[33] The main benefit of the zooxanthellae is their ability to photosynthesize which supplies corals with the products of photosynthesis, including glucose, glycerol, also amino acids, which the corals can use for energy.[34] Zooxanthellae also benefit corals by aiding in calcification, for the coral skeleton, and waste removal.[35][36] In addition to the soft tissue, microbiomes are also found in the coral's mucus and (in stony corals) the skeleton, with the latter showing the greatest microbial richness.[37]
The zooxanthellae benefit from a safe place to live and consume the polyp's carbon dioxide, phosphate and nitrogenous waste. Stressed corals will eject their zooxanthellae, a process that is becoming increasingly common due to strain placed on coral by rising ocean temperatures. Mass ejections are known as coral bleaching because the algae contribute to coral coloration; some colors, however, are due to host coral pigments, such as green fluorescent proteins (GFPs). Ejection increases the polyp's chance of surviving short-term stress and if the stress subsides they can regain algae, possibly of a different species, at a later time. If the stressful conditions persist, the polyp eventually dies.[38] Zooxanthellae are located within the coral cytoplasm and due to the algae's photosynthetic activity the internal pH of the coral can be raised; this behavior indicates that the zooxanthellae are responsible to some extent for the metabolism of their host corals.[39] Stony Coral Tissue Loss Disease has been associated with the breakdown of host-zooxanthellae physiology.[40] Moreover, Vibrio bacterium are known to have virulence traits used for host coral tissue damage and photoinhibition of algal symbionts.[41] Therefore, both coral and their symbiotic microorganisms could have evolved to harbour traits resistant to disease and transmission.
Corals can be both gonochoristic (unisexual) and hermaphroditic, each of which can reproduce sexually and asexually. Reproduction also allows coral to settle in new areas. Reproduction is coordinated by chemical communication.[clarify]
Corals predominantly reproduce sexually. About 25% of hermatypic corals (reef-building stony corals) form single-sex (gonochoristic) colonies, while the rest are hermaphroditic.[citation needed] It is estimated more than 67% of coral are simultaneous hermaphrodites.[42]
About 75% of all hermatypic corals "broadcast spawn"[citation needed] by releasing gametes—eggs and sperm—into the water where they meet and fertilize to spread offspring. Corals often synchronize their time of spawning. This reproductive synchrony is essential so that male and female gametes can meet. Spawning frequently takes place in the evening or at night, and can occur as infrequently as once a year, and within a window of 10–30 minutes.[43][44] Synchronous spawning is very typical on the coral reef, and often, all corals spawn on the same night even when multiple species are present.[45] Synchronous spawning may form hybrids and is perhaps involved in coral speciation.[46]
Environmental cues that influence the release of gametes into the water vary from species to species. The cues involve temperature change, lunar cycle, day length, and possibly chemical signalling.[45] Other factors that affect the rhythmicity of organisms in marine habitats include salinity, mechanical forces, and pressure or magnetic field changes.[44]
Mass coral spawning often occurs at night on days following a full moon.[43][47] A full moon is equivalent to four to six hours of continuous dim light exposure, which can cause light-dependent reactions in protein.[43][44] Corals contain light-sensitive cryptochromes, proteins whose light-absorbing flavin structures are sensitive to different types of light. This allows corals such as Dipsastraea speciosa to detect and respond to changes in sunlight and moonlight.[43][44][48]
Moonlight itself may actually suppress coral spawning. The most immediate cue to cause spawning appears to be the dark portion of the night between sunset and moonrise. Over the lunar cycle, moonrise shifts progressively later, occurring after sunset on the day of the full moon. The resulting dark period between day-light and night-light removes the suppressive effect of moonlight and enables coral to spawn.[43][47]
The spawning event can be visually dramatic, clouding the usually clear water with gametes. Once released, gametes fertilize at the water's surface and form a microscopic larva called a planula, typically pink and elliptical in shape. A typical coral colony needs to release several thousand larvae per year to overcome the odds against formation of a new colony.[49][50]
Studies suggest that light pollution desynchronizes spawning in some coral species. In areas such as the Red Sea, as many as 10 out of 50 species may be showing spawning asynchrony, compared to 30 years ago. The establishment of new corals in the area has decreased and in some cases ceased. The area was previously considered a refuge for corals because mass bleaching events due to climate change had not been observed there.[43][51] Coral restoration techniques for coral reef management are being developed to increase fertilization rates, larval development, and settlement of new corals.[52]
Brooding species are most often ahermatypic (not reef-building) in areas of high current or wave action. Brooders release only sperm, which is negatively buoyant, sinking onto the waiting egg carriers that harbor unfertilized eggs for weeks. Synchronous spawning events sometimes occur even with these species.[45] After fertilization, the corals release planula that are ready to settle.[35]
The time from spawning to larval settlement is usually two to three days but can occur immediately or up to two months.[53] Broadcast-spawned planula larvae develop at the water's surface before descending to seek a hard surface on the benthos to which they can attach and begin a new colony.[54] The larvae often need a biological cue to induce settlement such as specific crustose coralline algae species or microbial biofilms.[55][56] High failure rates afflict many stages of this process, and even though thousands of eggs are released by each colony, few new colonies form. During settlement, larvae are inhibited by physical barriers such as sediment,[57] as well as chemical (allelopathic) barriers.[58] The larvae metamorphose into a single polyp and eventually develops into a juvenile and then adult by asexual budding and growth.
Within a coral head, the genetically identical polyps reproduce asexually, either by budding (gemmation) or by dividing, whether longitudinally or transversely. Budding involves splitting a smaller polyp from an adult.[49] As the new polyp grows, it forms its body parts. The distance between the new and adult polyps grows, and with it, the coenosarc (the common body of the colony). Budding can be intratentacular, from its oral discs, producing same-sized polyps within the ring of tentacles, or extratentacular, from its base, producing a smaller polyp. Division forms two polyps that each become as large as the original. Longitudinal division begins when a polyp broadens and then divides its coelenteron (body), effectively splitting along its length. The mouth divides and new tentacles form. The two polyps thus created then generate their missing body parts and exoskeleton. Transversal division occurs when polyps and the exoskeleton divide transversally into two parts. This means one has the basal disc (bottom) and the other has the oral disc (top); the new polyps must separately generate the missing pieces.
Asexual reproduction offers the benefits of high reproductive rate, delaying senescence, and replacement of dead modules, as well as geographical distribution.[clarification needed][59] Colony division Whole colonies can reproduce asexually, forming two colonies with the same genotype. The possible mechanisms include fission, bailout and fragmentation. Fission occurs in some corals, especially among the family Fungiidae, where the colony splits into two or more colonies during early developmental stages. Bailout occurs when a single polyp abandons the colony and settles on a different substrate to create a new colony. Fragmentation involves individuals broken from the colony during storms or other disruptions. The separated individuals can start new colonies.[60]