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Lobophyllia pachysepta (Ehrenberg, 1834)
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Emma Phillips 2017
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Summary | |
Lobophyllia pachysepta is a scleractinian colonial coral of the Mussidae family. It is recognised for its large uniform dark green or grey fleshy polyps with yellow to cream coloured stripes. On coral reefs, L. pachysepta is best suited to the upper reef slopes and lagoons, although can exist between a depth of 3 to 30 m. These reef zones provide enough protection from wave energy, but still have enough circulation to take advance of the fresh nutrients being deliveredto the reef. Biographically, this species can be found in the Indo-Pacific region including the Great Barrier Reef.
L. pachysepta is a hermaphrodite that reproduces both asexually via budding, and sexually via spawning for external fertilization of eggs and sperm. They have a biphasic life-cycle starting with a free swimming planula larvae body form. Once the right substrate is found, the larvae metamorphose into a sessile adult polyp body form. It remains like this for the majority and remainder of its life, budding to form new colonies. Polyps have a few feeding methods including extending their tentacles at night to catchzooplankton and other various small marine invertebrates. This is achieved with the assistance of mucus. L.pachysepta is a zooxanthellate coral such that it has a mutualistic symbiotic relationship with algae, providing the coral with the majority of its nutrient requirements.
This species has a simple tubule body plan such that gas exchange and excretion occurs over the body wall. Polyps have longitudinal and circular muscles controlled by a double net nervous system. This species, as with all cnidarians, posses cnidocyte stinging cells, which can be seen in high density in the outer edges of tentacles.
L. pachysepta is identified as a near threatened species on the IUCN red list of threatened species. Studies have shown that although this species may still be able to survive on reefs that are already degraded, approximately 22% of its habitat has been destroyed by a combination of local and global threats such as global warming and development. More research in the areas of taxonomy,population trends, life history and ecology, threats, and actions is recommended by the IUCN.
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Physical Description | |
Scleractinian corals are largely described by their corallite
structure (see Fig 2) since the physical appearance of the polyp can differ significantly
with reef location, depth and other environmental factors. L. pachysepta is a colonial species with mostly a phaceloid arrangement, although sometimes
partly flabello-meandroid (see Fig 1). Corallites
are monocentric to slightly irregular, especially when in the process of
division (Veron and Pichon, 1980). Fully grown corallites are large, ranging
between 40 and 50 mm in diameter and are known for their fleshy polyps which
often appear uniform in colour, either dark green or grey. The branches are short and widely spaced (Veron and Pichon, 1980) forming either flat or
hemispherical colonies that grow no greater than 0.5 m across (Veron, 2000).
The Lobophyllia genus is recognised for its solid skeletal
structure with large corallites and large septa with long teeth or denations. As with most corals, L. pachysepta septa are
cylindrically arranged and of varying lengths. The septa exist within four orders. The first order are the thickest with 3-5 long,
irregular, lobate denations (Veron and Pichon, 1980). The following orders decrease in thickness
and have finer, more regularly occurring denations (Veron
and Pichon, 1980). The primary
septa of this species can often characteristically be seen pushing up from under
the polyp tissue forming yellowish to cream bumpy stipes. The surface layer of all septa sides are
finally granulated, and all tops of denations are granulated in texture (Veron and Pichon, 1980).
The septa meet in the centre of the corallite with a columĀnar
skeletal mass called the columella. In L. Pachysepta,
the columellae are large, irregular in size and shape, occasionally branched
and usually diffuse with a sponge like structure (Veron,
2000). Most of the thin and
porous dentations of the septa intertwine with the columellae, and the first order
of septa is often deeply entangled within the columellae. In some cases, the denations grow up above
the columellae. The costae (part of
septa located on the outside of the corallite wall) are inconspicuous and the
spines are echinulate. The endothec (the
inside part of the cup holding the aboral end of the polyp) is mostly
vesicular.
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Figure 1 |
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Figure 2 |
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Ecology |
Habitat | |
The coral species L. pachysepta
can live in most reef environments apart from high energy ones and within a
depth range of 3 to 30 meters. It is
however most common on the upper reef slopes and lagoons of coral reefs (Iucnredlist.org, 2017).
Flood, (1977) identifies coral reefs as having a
number of zones, rather than being one homogenous reef. These reef zones are exposed to varying
levels of energy from wave action and currents.
This factor along with varying water depths create microhabitats along a
reef cross section, and hence support corals of varying morphology. The reef crest (see Fig 3) is exposed to the
greatest energy of any reef zone as it has the highest elevation (apart from a
cay), which then dissipates in both the leeward and windward direction (Castro and
Huber, 2008).
The upper reef slope (or fore reef zone in Fig 3) is nested just
below the reef crest in a moderate energy environment, just above the deep fore
reef. The upper reef slopes generally
has high coral abundance and diversity since it is protected from high wave
energy by the reef crest, while still being shallow enough to access sufficient
sunlight for the corals photosynthesising algae symbionts. This zone also has
enough wave energy for a continual supply of nutrients (Castro
and Huber, 2008). The lagoon is a
similar environment that sits on the other side of the reef crest in a lower
energy environment ideal for delicate branching corals(Castro and Huber, 2008).
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Figure 3 |
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Symbiosis | |
Many corals including L. pachyseptaare able to thrive in nutrient poor waters due to their mutualistic relationship with zooxanthellae algae. These are single celled dinoflagellates that are housed in the gastrodermal tissue of the polyp, giving the coral its colour. It is a mutualistic symbiotic relationship since both parties are dependent on each other for survival. The algae perform photosynthesis using sunlight. This reaction produces photosynthates including sugars, lipids and oxygen which the algae donate to its coral host, sometimes providing it with up to 90% of its required nutrient. In return, the coral provide the algae with their by-products of cellular respiration including nutrients, carbon dioxide and a protective niche (Ruppert, Fox and Barnes,2009).
A study by Jackson, Miller and Yellowlees, (1989) found that corals that only extend their tentacles at night for feeding,such as L. pachysepta, tend to have low densities of zooxanthaelle in their tentacles. This may suggest that coral species such as L. pachysepta primarily use their tentacles for catching zooplankton at night when it is in its greatest abundance (Sorokin 1991), while during the day, algae symbiosis is utilized elsewhere in the polyp.
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Life History and Behaviour |
Reproduction | |
Stony corals have many strategies of reproduction and include both asexual and sexual reproduction, and have a generation length of 10 years (Iucnredlist.org, 2017). Many species have separate sexes where the whole colony is either male or female, and some like the genus Lobophyllia are sequential hermaphrodites(Fadlallah, 1983). Individual polyps produce both male and female gametes for external fertilization rather than brooding planulae for internal fertilization. This method is known as broadcast spawning.
Asexual
L. pachysepta is a colonial organism and is made up of
repeated, genetically identical individuals known as polyps. The species propagates asexually through
division (Veron and Pichon, 1980), however little
is known about this species asexual reproductive modes.
Sexual
Sexual reproduction involves gametogenesis- the process in
which cells undergo meiosis to form gametes.
The primary oocytes (female germ cell) of scleractinian corals first
emerge in the mesoglea of the septa (Fadlallah, 1983). These continue to develop within the
mesogleal linings (lamellae) of the septa mesenteries that grow within the
gastrovascular cavity known as the coelenteron (Fadlallah,
1983). During vitellogenesis when
nutrients is deposited in the oocyte, and the oocytes increase in number, the
entire mesogleal layer between the septa filaments and the polyp wall can
become packed with gonads (Fadlallah, 1983). Spermatogenesis however begins in the septal
endoderm, where it is later included within mesogleal gonadal cysts (Fadlallah, 1983).
The mesogleal lining remains around the cluster until spawining occurs (Fadlallah, 1983).
Mature gametes are released into the coelenteron before being spawned
through the mouth for external fertilization (Ruppert,
Fox and Barnes, 2009). The eggs
and sperm are released in buoyant bundles which drift towards the water
surface, breaking up on the way so that fertilization can be achieved with
other colonies (Hayward et al., 2011).
Broadcast spawning is an annual evening event which involved
the synchronised release of eggs and sperm into the water column. Exogenous stimuli such as the lunar cycle and
warming water temperatures can stimulate the event (Fadlallah,
1983), while day length, tide height and salinity can influence the
timing of the even (GRUMPA). The exact way
in which these stimuli trigger the annual event however is still not properly
understood (Twan et al., 2006). Lobophyllia species have been identified as
mid-tropical breeders, requiring 30oC waters for their annual spring
broadcasting (Fadlallah, 1983). Spawning lasts between a few days and a week
as various species spawn at different intervals to avoid hybrids (GRUMPA). On the Great Barrier Reef, inshore reefs tend
to spawn the night after the first full moon in October, while corals on the
outer reef tend to spawn in November or December (GBRMPA). After the release of eggs and sperm into the water
column, the boyant , the gametes drift to the surface where fertilization takes
place.
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Life-Cycle and Development | |
The class Anthozoa is unique from all other cnidarians since
they do not include a medusa stage in their life cycle. Corals have a biphasic
life cycle consisting of a juvenile planktonic phase, and a dominant benthic
adult phase which is separated by metamorphosis. Following fertilization, egg cleavage
commences at the animal pole which is the early beginning of the oral body
end. Following blastulation,
gastrulation takes place to form the two inner and outer germ layers (endoderm
and ectoderm), and the blastoceal becomes the inner layer of mesoglea (Ruppert, Fox and Barnes, 2009).
After further cell
differentiation, free swimming ciliated planula larvae are produced. Over several days, the planula elongates and swims
aboral end first with use of its flagella amongst the plankton and ocean
currents for a period ranging between a few days to a few months (Hayward et al., 2011).
During this time, the planula performs corkscrew swimming onto the ocean
floor in search of an appropriate substrate using sophisticated chemosensory
(Hayward et al., 2011). Early
metamorphosis occurs just prior to settlement with the morphogenesis of
tentacles, septa and pharynx (Ruppert, Fox and Barnes, 2009).
Metamorphosis is a critical stage in
the corals life altering the morphology of free swimming larvae into a sessile
polyp. Once a suitable substrate is
detected, metamorphosis continues as the larva settles aboral end down. New tissue is grown and the newly formed
polyp body begins calcification and at this stage also obtains symbiotic
zooxanthellae (Hayward et al., 2011). The single polyp asexually propagates
into a colony before the cycle begins again.
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Feeding | |
Corals such as L. pachyseptahave a range of feeding modes which differ within its lifecycle. Coral larvae are lecithotrophic meaning that they feed from a nutrient rich yolk sac, and do not have to actively search for food. The adult polyp form however is a predator with three main feeding mechanisms including nutrients from its symbiotic zooxanthellae, use of cnidae packed tentacles and mucus nets.
Mussid species are all zooxanthellates such that they house unicellular dinoflagellate zooxanthalle in their gastrodermal cells. Zooxanthalle algae photosynthesise during theday supplying the coral with photosynthates and up to 50 percent of theirenergy requirements (Ruppert, Fox and Barnes, 2009). During the night, Lobophyllia species predate by extending their tentacles packed with cnidocytes to immobilise and feed on a variety of marine organisms ranging from zooplankton to small fish (Ruppert, Fox and Barnes, 2009).
Mussids also utilise a third mode of feeding which involves mucus nets (Sorokin, 1993). These corals secrete large quantities of mucus from the mouth in strip forms. Cilia located from the oral disc out to the side of septa beat to form mucus ‘nets’ at their ends. When the nets are successful in catching prey, the polyp is triggered and opens it mouth, swallowing the mucus stipes along with its catch (Sorokin,1993).
Lobophyllia species have also been observed to use their mucus nets for sedimentary filter feeding also (Sorokin,1993). Negatively charged micells of colloids, bacteria cells and detritus particles within the sediments are attracted to the corals muscus which is positively charged by the presence of amino acid groups (Sorokin,1993). This method of feeding is used continuously, and not dependant on day or night likes the previous two feeding methods discussed.
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Anatomy and Physiology |
Internal Morphology | |
The coral polyp sits within a corallite (see Fig 4) possessing a tubular body shape. The oral end consists of an elongated mouth slit which sits within the oral disk, fringed by a whorl often tacles at its outskirts. The mouth is connected to the central gastrovascular cavity known as the coelenteron by the pharynx. Ciliated grooves run verticallydown the pharynx at each end of the mouth called siphonoglyphs. These create currents of water in the pharynx aiding with respiration and maintenance of internal pressure (Willey et al., 1971). The coelenteron is radially partitioned by septa- vertical blades that extend out from the corallite cup. The septa are a result of gastrodermis out folding such that the individual septa are made of three layers- mesoglear lined either side by gastrodermis (Ruppert, Fox and Barnes, 2009). The septa extend up to insert themselves onthe oral disk; a distinctive characteristic in L. pachysepta (see physical description for more detail). Tentacles are an extension of the body wall that grow out between a pair of septal insertions on the oral disc (Ruppert, Fox and Barnes, 2009). They consist of the three body wall layers of epidermis, mesoglear and gastrodermis that line a hollow centre.
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Figure 4 |
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Nutrient Transport | |
Following the capture of prey (see Feeding section under Life History and Behaviour), it is moved down into the gastrovasular cavity known as the coelenteron where extracellular digestion takes place. This is a blind gut with one opening to the exterior via the mouth. It is partitioned up by septa, increasing the surface area of the gastrodermis to assist in digestion. In all cnidarians including L. pachysepta, the coelenteron is the main internal transport system in which fluid is transported by the ciliated gastrodermis, muscular contraction, or both (Ruppert, Fox and Barnes, 2009).
Digestion can be initiated with the release of cnidae and injection of proteolytic enzymes during the initial prey capture; however most digestion occurs in the coelenteron (Ruppert, Fox and Barnes, 2009). Once there, extracellular digestion takes place as gastrodermal gland cells release enzymes, (primarily proteases) reducing prey to a soup like pulp over a period of a few hours. This enables the breakdown of lipids, carbohydrates and allows for the digestion of proteins. The nutrients is then absorbed by the gastrodermal cells as the pulp is circulated, and larger chunks are engulfed for intracellular digestion (Ruppert, Fox and Barnes,2009). This absorption phase of the extracellular digestion takes 8 to 12 hours, followed by intracellular digestion over a few days. Any indigestible material is ejected via the mouth in a coating of mucus (Ruppert, Fox and Barnes,2009).
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Muscular and Nervous System | |
Cnidarians have a wide range of movement due to their
muscular system. The polyp body form in
corals can impressively extend, contract and bend their bodies due to having
both longitudinal and circular muscle fibres.
The longitudinal muscles are typically associated with the epidermis,
while the circular muscles are associated with gastrodermis and are made of
epitheliomuscular cells. These cells are
of or relating to the epithelial cell in coelenterates that contains
contractile fibres thus allowing it behave like a true muscle cell. Anthozoans and scyphozoans are unique from
all other cnidarians in that that some of their epitheliomuscular cells from
the epithelia have migrated to the mesoglea, and evolved into true muscle cells
known as myocytes. Corals also have
radial muscle fibres extending from the polyps inner axis out towards the
exterior (Ruppert, Fox and Barnes, 2009).
Corals have a
decentralised nervous system consisting of sensory neurons that feel the
environment, motorneurons that activate muscles and other cells, and
interneurons that join the two together.
Cnidarians have a two layered net systems with one layer in the base of
the epidermis, and the other at the base of the gastrodermis. These two layers are joined by interneurons
that run through the mesoglea (Ruppert, Fox and Barnes,
2009). Unlike other medusae cnidarians,
corals do not have advanced nerve rings or ganglia.
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Respiration and Excretion | |
Cnidarians do not have a circulatory system since they only
have two true tissue layers (epidermis and gastrodermis) that are in contact
with the surrounding water. These
surfaces are ciliated, thus creating a current and making the exchange more
efficient. Therefore all gas exchange
simply occurs over the body wall of the individual. Excretion happens much the same way, such
that ammonia (the excretory product of cnidarians), passes through the body
wall by diffusion, and is dumped into the surrounding water where it is quickly
dissolved and removed by water motion (Ruppert, Fox and
Barnes, 2009).
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Cnidome | |
A key defining feature of all cnidarians is that they have stinging cells known as cnidocytes. A cnidocyte cell is an explosive combined sensory-effector cell which houses a cnida organelle. The cnida is a fluid filled sack containing a coiled hollow tubule which is discharged from the cell when signalled. Cnidocytes are common throughout the epidermis of cnidarians, and in higher density in tentacles and sometimes in localised regions of gastrodermis to assist with immobilising prey for digestion (Ruppert, Fox and Barnes, 2009).
There are three categories of cnidae including nematocysts, spirocysts and ptychocysts which are housed in three different types of cnidocytes including nematocytes, spirocytes and ptychocytes. Nematocysts occur in all cnidarians, but within anthozoans, the discharge site of the cnidocyte cells is covered by thre eapical flaps (Ruppert, Fox and Barnes, 2009). Spirocysts lack any barbs, and instead have sticky threads (see figure 5).
A cnidome refers the specific assortment of cnidae specific to a species. The cnidome of L. pachysepta has not yet been described to date, however the cnidome of a close relative Lobophyllia hemprichii was described by Cordie (2015) as having a low density and diversity(Cordie, 2015). Cordie (2015) also noted that in central sections of the polyps, spirocytes are few and far between, and nematocytes are uncommon. The edge sections of the L. hemprichii were similar, although as light decrease in cnidocytes was found (Cordie, 2015).
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Figure 5 |
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Tentacle Morphology | |
From a single L. pachysepta specimen, I clipped off a single tentacle and had it H&E stained and cross-sectioned through various points along the tentacle (see Fig 7). Slide 1 appears to be the tentacle tip containing only one homogenous layer of epidermis with cnidocytes. Some epidermis nuclei can also be observed asa vague ring of dark purple dots. Very few mucocytes (mucus glands) around the outskirts appear as transparent light purple vacancies as the mucus does not colour with H&E. There is no obvious mesoglea or gastrodermis layer in this slide.
The second slide is taken from the central region of the tentacle (see Fig 7),and all three layers are present- a thick epidermis (packed with cnidocytes and very few mucocytes), a very thin mesoglea and a central gastrodermis housing zooxanthellae. The gastrodermal cavity also starts to make an appearance in this section and is called the lumen. A vague ring of dark purple dots can also be seen here showing epidermis nuclei.
The third slide (see Fig 7) is located closest to polyp body (point furthest from the tentacle tip). The wide epidermis is packed full of cnidocytes and mucocytes. A clear, more compact ring of epidermis nuclei can be seen in this slide. The mesoglea appears very thin, zooxanthellae are slightly sparser, and the lumen almost consumes the gastrodermis. No slides show evidence of myocyte cells in or around the mesoglea(see section on Muscles and Nervous System above).
Figure 6 is a high magnification (40x) of a central section of tentacle with an uneven boarder. It shows clear tissue layers including gastrodermis housing zooxanthellae, a thick epidermis with mucocytes and cnidocytes and a thin mesoglea layer. At this maginification, the majority of cnidocytes appear to be thin walled spirocysts as the coiled tubules can be made out (see section on Cnidome). Mucocytes are large and common in the epidermis layer, and the epidermis nuclei are clearly scattered throughout epidermis layer.
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Figure 6 |
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Figure 7 |
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Biogeographic Distribution | |
The L. pachyseptaspecies is found in the Indo-Pacific region as highlighted in Fig 8. It is found throughout The Great Barrier Reef including Heron reef where studies specimen was found, and also in northern and western Australia. In the central Indo-Pacific region, it is found in South-east Asia, Japan and East China Sea. It also lives in the central and northern Indian Ocean of the Indo-West Pacific, and in Palau in the West Pacific(Randall, 1995).
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Figure 8 |
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Evolution and Systematics | |
Traditional methods of scleractinian phylogony have been based on corallite morphology. However with advancing technology, molecular analysis has become a more reliable method. Romano and Palumbi, (1996) produced a molecular phylogram of genera based on 34species of corals. The phylogram was produced based on the sequences from mitochondrial 16S ribosomal gene region 15(Romano and Palumbi, 1996), with Lobophillia genus marked with red star, (see Fig 9). The exact positioning of L. pachysepta is not well resolved.
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Figure 9 |
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Conservation and Threats | |
L. pachysepta is ranked as ‘near threatened’ on the IUCN Red List of Threatened Species (see Fig 10). This is largely due to the major threat from loss of coral reef habitat, but also a combination of global and local contributing threats (Iucnredlist.org,2017).
The population trend of this species is unknown, however Wilkinson (2004) proposes that it is best inferred from estimates of habitat loss in the regions that L. pachysepta is known to exist. Wilkinson notes that this species has an ‘assumed large effective population size that is highly connected and/or stable with enhanced genetic variability’ (Wilkinson,2004). This suggests that the species is likely to be resilient, and hence able to survive on coral reefs already experiencing a critical level of degradation. With these considerations, Wilkinson (2004) therefore estimates a habitat loss of 22% from all reefs within the geographical area of L. pachysepta (see section on Biogeographical Distribution), (Wilkinson, 2004).
L. pachysepta is also vulnerable to bleaching as it was recorded during the 1998 bleaching event in Palau, Micronesia. L. pachysepta species were observed in high stress states of bleaching and mortality (Brunno et al., 2001). Most corals can survive in a state of stress for a period of time, but if not alleviated, results in mortality. Along with habitat loss and coral bleaching, global climate change is contributing to many general threats such as increasing corals susceptibility to diseases, increasing events such as ENSO and storms, and causing ocean acidification (Iucnredlist.org,2017).
Local threats are largely a result of human population increase and development. Overfishing reduces fish populations and unbalances coral reef ecosystems, while fishing methods such as dynamite and chemical directly destroy coral reefs (Iucnredlist.org, 2017). Change of land use and over fertilization of crops can result in pollution. Land once held together by natural vegetation can be dislodged when converted to livestock grazing fields, resulting in sediments being washed down estuaries and accumulating on near shore reefs, and hence restricting photosynthesis by corals algae. Pollution can also come in the form of crop fertilizer. Land runoff can contain excess crop fertilizer, thus feeding the marine environment with a high nutrient supply that favours coral competitors such as algae (Great Barrier Reef Foundation, 2017). This type of pollution is particularly prevalent along the east coast of Australia bordering the Great Barrier Reef.
The combined effect of global and local threats is not known, and only a small percent of L. pachysepta’s geographical distribution lay within marine protected areas (Iucnredlist.org, 2017). The IUCN Red List of Threatened Species advices more research in the areas of taxonomy, population trends, life history and ecology, threats, and actions (Iucnredlist.org, 2017).
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Figure 10 |
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