Select the search type
 
  • Site
  • Web
Search

Student Project

Minimize
Micromussa lordhowensis


Ella Yeates Lejeune 2017

Summary

Micromussa lordhowensis (previously Acanthastrea lordhowensis) is a colonial hard coral found throughout the Indo-Pacific. They are a very colourful species which makes them popular in the aquarium trade. The classification of the species has recently undergone revision, with both the family and genus being changed.

A M. lordhowensis specimen was found in the University of Queensland (UQ) aquarium, and identified by Dr Merrick Ekins, Collection Manager of Sessile Marine Invertebrates at the Queensland Museum.

Physical Description

M. lordhowensis are colonial corals with large, fleshy polyps covered in fine papillae (Veron, 2000b). They are very brightly coloured, most commonly green, grey or, as is the UQ aquarium specimen, orange (Davie, 2011). Each polyp sits in an external skeleton known as a corallite. The corallites are 10-20mm across, polygonal in shape with acute walls, and of uneven heights. The corallite septa are thick, with large but fine septal teeth (Veron, 2000b), and the columella (central structure) is barely developed. Colonies are cerioid (corallites sharing walls) and less than 25cm across (Davie, 2011).

Ecology

Habitat

M. lordhowensis is found in shallow reef habitats, most commonly in subtropical environments (Veron, 2000b). While found across the Great Barrier Reef, it is most predominant in the southern part (DeVantier et al., 2006).

Zooxanthellae

Hard corals have a symbiotic relationship with dinoflagellate algae, known as zooxanthellae. Corals can live without zooxanthellae, but they are important for reef building as they produce the calcium carbonate of which the coral skeleton is made. They also give the corals their colour, and produce sugars which supplement the coral’s diet. The corals provide the zooxanthellae with protection in their polyp tissue, as well as carbon dioxide, ammonia, phosphate and trace minerals; waste products which are toxic to the coral, but required by the zooxanthellae. (Faulkner & Chesher, 1979)

Life History and Behaviour

Sexual Reproduction

M. lordhowensis is a hermaphroditic broadcast spawner (Kerr et al., 2011). Each individual releases both eggs and sperm through its mouth into the water, and fertilisation is external. The planktonic planula larvae have few energy reserves and do not feed, resulting in mostly short-distance dispersal (Wilson & Harrison, 1998). The time taken for a coral larva to settle depends on locating a substrate appropriate for settling. In a study investigating settlement competency periods, Wilson and Harrison (1998) found M. lordhowensis larvae to have minimum and maximum settling periods of 12 and 78 days, with the majority of settlements occurring within the first 20 days. The maximum settling period indicates the potential for long-distance dispersal.

Development

Once a larva has found a suitable settlement surface, it attaches, with the skeleton forming soon after attachment. The skeleton is produced by depositing calcium carbonate layers on top of each other. At the same time, the larval cells start to flatten, and the top forms a dome. The tentacles sprout from the body wall around the oral disk. The polyp then reproduces asexually by budding, to form a colony (Faulkner & Chesher, 1979).

Two forms of budding occur in M. lordhowensis – intracalicular and extracalicular (Arrigoni et al., 2016). Intracalicular budding occurs within a corallite, with the parent and daughter corallites splitting, while extracalicular budding occurs externally to the corallite (Budd et al., 2012).

Feeding

M. lordhowensis are opportunistic predators of small prey and plankton. Prey which swim into contact with the coral’s tentacles or oral disk are caught and immobilised by cnidocytes (specialised sensory-effector cells) and mucus, and then pushed into the mouth by the tentacles and tentacular cilia.

The prey is swallowed by the pharynx as a result of ciliary action and peristaltic movements of the pharyngeal wall. Once in the coelenteron, extracellular digestion occurs by enzymes. The molecules resulting from enzymatic digestion circulate the coelenteron by movement of gastrodermal flagella, and are phagocytosed by gastrodermal cells. Nutrients are shared among the polyps of a colony, via coelenteric tubes. (Ruppert et al., 2004)

Gas Exchange and Excretion

Gas exchange in corals occurs across the body wall and tentacles. It is facilitated by ciliary flow over both the gastrodermis and ectodermis. Ammonia, the coral’s waste product, is also excreted via the body surface, as well as being used by zooxanthellae. (Ruppert et al., 2004)

Anatomy and Physiology

Polyp

Like all Cnidaria, M. lordhowensis are radially symmetrical. Their body wall consists of two layers, the ectodermis and gastrodermis, separated by a jelly like layer, the mesoglea. The gut is a sac-like cavity known as the coelenteron, with the mouth being the only opening (Figure 1.a). The mouth is surrounded by tentacles (Figure 1.b), with the tissue in between known as the oral disc (Figure 1.c). While cnidarians can take a medusa or polyp form, corals, as part of the class Anthozoa, are purely a sedentary polyp form. Many corals, including M. lordhowensis, are colonial. (Veron, 2000a)

Corals have well-developed nervous, muscular and reproductive systems. The nervous system consists of a simple nerve net spread throughout the body, with both ectodermal and gastrodermal cells. The nerve net connects to specialised cells responsible for sensing mechanical, chemical and light stimuli. The muscular system allows the polyp to extend and retract in response to signals from the nerve net. In colonial corals, these signals are transmitted throughout the entire colony. Coral gametes arise from the mesoglea in the coelenteron mesenteries. During spawning they are shed into the coelenteron to be released through the mouth. (Veron, 2000a)
1
Figure 1

Skeleton

Hard corals secrete an external calcareous skeleton for support. The skeleton of an individual polyp is known as a corallite, shown in Figure 2. It takes the form of a tube with vertical plates, called septa (Figure 2.a), radiating inwards from the corallite wall (Figure 2.b). The septa are covered in projections known as teeth (Figure 2.c). Inward projecting teeth intertwine in the centre of the corallite to form a structure called a columella (Figure 2.d). This is barely developed in M. lordhowensis (Veron, 1986). M. lordhowensis colonies are cerioid (Veron & Pichon, 1980), meaning walls are shared between corallites, but other hard corals may have individual corallites. The corallites are joined by other structures known as coenosteum (Veron, 2000a).
2
Figure 2

Corallite Wall

Three characteristic components of the corallite wall which are found in M. lordhowensis are the coenosteum, sterome and dissepiments. The coenosteum is a porous skeletal material found in between each corallite. The sterome is a solid sheet which forms the inner lining of corallite wall. Dissepiments are thin horizontal layers of skeleton similar in structure to the sterome, situated between corallites. (Veron, 2000a).

Coelenteron

The mouth of the polyp leads to a pharynx, which opens up into the coelenteron. Each coelenteron in a colony is linked to the others by coelenteric tubes. The tubes transport nutrients, as well as water for respiration. The coelenteron is partitioned by vertical mesenteries, arranged radially. These increase the gastrodermis surface area for digestion, photosynthesis and respiration. They also contain the reproductive organs. The mesenteries are lined with coiled mesenteric filaments to further extend the gastrodermis surface area. (Veron, 2000a)

The tentacles surrounding the oral disc are extensions of the coelenteron. They contain stinging cells called nematocytes, and adhesive cells called spirocytes, for defence and food capture (Ruppert et al., 2004). Other ectoderm cells secrete slimy mucus which coats the polyp. This is used to remove sediment from the polyp surface, and to assist in food capture (Veron, 2000a).

Cnidocytes

Nematocytes and spirocytes are types of cnidocytes, sensory-effector cells characteristic of cnidarians, and of which there are various types with different functions. A cnidocyte houses a cnida, a membranous capsule containing a long everted tubule.

The cnida of a nematocyte is known as a nematocyst. The tubule has barbs on the surface for penetrating prey or predator, and releases toxins to sting or paralyse. Nematocytes have a sensory cilium known as a cnidocil.

Spirocytes are unique to the class Anthozoa, and do not have a sensory cilium. The cnida is known as a spirocyst, and the tubule lacks barbs, but releases sticky, adhesive threads. (Ruppert et al., 2004)

Biogeographic Distribution

M. lordhowensis was named for the location it was first found, Lord Howe Island (Veron & Pichon, 1980). It is most common in subtropical locations, and is found in the Red Sea and Gulf of Aden, southwestern and northern Indian Ocean, central Indo-Pacific, north, west and east Australia, Southeast Asia, Japan, the East China Sea and the Solomon Islands (Figure 3) (Turak et al., 2008).
3
Figure 3

Evolution and Systematics

Classification

Kingdom: Animalia
Phylum: Cnidaria
Class: Anthozoa
Subclass: Hexacorallia
Order: Scleractinia
Family: Lobophylliidae
Genus: Micromussa
Species: lordhowensis

Systematics

The phylum Cnidaria contains the jellyfish, corals and sea anemones, and are characterised by the presence of cnidocytes. They are sister group to the Bilateria, and have radial symmetry, tissues, muscles, nerves, and a gut. Cnidaria can take a polyp or medusa (bell-shaped) form, but Anthozoa are the only class without a medusa stage in their lifecycle. (Ruppert et al., 2004)

Anthozoa include corals and sea anemones. Hexacorallia separates the hard corals and anemones from the Octocorallia, which contains the soft corals.

The Scleractinia are also known as the hard, or stony corals, due to the calcareous skeletons produced by the polyps. The skeletons contribute to the building of reefs, therefore these corals are also referred to as reef-building, or hermatypic. (Ruppert et al., 2004)

Change in Classification

First described as Acanthastrea lordhowensis in the family Mussidae by Veron and Pichon in 1982, the species has recently been moved to the family Lobophylliidae, and only last year to the genus Micromussa. These changes are due to recent advancements in the classification system, now incorporating molecular data as well as morphological data.

Traditional methods of classification are based largely on the macromorphology of the skeleton through a binocular microscope. Suborders and families are distinguished by septa, columella and corallite wall structure, and genera are distinguished by corallite budding and integration (Budd et al., 2012). Scanning Electron Microscopy (SEM) is now being used to examine the skeletal micromorphology: teeth and granule shape (Budd et al., 2012). This morphological data is now also being used alongside molecular data (Arrigoni et al., 2016).

In 2012, several genera including both Acanthastrea (containing A. lordhowensis at the time) and Micromussa were assigned to the new family Lobophylliidae (Budd et al., 2012). This change was based on both morphological and molecular data.

Lobophyllidae is characterised by:
  • lamellar columella linkage
  • lack of epitheca
  • irregular lobate or bulbous teeth with elliptical bases
  • rounded granules (elevations on septal surface) enveloped by extensive thickening deposits with vertical palisade-like structures between teeth
  • differences in size/shape of teeth amongst septal cycles
(Budd et al., 2012; Budd & Stolarski, 2009)

M. lordhowensis can no longer be included in the revised Mussidae description on account of several morphological characteristics. These Mussidae characteristics include exclusively intracalicular budding, epitheca present, spiky granules, and equally sized teeth. Mussidae also does not include any Indo-Pacific taxa, based on molecular data. (Budd et al., 2012)

In 2016, Acanthastrea lordhowensis was changed to Micromussa lordhowensis (Arrigoni et al., 2016). The genus Micromussa was first created by Veron in 2000, with the only characteristic separating it from Acanthastrea being that it had corallites less than 5mm in diameter, therefore excluding A. lordhowensis. Arrigoni et al. (2016) examined lobophyllid species in regards to corallum macromorphology, corallite size and organisation, and phylogeny. This resulted in the reclassification of several species including Acanthastrea lordhowensis being changed to Micromussa lordhowensis. M. lordhowensis was grouped with similar species based on characters including corallite budding and integration, number of septa, tooth base and shape of granules.

Micromussa is characterised by:
  • medium tooth spacing (0.3-1mm)
  • strong, pointed, scattered septal granulation
  • circular or angular corallites
  • septa thickened at the corallite wall
(Arrigoni et al., 2016; Veron, 2000b)

Scleractinia Evolution

While reef-like structures of bacteria and algae occurred long before the evolution of animals, the first corals appeared in the Ordovician period (488-443 million years ago). These tabulate and rugose corals had calcite skeletons, a different form of calcium carbonate to the aragonite skeletons of Scleractinia. These skeletons were preserved well in fossils, but the corals did not survive the Paleozoic mass extinction (251 million years ago). (Veron 2000a)

The first Scleractinia appeared in the Triassic period (250-210 million years ago). These corals did have some sort of skeleton, but they were not reef-building. Their ancestors were likely askeletal, anemone-like organisms. (Veron, 1995)

The proliferation of scleractinian reefs began 20-25 million years after the first appearance of Scleractinia. Skeletogenesis, reef formation and zooxanthella symbiosis likely evolved together to exploit resources such as light and calcium carbonate. By the end of the Cretaceous period (67 million years ago), extensive scleractinian reefs occurred worldwide, and all major families which are extant today were established. (Veron, 1995)

Conservation and Threats

Status

The most recent assessment by the IUCN Red List of Threatened Species in 2008 listed M. lordhowensis as near threatened (Turak et al., 2008), however it has since been referred to as abundant in Moreton Bay, and with a widespread distribution in the Indo-Pacific (Davie, 2011). The major threat to corals worldwide is climate change. Increasing temperatures lead to coral bleaching as well as increased susceptibility to disease. Corals can also be threatened by species changes in the local environment. These include competitors, predators, pathogens and parasites. Anthropogenic stressors including fisheries, developments, pollution, and recreation and tourism activities, are also a threat to corals (Turak et al., 2008).

Diseases

Coral diseases have become frequent throughout the Indo-Pacific, with outbreaks recorded in the Great Barrier Reef, Marshall Islands and Hawaiian Islands (Turak et al., 2008). Onton et al. (2011), investigating the drivers of coral disease, identified seven diseases on Ningaloo Reef on the Western Australian Coast. The most common of these was skeletal eroding band, a prevalent coral disease of the Indo-Pacific.

Skeletal eroding band is caused by the protozoan 
Halofolliculina corallasia, which embed themselves in the coral skeleton, forming a black band. The ciliates asexually reproduce to form a motile swarming phase, the movements and chemical secretions of which damage and kill the coral (Page & Willis, 2008). Onton et al. (2011) found that coral disease frequency increased with the presence of the corallivorous sea snail genus Drupella, as well as anoxic events.

Corallivores and anoxic events are also threats to coral populations when disease is not present. 
Other corallivores include crown-of-thorns starfish, and species of fish. Anoxic events occur when calm wind and sea conditions coincide with spawning, and the available oxygen is depleted by the respiratory demands of the spawn (Onton et al., 2011).

Bleaching

Coral bleaching occurs when corals lose their endosymbiotic algae, zooxanthellae. Zooxanthellae provide corals with much of their nutritional requirements, without which the corals either die or are put under great stress. A lack of nutrients disrupts the coral’s symbiotic interactions with microbes in the surface mucous layer, making the coral more susceptible to pathogen attack and diseases. Bleaching events can be caused by a variety of unfavourable conditions, most commonly high temperatures and low salinity. Low salinity can be caused by high levels of rain and heavy land run-off. Bleaching events have been correlated with disease outbreaks and reef mortality on the Great Barrier Reef (Haapkylä et al., 2013).

Conservation

According to the IUCN Red List, conservation methods for M. lordhowensis include identification, establishment and management of protected areas, as well as disease, pathogen and parasite management. This can be made possible through research into ecology and habitat status, threats and resilience to threats. (Turak et al., 2008)

Due to the popularity of M. lordhowensis in aquariums, trade should be monitored through the use of CITES (Convention on International Trade in Endangered Species) analysis reports. Fisheries management is also required, including quotas and size limits, and population surveys to monitor the effects of harvesting. (Turak et al., 2008)

References

Arrigoni, R., Benzoni, F., Huang, D., Fukami, H., Chen, C.A., Berumen, M.L., Hoogenboom, M., Thompson, D.P., Hoeksema, B.W., Budd, A.F., Zayasu, Y., Terraneo, T.I., Kitano, Y.F., & Baird, A.H. (2016) When forms meet genes: revision of the scleractinian genera Micromussa and Homophyllia (Lobophyllidae) with a description of two new species and one new genus. Contributions to Zoology, 85, 387-422.

Budd, A.F., Fukami, H., Smith, N.D., & Knowlton, N. (2012) Taxonomic classification of the reef coral family Mussidae (Cnidaria: Anthozoa: Scleractinia). Zoological Journal of the Linnean Society, 166, 465-529.

Budd, A.F. & Stolarski, J. (2009) Searching for new morphological characters in the systematics of scleractinian reef corals: comparison of septal teeth and granules between Atlantic and Pacific Mussidae. Acta Zoologica, 90, 142-165.

Davie, P. (2011) Wild guide to Moreton Bay and adjacent coasts, 2nd edn. Queensland Museum, South Brisbane Qld.

DeVantier, L.M., De’ath, G., Turak, E., Done, T.J., & Fabricius, K.E. (2006) Species richness and community structure of reef-building corals on the nearshore Great Barrier Reef. Coral Reefs, 25, 329-340.

Faulkner, D. & Chesher, R. (1979) Living Corals. Clarkson N. Potter, New York.

Haapkylä, J., Melbourne-Thomas, J., Flavell, M., & Willis, B.L. (2013) Disease outbreaks, bleaching and a cyclone drive changes in coral assemblages on an inshore reef of the Great Barrier Reef. Coral Reefs, 32, 815-824.

Kerr, A.M., Baird, A.H., & Hughes, T.P. (2011) Correlated evolution of sex and reproductive mode in corals (Anthozoa: Scleractinia). Proceedings: Biological Sciences, 278, 75-81.

Onton, K., Page, C.A., Wilson, S.K., Neale, S., & Armstrong, S. (2011) Distribution and drivers of coral disease at Ningaloo reef, Indian Ocean. Marine Ecology Progress Series, 433, 75-84.

Page, C.A. & Willis, B.L. (2008) Epidemiology of skeletal eroding band on the Great Barrier Reef and the role of injury in the initiation of this widespread coral disease. Coral Reefs, 27, 257-272.

Ruppert, E.E., Fox, R.S., & Barnes, R.D. (2004) Invertebrate zoology: a functional evolutionary approach, 7th edn. Thomson - Brooks/Cole, Belmont Calif.

Turak, E., Sheppard, C., & Wood, E. (2008). Acanthastrea lordhowensis, The IUCN Red List of Threatened Species. http://dx.doi.org/10.2305/IUCN.UK.2008.RLTS.T133366A3709692.en 12 May 2017.

Veron, J.E.N. (1986) Corals of Australia and the Indo-Pacific. Angus & Robertson, North Ryde N.S.W.

Veron, J.E.N. (1995) Corals in space and time: the biogeography and evolution of the Scleractinia. UNSW Press, Sydney.

Veron, J.E.N. (2000a) Corals of the World Volume 1. Australian Institute of Marine Science, Townsville.

Veron, J.E.N. (2000b) Corals of the World Volume 3. Australian Institute of Marine Science, Townsville.

Veron, J.E.N. & Pichon, M. (1982) Scleractinia of Eastern Australia. Part 4. Family Poritidae. Australian National University Press, Canberra.

Wilson, J.R. & Harrison, P.L. (1998) Settlement-competency periods of larvae of three species of scleractinian corals. Marine Biology, 131, 339-345.