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Summary | |
Metandrocarpa manina is a colonial ascidian species which inhabits tropical coral reef areas including reef crests, outer reef slopes and lagoons. Colonies typically live amongst cryptic communities, attached to the underside of structures such as corals (Figure 1). Zooids of a colony are bright red with white-yellow colouration between the siphons and are connected by a stolon. It is a filter feeding organism and zooids of a colony are commonly hermaphroditic, however, reproductive structures have been observed to vary between zooids.
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| Figure 1 |
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Physical Description | |
Metandrocarpa manina is a species of colonial, bright red zooids with white-yellow coloration between the atrial and buccal siphons. Individual zooids appear flattened and are approximately 3 millimetres (mm) in height and 2 mm wide (Monniot & Monniot, 1987). The colony studied from Heron Island consisted of zooids ranging in size from 1-5 mm (Figure 2). The zooids are covered by a tough, transparent membrane (tunic) with small red dots. The colony is known as a social colony as the zooids are not connected by a common tunic but by a stolon, a thin ‘tunical string’ (Monniot & Monniot, 1987).
Identification of Resources
The literary resources used to assist identification of this species were:
• Coral Reef Ascidians of New Caledonia (Monniot, et al., 1991)
• Mémoirs du Muséum National D’Histoire Naturelle (Monniot & Monniot, 1987).
• Memoirs of the Queensland Museum (Knott, 1985).
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| Figure 2 |
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Ecology |
Micro-habitats and Associations | |
Metandrocarpa manina are commonly found in shallow water, attached to the underside of rocks, bivalve mollusc shells, or coral (Monniot & Monniot, 1987). These habitats provide a colony with protection from predators and shade from excess sunlight. The colony from Heron Island was found attached to the underside of a coral boulder. It was part of a cryptic community with at least five other ascidian species (see blue circled areas on Figure 3). Other organisms within the cryptic community included calcium carbonate tube producing polychaete worms, bryozoan colonies and large amounts of purple coloured coralline algae (Figure 3). The diversity and abundance of organisms on this single piece of coral rubble shows there is competition for limited space.
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| Figure 3 |
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Crypsis | |
Metandrocarpa manina zooids were observed on the surface and in crevices of coral rubble from Heron Island. Like many ascidians, Metandrocarpa manina have been previously reported on the underside of coral (genus Pachyseris) or on the shells of bivalve molluscs (genus Arca) (Monniot & Monniot, 1987).
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Life History and Behaviour |
Behaviour | |
Metandrocarpa manina is a colonial filter feeding species and therefore, does not show any active predatory behaviour. However, if a zooid feels threatened (i.e. if it is contacted by another animal or object in the water) it will retract. This behaviour is evident in the video below as one zooid is brushed with broken up rubble as the water is disturbed. It retracts very quickly and slowly begins to expand when it can no longer sense the foreign object.
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Metandrocarpa manina retraction behaviour
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Feeding | |
Like all ascidians Metandrocarpa manina zooids filter feed by ciliary action using stigmata (ciliated gill slits) (Figure 8). Frontal cilia lining the pharynx (pharyngeal basket) and lateral cilia capillaries create water currents which draw water and food particles into the buccal siphon (Ruppert, et al., 2004). Food particles are trapped in a mucous net continuously secreted by the endostyle and transported to the oesophagus and stomach where it is digested (Ruppert, et al., 2004).
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Reproduction | |
Most colonial species are viviparous and produce lecithotrophic larvae (larvae has sufficient yolk to feed on before metamorphosis). Viviparous species produce tadpole larvae which swim to a suitable location and attach to the substratum using adhesive papillae before undergoing metamorphosis into the adult body form (Ruppert, et al., 2004). Metandrocarpa manina zooids fit this description as according to Monniot & Monniot (1987) there was evidence or larvae developing in the ‘cloacal cavity’.
After fertilisation of an egg, cleavage is holoblastic and bilateral (Ruppert, et al., 2004). After gastrulation, it eventually lengthens along the anterior-posterior axis and the archentron gives rise to the notochord (Ruppert, et al., 2004). Mesodermal cells are also created by lateral cell multiplications of the archentron which later develop into coelomic cavities (Ruppert, et al., 2004). Other organs then begin to develop such as a heart, pharynx, stomach and siphons, giving rise to an ascidian tadpole larva.
Metamorphosis of the tadpole larva, once attached to the substrate, involves a number of significant development processes. The tail is dramatically shortened and absorbed which results in loss of the notochord, dorsal hollow nerve cord and swimming musculature (Ruppert, et al., 2004). The body also rotates 90 degrees due to the rapid growth between the adhesive papillae and buccal siphon which allows the siphons to face in the opposite direction to the substratum (Ruppert, et al., 2004). The atrium expands which encloses the anus and pharynx, the number of gill slits increase and the siphons open as the larval cuticle is moulted, allowing the juvenile to begin feeding (Ruppert, et al., 2004).
Reproduction in colonies occurs by asexual reproduction (budding) as well as sexual reproduction. Budding can occur on the abdomen, postabdomen or stolon in stolonate species. Budding was not noted in the description of Metandrocarpa manina (Monniot & Monniot, 1987), however, it may be evident in Figure 4 below, further studies would be required to monitor and determine if this is evidence of the colony budding.
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| Figure 4 |
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Anatomy and Physiology |
External Morphology | |
Metandrocarpa manina zooids are typically flat, and between 1-5 mm in length although have been most commonly reported as approximately 3 mm in length (Monniot & Monniot, 1987). The zooids are identifiable from the bright red body with white-yellow colouration between the siphons. However, morphological differences between zooids of the same colony were evident as the length of siphons and spread of colouration between the siphons varied among individuals. Zooids also have a transparent tunic that spreads slightly onto the substratum. Individuals within the colony appear to be solitary, however on closer inspection, it is evident the zooids are connected by a stolon which use a vascular network to transport blood cells (haemocytes) around the colony (Monniot & Monniot, 1987).
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Internal Anatomy | |
Limited studies have been undertaken to describe Metandrocarpa manina and some details of their internal anatomy remain to be described. The internal anatomy of the species is described below, according to that of a typical ascidian, with additional information from personal observations (see dissection of Metandrocarpa manina in Figure 5 below) and the description by Monniot & Monniot (1987).
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| Figure 5 |
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Tunic Body Wall and Musculature | |
The tunic of zooids is the outer-most layer of the body, covering the epidermis (Figure 5). The tunic of Metandrocarpa manina is thick, tough and transparent with red dots, comprised of proteins and carbohydrates (Figure 6). Additional support and protection is provided by structural fibres known as tunicin, specifically arranged in parallel sheets that are layered at angles (Ruppert, et al., 2004). The tunic also connects the individual zooids at regular spacing via a stolon (Monniot & Monniot, 1987). The stolon provides a vascular network containing blood cells (haemocytes) that are generated below the epidermis to assist tunic growth at the same rate as the zooid (Ruppert, et al., 2004). The stolon also provides extra anchorage.
The epidermis overlies the basal lamina and connective tissue which comprises muscles, nerve cells and haemocytes. Muscles are circularly arranged around the siphons to control the size of siphon openings. Longitudinal muscles run along the length of the body and connect it to the siphons, which facilitates contraction of the whole body when disturbed (Ruppert, et al., 2004).
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| Figure 6 |
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Pharynx and Atrium | |
Metandrocarpa manina feeds and respires via the buccal and atrial external siphons (Figure 7). The buccal siphon empties into the pharynx, which allows gas exchange and food capture. The buccal tentacles at the siphon opening prevent large, unwanted particles from entering the pharynx (Ruppert, et al., 2004) (Figure 7). Water that passes through the pharynx is further filtered by small, ciliated gill slits (stigmata) (Figure 8) that capture food particles in a mucous net, secreted by the endostyle, on the pharynx surface (Ruppert, et al., 2004). Water flows though the pharynx to the atrium, a large, water-filled cavity, where it exits via the atrial siphon. The atrium also provides structural support to the pharynx and receives developing larvae from the gonoducts and waste products from the anus, for dispersal (Ruppert, et al., 2004).
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| Figure 7 |
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| Figure 8 |
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Digestive and Excretion System | |
Metandrocarpa manina feed on particles such as plankton present in the water they filter. Food particles collect in the mucous net, on the stigmata before being rolled into digestible cords and transported to the oesophagus (Ruppert, et al., 2004). Food particles in the mucous net are delivered from the oesophagus directly to the stomach and intestine in a U-shaped loop that houses secretory cells for extracellular digestion. With a U-shaped gut, the anus is positioned outwards within the atrium to transport waste out through the atrial siphon (Ruppert, et al., 2004).
Metandrocarpa manina, like all ascidians, does not have a metanephridial system to remove waste products. Therefore, they use two processes, the first is to convert metabolic products to ammonia which is diffused across the pharynx surface, out of the body (Ruppert, et al., 2004). Whereas, for uric acid and urates the waste is stored in nephrocytes which accumulate in tissues within the zooid,such as the body wall and digestive loop (Ruppert, et al., 2004). The waste is only released when the zooid dies.
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Internal Transport and Gas Exchange | |
The haemal system of Metandrocarpa manina is well set out and comprises of a heart (short and curved) located at the base of the digestive loop, vessels and sinuses (Figure 5). Gill slits within the pharynx form a respiratory surface for gas exchange to occur. The blood is rich in haemocytes, however unlike humans, the haemocytes lack a respiratory pigment and gas is transported via plasma (Ruppert, et al., 2004). The heart pumps blood around the zooid through contractions of myocardium filaments that force blood flow in one direction and prevent backflow (Ruppert, et al., 2004). Interestingly, ascidians use heartbeat reversal, where the heartbeat briefly stops every few minutes before reversing direction. Since body tissues are supplied by blood in a series, rather than a parallel circuit, this ensures that nutrient-dense blood is not always delivered first to the same organs (Ruppert, et al., 2004).
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Nervous System | |
While larvae have a dorsal hollow nerve cord, this is lacking in the adult form. The remnant is the cerebral ganglion (brain) located within the connective tissue, between the buccal and atrial siphons (Ruppert, et al., 2004). The cerebral ganglion controls the contraction and expansion of the majority of the body, including the buccal and atrial siphons. Furthermore, no sensory organs exist in adult ascidians, however, there are large numbers of sensory cells on internal and external surfaces of the buccal tentacles and the atrium. These cells are believed to be important in directing water flow through the pharynx (Ruppert, et al., 2004).
It is thought that zooid colonies are connected via a nervous system and all contract when one zooid does so. However, in the video of Metandrocarpa manina in Life History and Behaviour this was not evident. Further studies are required to determine the nerve connectivity between zooids.
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Reproductive System | |
Typically ascidians are hermaphroditic with separate male and female gonads which allow for cross fertilisation (Figure 5). This is facilitated by the oviduct and spermiduct which are positioned next to the intestine and also end near the anus, which release eggs and sperm through the atrial siphon (Ruppert, et al., 2004).
The reproductive system of Metandrocarpa manina varies between zooids, as previously shown by Monniot & Monniot (1987). Metandrocarpa manina are hermaphroditic zooids with either one or two testis on either side and ovaries present on one or both sides. Larvae were found to be brooded in the ‘cloacal cavity’ (Monniot & Monniot 1987). Further details of reproduction and development are outlined in Life History and Behaviour.
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Evolution and Systematics | |
Fossil History
The fossil history is limited for all ascidians as they are soft-bodied organisms (Ruppert, et al., 2004). Without a calcium carbonate or phosphate carbonate test or exoskeleton it is not well preserved throughout history.
Systematics or Phylogenetics
There are five key characteristics of chordates that ascidians portray either in the larval or adult phase. During the larval stage all characteristics are present, including the notochord, dorsal hollow nerve cord, pharyngeal slits, endostyle and post-anal tail (Noriyuki, et al., 2014). The only characteristics present in the adult body plan are the pharyngeal slits (pharynx) and endostyle (Ruppert, et al., 2004).
The evolution of Metandrocarpa manina is not well studied, however, it is known the colonial body plan has evolved independently several times within Class Ascidiacea (Ruppert, et al., 2004).
The phylogeny of Metandrocarpa manina is listed below;
Kingdom Animalia
Phylum Chordata
Subphylum Urochordata
Class Ascidiacea
Order Stolidobranchia
Family Styelidae
Genus Metandrocarpa
Species manina
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Biogeographic Distribution | |
The Metandrocarpa manina colony was identified on a piece of rubble in the north-western region of Heron Island, on the outer reef. Metandrocarpa manina has previously been recorded at Nouméa, New Caledonia, Papeete, Tahiti (Monniot, et al., 1991) and Ibo Island, Mozambique (Monniot, 2002). In these areas Metandrocarpa manina inhabits reef crests, outer reef slopes and lagoons (up to 5 m depth) (Monniot & Monniot, 1987). The species is difficult to identify, being only 1-5mm in length, which may account for the few recorded sightings of colonies. Metandrocarpa manina are closely related to Metandrocarpa sterreri, which has been identified in Bermuda and on the Guadeloupe coast (Monniot & Monniot, 1987). Consequently, there is a possibility that Metandrocarpa manina may also be widespread across the Pacific and Indian Oceans like Metandrocarpa sterreri. However, without surveys to test abundance of Metandrocarpa manina around New Caledonia the geographic range of this species is largely unknown.
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Conservation and Threats | |
Metandrocarpa manina has not been studied in great detail, therefore, current threats are unknown as they have not been assessed (International Union for Conservation of Nature and Natural Resources, 2015).
Climate change could potentially impact this species. The larval stage is the most susceptible aspect of the lifecycle to environmental changes. As sea surface temperature increases and anthropogenic effects such as increased nutrients reach coral reefs, then ascidian reproduction, larval settlement and metamorphosis may be impacted (Bates, 2005).
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References |
Acknowledgements | |
I would like to acknowledge Dr Merrick Ekins, Collection Manager of Sessile Marine Invertebrates at the Queensland Museum for his assistance in identifying the Metandrocarpa manina specimens. Also, to Marie-Helene Hegarty for translating the Monniot & Monniot (1987) description of Metandrocarpa manina from French to English.
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Bibliography | |
Bates, W., 2005. Environmental factors affecting reproduction and development in ascidians and other protochordates. Canadian Journal of Zoology, Volume 83, pp. 51-61.
International Union for Conservation of Nature and Natural Resources, 2015. Search Results. [Online]
Available at: http://www.iucnredlist.org/search [Accessed 28 May 2015].
Knott, P., 1985. The Australian Ascidiacea Part I, Phlebobranchia and Stolidobranchia. Memoirs of the Queensland Museum, Volume 23, pp. 1-438.
Lacalli, T., 1999. Tunicate tails, stolons, and the origin of the vertebrate trunk. Biological Reviews, Volume 74, pp. 177-198.
Monniot, C., 2002. Stolidobranch ascidians from the tropical western Indian Ocean. Zoological Journal of the Linnean Society, Volume 135, pp. 70-72.
Monniot, C. & Monniot, F., 1987. Les Ascidies de Polynésie française. s.l.:Mémoires du Muséum National D'Histoire Naturelle.
Monniot, C., Monniot, F. & Laboute, P., 1991. Coral Reef Ascidians of New Caledonia. s.l.:ORSTOM.
Noriyuki, S., Rokhsar, D. & Nishikawa, T., 2014. Chordate evolution and the three-phylum system. Proceedings of the Royal Society Biological sciences, Volume 281.
Ruppert, E., Fox, R. & Barnes, R., 2004. Invertebrate Zoology: A Functional Evolutionary Approach. 7th ed. s.l.:Brooks/Cole, Cengage Learning.
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