Marine Invertebrates of Bermuda Green Sea Mat (Zoanthus sociatus) By Emily Field and James B. Wood (Ed) Abstract Taxonomy  Habitat  Ecology  Recent Research  Commercial Importance  Bermuda Laws  Personal Interest  References  Links

Abstract

Zoanthus sociatus is a zoanthid commonly known as the green sea mat, or the button polyp. Zoanthids (Class: Anthozoa) are found in tropical regions from Japan to Central America and the Caribbean. Z. sociatus is found in shallow reef zones, and has a symbiotic relationship with zooxanthellae. Since Z. sociatus gets most of its required energy from the symbiotic zooxanthellae, very little feeding is required. Zoanthids mainly reproduce asexually, although there is a limited amount of sexual reproduction. Sexual reproduction is also size-dependent, so that a colony can grow large enough to reduce mortality risks. Although some zoanthids use chemical compounds for defense and competition, Z. sociatus does not. Therefore, Z. sociatus must use other methods to compete. One such method is its use of the low intertidal zone to avoid high levels of predation and competition. In addition, Z. sociatus has higher resistance to dessication and a stronger attachment to the sediment than some other zoanthids. Z. sociatus is currently being studied for its use against human lymphatic parasites.

Taxonomy


Phylum: Cnidaria
  Class: Anthozoa
    Subclass: Hexacorallia
      Order: Zoanthidea
        Family: Zoanthidae

Zoanthus sociatus has nematocysts, which makes it a cnidarian. As it has polyp morphology, it is an anthozoan. It also has tentacles in multiples of 6, it falls under the sublass Hexacorallia. It is in the order Zoanthidea due to its lack of a calcium carbonate skeleton.

Habitat

Zoanthus sociatus lives in the lower intertidal and upper subtidal zones on protected Caribbean reefs (Karlson 1986). It is a sessile, colonial organism (Karlson 1988). Z. sociatus grows in the reef understory and on disturbed substrate (Karlson 1983). Z. sociatus can survive dessication and lower levels of salinity than other species (Karlson 1983). Also, it appears to dominate other zoanthids at intermediate levels of disturbance (Karlson 1983).

Ecology

Zoanthus sociatus grows in a colonial, green mat (Goreau 1959). Its polyps are connected via stolons (Karlson 1986). The diameter of its oral disc is usually around 3mm (Sterrer 1986). Colonies are small, generally about 10cm squared (Sterrer 1986). Z. sociatus has no calcareous skeleton, and so it must grow laterally in a sheet or runner morphology (Karlson 1983; Sorokin 1991). Its lack of a hard skeleton makes it more susceptible to shading by other reef organisms, since it cannot grow “upwards” like hard corals (Karlson 1983). However, this trait allows colonies to respond more quickly to disturbances (Karlson 1983).

Z. sociatus has a symbiotic relationship with the zooxanthellate dinoflagellate Symbiodinium pilosum (Iglesias-Prieto and Trench 1994). The zooxanthellae provide the polyps with 48.2% of the fixed carbon required for respiration, which is 95.1% of total fixed carbon they produce (Steen and Muscatine 1984). Since Z. sociatus grows in shallow reef areas, they have clade A zooxanthellae. These dinoflagellate are most effective in higher light regimes (Santos 2002). Iglesias-Prieto and Trench (1994) found that S. pilosum could not photo-acclimate to low light.

When a polyp is shaded, it will decrease in density and increase in height and diameter in order to increase surface area for photosynthesis (Karlson 1983). When it is shaded, bitten, overturned, or otherwise disturbed, Z. sociatus has a higher rate of regeneration than other zoanthids, such as Zoanthus solanderi (Karlson 1983). This ability to regenerate quickly is important for Z. sociatus, as it grows in the low intertidal zone and is thus more susceptible to storm damage (Karlson 1988). Z. sociatus is also more resistant to dessication due to adaptations to its zonation (Karlson 1988). While the polyps are strongly attached to the substrate to increase storm resistance, Z. sociatus also has higher rates of fragmentation compared to other zoanthids (Karlson 1983; Karlson 1986). High levels of fragmentation prevent the stolons from ripping during a storm, as well as increase the dispersal of genetically identical polyps (Karlson 1986). Z. sociatus recovers from a disturbance best when there is macroalgae present and low densities of Diadema antillarus (Karlson 1983; Bastidas and Bone 1996). This is most likely because Z. sociatus can withstand the shading and space competition with the macroalgae better than other species of zoanthids, but that it cannot grow quickly when being eaten by Diadema sp. (Karlson 1983). In addition, Karlson (1983) found that Z. sociatus is most abundant under intermediate levels of disturbance. However, Bastidas and Bone (1996) suggest that Palythoa caribaeorium is a stronger competitor and grows faster after a disturbance than Z. sociatus does.

While P. caribaeorium appears to have a competitive advantage over Z. sociatus, Z. sociatus is still an abundant and competitive species. When it is growing in the low intertidal zone, there is less competition and predation (Karlson 1988). Z. sociatus grows faster and is more abundant than Z. solanderi (Karlson 1983). Its growth rates have been debated though: Karlson (1988) found that Z. sociatus had higher growth rates with decreasing fragment size, while Bastidas and Bone (1996) found that it grew slower with decreasing size. When Z. sociatus and P. caribaeorium were placed next to each other, a stand-off occurred (Bastidas and Bone 1996). However, P. caribaeorium is sometimes observed to overtop, or grow above and shade, in the field (Bastidas and Bone 1996).

Some known predators of Z. sociatus are sea urchins and the corallivorous gastropod Coralliophila abbreviata (Moore 1972; Karlson 1983). A high abundance of Diadema sp. is known to decrease a colony’s ability to recolonize an area (Karlson 1983). Since Z. sociatus does not use allelopathy, its only method of predator avoidance is growth in shallow areas, and thus it is able to escape some of the other predators that deeper zoanthids must combat (Karlson 1988).

As previously stated, the Z. sociatus polyps obtain nearly half of their required energy from the zooxanthellae (Steen and Muscatine 1984). Therefore, the rest of the energy must be obtained through feeding. Zoanthids have nematocysts on their mesenterial filaments that are used for prey capture (Trench 1974). Z. sociatus will eat most anything that is the right size, from Artemia cysts to dissolved organic matter (Sorokin 1991). While zoanthids are less efficient heterotrophs than scleratinians, they produce more energy photosynthetically due to their lack of a calcified skeleton (Sorokin 1991). The lack of a skeleton allows more light to reach the chloroplasts (Sorokin 1991). To digest prey, Z. sociatus uses both extracellular and intracellular methods (Trench 1974).

The majority of reproduction by Z. sociatus is asexual (Fadlallah et al. 1984). There is extratentacular budding, which is the creation of a new polyp from an old polyp, and fission, where the stolons disintegrate and a new fragment in formed (Karlson 1988, Karlson 1989). Fission rates are modified by recruitment and mortality rates of the colony (Karlson 1989). The size of a fragment is also controlled by the increasing rate of mortality with decreasing fragment size (Karlson 1988). A colony is generally genetically the same, as there is little fusion of colonies or recruitment to a old colony (Goreau 1959). Even when a colony is sexually reproductive, a large proportion of polyps remain infertile, which demonstrates the greater importance of asexual reproduction and growth (Fadlallah et al. 1984).

Z. sociatus colonies do not become reproductive until they reach a certain size (Karlson 1988). They are broadcasters, meaning that they use external fertilization, and are mostly hermaphroditic, although some are male or protogynous (female and then male) (Fadlallah et al. 1984). Z. sociatus was found to reproduce seasonally in Panama, and synchronizes the release of gametes with extremely low tides (Fadlallah et al. 1984). Z. sociatus carries more eggs than either P. caribaeorium or Z. solanderi, which could add to its competitive ability (Fadlallah 1984).

Recent Research

Most of the recent research on Zoanthus sociatus focuses on the classification of their zooxanthellae (LaJeunesse 2002; Santos et al. 2002). However, some studies are also being done on the phylogenetics of zoanthids. Reimer et al. (2006) described two new species in southwest Japan.

Other studies being performed are on the medicinal benefits of zoanthid chemicals. Lakshi et al. (2004) discovered that the chloroform methanol from zoanthids killed a parasite that infests the human lymphatic system. It also sterilizes the surviving females (Lakshi 2004). Another possible medicinal use of zoanthids is for osteoporotic drugs (Kuramoto et al. 2004). A chemical isolated from Zoanthus spp. has been shown to prevent bone density decrease (Kuramoto et al. 2004).

In addition, zoanthids fluoresce under specific wavelengths. Recently, the proteins responsible for this fluorescence have been isolated (Miyawaki 2002).

Commercial Importance

The main commercial importance of Zoanthus sociatus is in the aquarium trade. The Green Sea Mat is considered to be aesthetically pleasing and easy to maintain in a tank (Sprung 2003). Recently, chemical compounds in zoanthids are being examined as medicinal sources, as described above (Kuramoto et al. 2004; Lakshi et al. 2004).

Bermuda Laws

There are no specific laws pertaining to Zoanthus sociatus in Bermuda. However, in the 1972 Fisheries Act (Order 2000), 29 marine protected areas (MPAs) were created in Bermuda. Colonies located within an MPA cannot be touched or removed, depending on the MPA (Wood and Jackson 2005).

Personal Interest

I first saw a colony when we snorkeled at Concrete Beach, and was very curious as to what these small, bright green polyps were. I think that Zoanthus sociatus is a very interesting anthozoan. Since it does not have allelochemicals, it must be a hardier species to increase its competitive ability. For example, it is able to grow better than other species of zoanthids when there is an abundance of macroalgae.

References

Bastidas C and Bone D. Competitive strategies between Palythoa caribaeorium and Zonathus sociatus (Cnidaria: Anthozoa) at a reef flat environment in Venezuela. Bull. Mar. Sci. (1996) 59(3):543-555

Fadlallah YH, Karlson RH, Sebens KP. A comparative study of sexual reproduction in 3 species of Panamanian zoanthids (Coelenterata: Anthozoa). Bull. Mar. Sci. (1984) 35(1):80-89

Goreau TF. The ecology of Jamaican coral reefs I. Species composition and zonation. Ecol. (1959) 40(1):67-90

Iglesias-Prieto R and Trench RK. Acclimation and adaptation to irradiance in symbiotic dinoflagellates 1. Responses of the photosynthetic unit to changes in photon flux density. Mar. Ecol. Prog. Ser. (1994) 113:163-175

Karlson RH. Disturbance and monopolization of a spatial resource by Zoanthus sociatus (Coelenterata, Anthozoa). Bull. Mar. Sci. (1983) 33(1):118-131

Karlson RH. Disturbance, colonial fragmentation, and size-dependent life history variation in 2 coral reef cnidarians. Mar. Ecol. Prog. Ser. (1986) 28:245-249

Karlson RH. Size-dependent growth in 2 zoanthid species: A contrast in clonal strategies. Ecol. (1988) 69(4):1219-1232

Kuramoto M, Arimoto H, Uemura D. Bioactive alkaloids from the sea: A review. Mar. Drugs (2004) 2:39-54

LaJeunesse T. Diversity and community structure of symbiotic dinoflagellates from Caribbean coral reefs. Mar. Biol. (2002) 141(2):387-400

Lakshi V, Saxena A, Pandey K, Bajpai P, Misra-Bhattacharya S. Antifilarial activity of Zoanthus species (Phylum Coelenterata, Class Anthozoa) against human lymphatic filarial, Brugia malayi. Paras. Res. (2004) 93(4):268-273

Miller Ac. Observations on the associations and feeding of six species of prosobranch gastropods on Anthozoans. Atoll Res. Bull. (1972) 152:4

Miyawaki, A. Green Fluorescent Protein-like Proteins in Reef Anthozoa Animals. Cell Struc Func. (2002) 27(5):343-347

Reimer JD, Ono S, Atsushi I, Takishita K, Tsukahara J, Maruyama T. Morphological and molecular revision of Zoanthus (Anthozoa: Hexacorallia) from southwest Japan, with descriptions of two new species. Zool. Sci. (2006) 23(3):261-275

Santos SR, Taylor DJ, Kinzie III RA, Hidaka M, Sakai K, Coffron MA. Molecular phylogeny of symbiotic dinoflagellates inferred from partial chloroplast large subunit (23S)-rDNA sequences. Molec. Phylog. & Evol. (2002) 23:97-111

Scheuer PJ. The chemistry of toxins isolated from some marine organisms. Fortschritte der Chemie Organischer Naturstoffe, Wein. (1964) 22:265-277

Sorokin YI. Biomass, metabolic rates and feeding of some common reef zoantharians and octocorals. Aust. J. Mar. Freshwater Res. (1991) 42:729-741

Sprung, Julian. Aquarium Invertebrates. Advance Aquarist Online Magazine. 10.11.2007; 2003. http://www.advancedaquarist.com/issues/feb2003/invert.htm

Steen RG and Muscatine L. Daily budgets of photosynthetically fixed carbon in symbiotic zoanthids. Biol. Bull. (1984) 167:477-487

Sterrer, W(ed). Marine Fauna and Flora of Bermuda. John Wiley & Sons, Inc. NY 1986

Trench RK. Nutritional potentials in Zoanthus sociatus. Helgolaender Wiss. Meeresunters (1974) 26(2):174-216

Wood, J. and K. Jackson. 2005. Bermuda, In: Caribbean marine Biodiversity., Miloslavich, P. and E. Klein Eds. De Stech Publications Inc. pp 19-35


Links

Aquarium Invertebrates: Zoanthids, Polyps as cute as a button
Mostly Colonial & Very Hardy; The Sea Mats & Polyps That Are the Zoanthids