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Sansibia spp. (Alderslade 2000)
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Megan Skelton 2016
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Summary | |
The octocoral Sansibia differs from other genera present in the Family Xeniidae due to the high concentration of zooxanthellae within its polyps, which are only semi-retractable when disturbed, and the morphology of sclarites (Janes and Wah, 2005). Sansibia's ecological niche allows fast colonisation of disturbed reefs, allowing stabilisation of substrates (Chanmethakul et al 2009), however the true extent of their distribution is not yet known (Alderslade, 2000). Spheroid-like platelet sclerites were extracted from the polyp and inferences on their role were made based on findings available in the scientific literature. As this colony was collected from the University of Queensland's Aquarium, its origins are unknown, which resulted in some lack of conclusion for the morphology and symbiosis, as ecological context would have allowed further inferences of characteristics.
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Physical Description | |
The genus Sansibia was recently introduced to incorporate members of Anthelia and Clavularia and provide clarity and consistency to the standing genus descriptions (Alderslade 2000). This specimen was not identified down to species level, as revised descriptions of the species compiled in the genera have not been conducted and pictorial identification methods such as field guides do not include Sansibia yet. Morphology within the family Xeniidae is quite consistent, so dividing characteristics are often cryptic (Alderslade, 2000).
Sansibia genus is characterised by slender stalks, thin coenocarc and non-retractile polyps, which curl when disturbed (Alderslade 2000; Janes and Wah, 2005; figure 1). The stalks are highly iridescent, with opaque skin and sclerites visible through the epithlium under a dissection microscope (figure 2). While members of the genus are predominately brown, though can incorporate blue or green iridescence in the tentacles (Alderslade, 2000). Polyps – the tentacles and pinnules in particular – are of a much darker colouration which can be attributed to the high concentration of zooxanthellae (Janes and Wah, 2005). Sansibia do not have dimorphic polyp structure as they lack siphinozooids. Autozooids (responsible for feeding and reproduction) are the only form found on this coral (Alderslade, 2000; figure 3).
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| Figure 1 |
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| Figure 2 |
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| Figure 3 |
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Ecology |
Morpho-functional Ecology | |
Water clarity, food availability, angle of the slope and sediment type are tied to the distribution and richness of Octocorals (Hutchings et al 2008; Chanmethakul et al 2009). Ecological context is also the most effective way to determine genus due to the high site-specificity exhibited in this taxonomic grouping (Janes, 2013). Based on life-history characteristics, it is inferred that Sansibia - like other Xeniidae species – are important in recovering disturbed reef ecosystems as they may stabilise the substrate through asexual reproduction (Chanmethakul et al 2009).
Typically found between four and seven meters depth, Sansibia occupies habitat which is usually only accessible to azooxanthellae species, due to the turbidity and low light penetration. Found only on sand within reef patches, preferring a gradient between ~45 degrees and horizontal on the reef-slope or lower zone. It is currently inferred Sansibia can cope with high sediment loads based on complementary morphology with well-studied species in the same niche (Chanmethakul et al 2009).
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Symbiosis | |
Internal
This combination of the heterotrophic animal and autotrophic algae creates a unique holobiant, extending the physiological capacities of the organism (Rowan 1998). Five of the eight clades from the dinoflagellate genus Symbiodinium have endosymbiosis with coral and the microalgae allow tight nutrient cycling in exchange for shelter within the host coral’s tissue, maintaining the high productivity of coral reef ecosystems (Horgh-Guldberg, 2006).
Zooxanthellae is related to geographic distribution over taxonomic relationship, due to the continuous losses and gains of the symbiont over evolutionary history; the condition of the ancestor is still debated. Tropical environments have greater richness of symbiont species in comparison to temperate (Oppen et al 2005).
Ocotocorals have a high proportion of azooxanthellae species, and while these can often be identified through the overall bright colony colouration, habitat specifics are also essential, as photosynthetic organisms require high light penetration (Oppen et al 2005). While Sansibia is described as zooxanthellate, they also occupy shallow environments with low-light and high flows, which typically is not ideal habitat for successful photosynthesis.
Zooxanthellae was extracted from the coral polyp to ensure the genus description was correct and confirm the presence. The polyp was removed from the colony and squished between a cover slip and slide and viewed under a compound microscope. The microalgae was present and was golden to dark brown, giving the coral its characteristic colour (figure 6).
It has been noted that deep-water corals can improve the photosynthesis of algae through light-amplifying systems (Schlichter 1990), increasing efficiency. While this system was described for species at over 100 meters depth, the iridescence produced by the sclerites in Sansibia species stalk and tentacles may produce a similar effect, allowing the high concentration of zooxanthellae to absorb maximum light. As this is only an assumption based on traits present, further experiments need to be conducted to determine if there is an interaction.
External
Small brittle star was observed emerging from the colony, indicating there are external symbiotic relationships with this species. Many species also feed upon the mucus that is excreted by the coral (Oppen et al 2005).
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| Figure 6 |
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Secondary Compounds | |
These species-specific compounds are primarily used over cnidocytes in Octocorals (due to their underdeveloped state) and are used for defence reproduction, competition for space and antifouling (Sammaro and Coll, 1992; Hutchings et al 2008). Many species are icthyotoxic, using terpanoid toxins as predator defence, although it has been suggested that some species have adopted Barasian mimicry to enhance protection with reduced energetics by only ‘pretending’ to contain toxins (Sammaro and Coll, 1992).
As an unidentified hermit crab was discovered feeding on the Sansibia colony, it has been assumed that either this species (or genus) is not icthyotoxic, or the predator has evolved the capacity to withstand these terapnoids.
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Life History and Behaviour |
Feeding | |
Zooxanthellate cnidarian species all feed in a similar manner, relying on both autotrophy (from the microalgae) and heterotrophy (particulate ingestion). Zooxanthellae provide as much as 95% of the photosynthetic products across the symbiont-host barrier, providing essential compounds and energy for the coral. In turn, the zooxanthellae are provided with the coral’s waste products which cannot be acquired through the water column (Hoegh-Guldberg, 2006). Tentacles capture particles of inorganic and organic dissolved matter as well as bacteria and plankton from the water column (Goreau et al 1971). Mucus is secreted help allow this matter to be captured on the tentacles; nematocysts are not used in prey capture (Hutchings et al 2008).
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Reproduction and Development | |
Asexual
Throughout a coral's lifetime, it will continuously reproduce asexually through colony fission or budding off the adult (McFadden et al 2014). This method of reproduction allows high colonisation rates of an area, though sexual reproduction remains the primary method of population growth (Khang et al 2014). No larval stage occurs with asexuality.
Sexual
Two forms of sexual reproduction observed in Octocorals are brooding and spawning (Sammarco and Coll, 1992). Sansibia reproduction has not been investigated, although internal brooding is the dominant form present in the Xeniidae family. This may also be related to the climate and distribution, as gonochoristic (single sex) brooders are linked to warmer waters (Khang et al 2011).
In brooding species, sperm is released into the water column, fertalising the eggs internal or externally on the maternal colony. The larvae are retained within while developing, before detaching a few days after a full moon and settling (Alino and Coll, 1989). Brooding pouches can be invaginations of the surface epithelium (external) or (internal), and either permanent or seasonal structures (Achituv et al 1992). In other Xeniidae species, the symbiotic zooxanthellae has been transferred to the endoderm of the planula larvae so it enters the water column with the algae; it is unknown if this occurs in Sansibia, or if juveniles must acquire the zooxanthellae from the water column (Khang et al 2011).
Synchronius mass spawning on the Great Barrier Reef has also been observed for the Order Alcyonacea (Alino and Coll, 1989), although since Sansibia has not been recorded in this region, it is unknown if this is applicable to the genus.
Development
Unlike other Cnideria, Anthrozoa lack the triphasic life cycle due to the absence of the medusa stage (Ruppert et al 2004). Therefore, the life cycle moves from polyp to planula and back to polyp, beginning metamorphosis into sessile adult once the mobile larvae form settles on the benthos (Hutchings et al 2008).
Xeniidae species are fast-growing with a short life expectancy, though can quickly colonise soft-bottom habitats (Hutchings et al 2008). However, there is little relationship between size and age, as often corals can shrink in size due to presence of stresses including predation, movement of substrate or unusual wave action (Hutchings et al 2008). This was exhibited on the Sansibia specimen, which had shrunk to half its size in the course of a week due to predation.
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Mucus Expulsion | |
Mucus is produced via the polyp epidermis in all coral, although the composition varies significantly between species (Meikle et al 1988). Desiccation protection, resistance to environmental change or disease and a source of nutrition for other reef inhabitants are a few of the functions that mucus provides for both the coral and reef ecosystem (Meikle et al 1988).
Upon disturbance, the polyp tentacles curled in on themselves and on occasion would release strands of white mucus, as seen in Figure 7. As disturbance stimulus remained constant, this ruled out the possibility that this was in response to a specific stress. The mucus strands were extracted and subsequently observed under a compound microscope. Despite only seen under response to stress, no nematocytes or zooxanthellae were observed from over 13 samples (figure 8). This suggests that this species does not employ nematocytes in inter-specific composition (outlined further in Cnidocytes), however comparison of mucus samples of disturbance by coral and non-coral stimulus will confirm this. Instead, there may be secondary compounds released, though once again, further investigation is required.
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| Figure 7 |
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| Figure 8 |
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Additional Behaviour Observations | |
Sansibia species coral polyps responded only to mechanosensory stimulation (video), coiling the tentacles in on themselves. Light increase/decrease and altering current intensity had no effect. Fluro and DAPI staining was attempted in order to observe the structure of the polyps, as this has not been investigated in Sansibia species, but this was unsuccessful both times.
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Video of Sansibia spp. contracting and uncoiling it's polyps after mechanosensory stimulation. Video by Megan Skelton (2016)
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Anatomy and Physiology |
Tissues and Body Compartments | |
Like other sessile benthic organisms, Anthrozoa are radially symmetrical, thus allowing them to sense the environmental from all directions (Sammarco and Col, 1992; Ruppert et al 2004). Cnideria are considered diploblastic, containing the epidermis and gastrodermis and sandwiched between these layers is a gelatinous, non-tissue matrix known as mesoglea (Ruppert et al 2004). As modular organisms, Sansibia are comprised of individual polyps forming a colony which is linked though the gastrovascular system, overlayed by a cenoscarc, which allows the sharing of waste, nutrients and water (Ruppert et al 2004).
Octocorals have eight tentacles and mensentaries in multiples of eight, which bear retractor muscles as well as gastrovascular cavity divided into eight through partitioning of septa (Daly et al 2007). Polyps are attached via a pedal disk, with mouth facing upwards, surrounded by eight tentacles bearing pinnules. Other species polyps have the ability to retract towards the pedal, though it is not known if this inability in Sansibia is due to lack of appropriate musculature or another factor (Daly et al 2007).
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Cnidocytes | |
The combined sensory-effector cells are the uniting characteristic of the Cnideria phylumn, used primarily for defense and predation (Ruppert et al 2004). Anthrozoa are the only Cnideria class to contain all three forms of nematocytes, which are embedded in the epithelium (Ruppert et al 2004); no loss of nematocytes have been recorded across any of the taxa, which has ensured the trait remains diagnostic for admittance in the phylum (Sammarco and Coll, 1992). However, these are not well developed in the Octocorals, which suggest they rely upon the secondary compounds they produce as a primary feeding and/or defence strategy. Some species may employ these nematocytes when competing for space, though are generally considered ‘non-toxic’ to most organisms (Sammarco and Coll, 1992).
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Sclerites | |
One of the major indicators of Octocoral taxonomy is through the structure and concentration of sclerites, due to the wide variation observed (Alderslade, 2000). These magnesium-calcite structures form within the scleroblasts and distributed throughout the colony, providing hydrostatic support through skeletal elements (Gabay et al 2014).
Size relates to the particular function, with larger sclerites used to deter predators, though smaller is related to stiffness or structural integrity. Greater concentrations of the smaller sclerites allow increased rigidity, due to the surface area provided to allow tissue attachment (Clavecio et al 2007). Reflected light produces an opalescent effect in the Sansibia species (Alderslade, 2000) and may have an interaction with light attenuation and zooanthellae.
Sclarite extraction and analysis allows the confirmation of species as well as the role in which they play. Three polyps were extracted from the coral colony, placed in a petri dish and covered with bleach for approximately 60 minutes; every 10 minutes the sample was shaken to ensure the tissue was dissolving. The resulting materials was then placed into an Eppendorf tube and washed with distilled water before spinning on a centrifuge. This process was repeated before the pellet was extracted, placed on a cavity slide and analysed under a compound microscope.
Two spheroid-like platelet sclerites were extracted (figure 10 and 11), confirming that this coral was indeed Sansibia. Sclerites visible within the polyps through microscopic magnification have the appearance of elongated rods, though this is only due to being viewed from another angle (Janes and Wah, 2005; figure 9). These polyp sclerites were 0.02mm in length, indicating they are the smallest of these structures from the Xeniidae family:
- Xenia: 0.04mm
- Heteroxenia: 0.03mm
- Effkatounaria: 0.05mm
- Cespitularia: 0.004mm
- Anthelia: 0.03 – 0.2mm
- Sympodium: 0.03mm (Janes and Wah, 2005).
Due to the size, this indicates that the sclerites within Sansibia coral poylps are used for structural integrity. Further work using electron scanning microscopy to relate surface ultrastructure to attachment potential would be required to confirm the function and performance of the sclerites.
In other species, the insufficient quantity of sclerites may indicate further grinding or manipulation of the sample is required, though Sansibia is characterised by few or absent sclerites. Smaller sclerites are often found in greater quantities for integrity coupled with flexibility, though perhaps due to the lack of retraction of the polyps Sansibia does not . This may need to be investigated in relation to strength of hydrostatic skeleton or (environments as may be phenotypic plasticity).
It is important to note that sclerite morphology can be altered due to aquarium or laboratory conditions, so what is present may not reflect the true morphology of the genus (Janes and Wah, 2005); the time frame for this change has not been mentioned, although Sansibia spp. are fast-growing and short-lived, so this effect may be amplified due to the rate at which sclarites are produced. As the specimen was retrieved from the University of Queensland Aquarium, the length in captivity is unknown.
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| Figure 9 |
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| Figure 10 |
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Biogeographic Distribution | |
Due to the relatively recent formation of the Sansibia genus, it is currently described as ‘scattered’ (Alderslade, 2000), as genus distributions are more commonly recorded than species (due to high site-specificity) so it is difficult to discern which sightings recorded as others were actually Sansibia. Therefore, the current range of Sansibia is likely to be conservative. In general, Ocotocorals have a broader distribution than hard corals, due to the number of species azooxanthellae species which aren't constrained by the ability to photosynthesize (Alderslade, 2000).
Sansibia occupies sub-tropical and tropical waters of the Indo-pacific including Africa, Hawaii, Taiwan, Thailand, Indonesia and Australia (Alderslade, 2000). Samples deposited in museums have been recorded from Western Australia, Northern Territory, South-East Queensland and Northern New South Wales (figure 12). As of yet, no species have been sighted in the Great Barrier Reef.
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| Figure 11 |
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Evolution and Systematics | |
Phylum: Cnidaria
Class: Anthrozoa
Subclass: Octocorallia
Order: Alcyonacea
Family: Xeniidae
Genus: Sansibia
It is widely accepted that Anthrozoa are the basal class of Cnideria, although the relationships of the remaining classes, and the position of Cnideria in relation to the evolution of the metazoan is still under debate.
The order Alcoynaria are exclusively marine (Hutchings et al 2008). Although Octocorallia is considered a monophyletic
group with the Hexacorallia, there is little consensus of the phylogeny within
the subclass Octocorallia (Daly et al 2007). This is primarily due to the lack of
morphological characteristics required to separate or unite these groups at a
higher level and the continued existence of intermediate forms (Hutchings et al 2008).
DNA barcoding for members of Xeniidae based on four loci has determined that this family maybe polyphyletic, though the standing work has poor resolution (McFadden et al 2014). While the current understanding of this relationship may be clouded due to the exclusion of some genera, this is the most up to date work. It has been noted that significant revisions and descriptions are required for Octocorals (Hutchings et al 2008). Despite no obvious morphological differences,three clades within Xeniidae were determined, with Sansibia appearing to be paraphyletic with Sarchothelia and a number of Xenid species (McFadden et al 2007; figure 13).
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| Figure 12 |
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Conservation and Threats | |
No threats have been identified that focus specifically on Sansibia, although there are a number of factors which globally impact the Anthrozoa. IUCN assessments, and therefore special conservation status, do not exist for family level or below within the order Alcyonacea, highlighting the lack of detailed knowledge for soft corals.
Environments in which coral reefs exist are typified as stable, with seasonal fluctuations in abiotic variables quite small. However, changes in the environment, especially temperature can cause stress, leading to the intracellular zooxanthellae fleeing from the coral host, known as bleaching. Mass bleachings have caused the mortality of many ocotocoral species worldwide and is particularly concerning in relation to climate change increasing the severity and frequency of these events (Hoegh-Guldberg 2006). Damage or reduction of corals reefs is projected to have a substantial impact upon the economy, due to the resources and services they provide, including tourism, support for fisheries, drug discovery and coastal protection against extreme weather (Hoegh-Guldberg, 2006).
With increasing atmospheric carbon, the saturation state and concentration of carbonate declines, reducing the calcification ability for marine organisms who build skeletons and shells (Gabay et al 2014). Dissolution and thinning shells have been reported across a wide geographic and taxonomic range. In contrast, Octocoral tissue may provide protection for sclerites against elevated oceanic pH, particularly in comparison to other coral groups with greater skeletal structural integrity. The mechanism for the defence is not yet understood, although species with zooxanthellae are assumed to receive the greatest advantage (Gabay et al 2014).
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References | |
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