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Student Project

Tethya robusta, (Bowerbank, 1873)

Fiona Naja Hoegh-Guldberg 2018


The Specimen:

The sponge I have obtained from the UQ Lab Aquarium for this species project is a hard-bodied sponge in the genus of Tethya sponges, within the class Demospongiae. Demosponges have a spongin skeleton, with or without siliceous spicules. The specimen also belongs to the order Hadromerida; those demospongiae with radially patterned monactinal (single-ended ray) spicules (Vacelet & Boury-Esnault, 2013; Lévi, 1998). It belongs to the Phylum Porifera, within Kingdom Animalia and family Tethyidae (Heim et al., 2007. The specific species, I have classified as Tethya robusta, on account of its distinguishing physical characteristics; spherical shape, cortical spiculation, distribution and spicule types (Sarà, 1992).


KINGDOM: Animalia

PHYLUM: Porifera

CLASS: Demospongiae

ORDER: Hadromerida

FAMILY: Tethyidae

GENUS: Tethya

SPECIES: robusta

Physical Description

Colouration & Appearance:

Tethya sponges are well known for their globular-to-spherical body morphs and are generally divided into two morphofunctional groups; hard-bodied and soft-bodied. Tethya robusta is a hard-bodied sponge, with a compact and tough external cortex that is typically yellowish, pale orange or pale brown (Sarà & Sarà, 2004; Sarà, 1990). Sarà et Sarà (2004) characterises T.robusta as notably spherical and spiculated, with very distinct rounded tubercles (~2-4mm, Sarà 1990). Flattened papillae and externally protruding spicules also define the species (Sarà & Sarà, 2004). The internal medulla is usually ochraceous yellow, or ‘apple’ to olive-green (Sarà, 1992). The cortex of my collected specimen was a pale orange and the medulla, a bright, potentially ‘apple’ green (see Figure 1). An extracted portion of the specimen analysed under dissection (x10) and compound microscope (x40-60), indicates a well-defined cortex-medulla boundary and prominent cortical spiculation, as described for T.robusta (Figures 1-4, Sarà, 1992).  

Figure 1
Figure 2
Figure 3
Figure 4
Figure 5


Within T.robusta’s globose spongin form, silaceous spicules called strongyloxeas (Figure 3), spherasters (Figure 3) and micrasters (figure 6) were found, as expected for most Tethya species (Sara, 2002). Strongyloxeas (Figure 1-4 & 5A) are stylote (elongated) megascleres that radiate outwards from the choanosomal core, as visible to the naked eye (Figure 1, Sarà, 2002). Specifically, strongyloxeas are short and apically rounded in some areas for this species (Figure 4 & 5A) with dense packing of longer, less-rounded strongyloxeas extending outward from the central region of the medulla (Figure 1-4, Sarà, 1990).

Megascleres and microscleres:

Strongyloxeas provide the majority of structural support within T.robusta, while the spherasters and micrasters, known as microscleres, have a lesser known auxiliary function; maintaining inhalant pores on the sponge surface and surrounding inhalant and exhalant canals within body tissue (Bavestrello et al., 2000; Sarà & Manara, 1991; Ribeiro & Muricy, 2011). The following scanning electron micrographs (SEM's) in figure 6 (below) identify the spherasters and micrasters observed within the medulla canals of my collected specimen.

Architectural skeleton:

Different Tethya species have different megasclere and microsclere shapes and distributions within the cortex and medulla (Sarà, 1992; Sarà, 1990; Sarà & Gaino, 1987). The distribution for T.robusta observed by Sarà (1992) at Laing island in Papua New Guinea (figure 7), shows radial divisions of stongyloxeas as the main category of spicules, with spherasters (~20-80μm) in the medulla being smaller than those in the cortex, and cortical and medulla micraster forms being different (Figure 8, Sarà, 1992). Micraster forms are defined primarily by the 'sharpness' of their ray tips, from tylasters (most sharp) to chiasters (intermediate) and oxyasters (blunt, Van Soest et al., 2012). According to Sarà (1992) cortical micrasters include; oxyasters, chiasters and tylasters (8-12μm), and medulla micrasters include; oxyasters-chiasters (12-16μm). This is compared with the scanning electron microscope (SEM) images of the medulla and cortex for the collected specimen (Figure 8, Sarà, 1992). Notable size variation of megasters (spherasters) within T.robusta is expected between populations in Papua-New Guinea, the Maldives and Australia, with Australian T.robusta having generally larger megasters (Sarà, 1993). Unlike T.boeroi, no medulla-nucleus is found in T.robusta, as would be signified by transverse strongyloxeas in figure 9 (Sarà, 1992). The cortical layer is well developed with spicule layering, and small sub-dermal lacunes are observed below (Sarà, 1990), as shown in Figure 1.  

Figure 6
Figure 7
Figure 8
Figure 9


Sympatry & Ecological Niche:

Sara (1992), describes the morpho-functional differences observed between soft-bodied and hard-bodied Tethya at Laing Island, regarding ecological niche. Soft-bodied Tethya, T.viridis and T.microstella, appear to coexist in a majority of habitats with hard-bodied T.robusta; whereby soft-bodied species are found in sheltered areas, and the hard-bodied T.robusta, in exposed regions (Sarà, 1992, figure 10). A similar dynamic is also observed on a north Australian reef, where T.robusta again occupies exposed areas of the habitat, while T.orphei and T.microstella occupy the sheltered niche (Sarà 1990). Another potential example of this co-existence is seen in Mediterranean reefs, between T.citrina (soft-bodied) and T.aurantium (hard-bodied) (Corriero et al., 1989). Sarà (1990) suggests that within soft-bodied Tethya species, flattening seems to be an adaptation to inhabiting tight interstitial areas. Clear differences in the skeletal architecture and aquiferous systems present questions about the resource partitioning between similar species on the same reef and how differences in feeding modalities and locomotion between soft-bodied and hard-bodied Tethya may facilitate sympatry (Sarà, 1992). Additionally, the tank that my T.robusta specimen was collected from also possessed two other Tethya species (figure 11) that were soft-bodied, although obvious differences between reef-habitat and tank-habitat should be noted.   

Figure 10
Figure 11

Life History and Behaviour


Tethya are well-known for their unique ‘rolling’ locomotive ability. Unlike other sponges, Tethya do not require whole-body morphogenic reorganization to relocate (Nickel, 2006). The general morphology of the body-plan remains stable, and only local rearrangements and cortical tissue are thought to be involved (Nickel, 2006). 'Rolling' has been recorded for different Tethya species, varying from speeds of 24um hr-1 to 2mm hr-1 (Nickel, 2006). However, there is very little literature characterising the specific locomotion of T. robusta. Instead, other species such as T. willhelma have been used as model-specimens. A recent time-lapse study on T. wilhelma (Nickel, 2006) also observed that rolling can occur in straight lines for significant periods of time and that immediate directional changes can occur, although sudden movements are seemingly rare. Rolling behaviour has been observed for over 150 years (Nickel, 2006) and in addition to the periodic nature of contractions also observed, has raised questions about the involvement of contractile waves or peristalsis in ‘rolling’ movements (Bond & Harris, 1988; Ellwanger et al.,2006; Nickel, 2006). Nickel (2006), however, demonstrates that contraction is not solely responsible for the skeletal rotation observed, but only for discrete modifications to local tissue (Nickel, 2006). Furthermore, changes to the angles of megasclere bundles demonstrate that movement and contraction only modify morphology temporarily (Nickel, 2006). Instead of cell-by-cell reorientation, local cell contractions appear to deform the Tethya shape, and thus, shift the trace-point (figure 11), causing rotation (Nickel, 2006). Internal cell movements that cause long-anchored body extensions and stretching have also been shown to determine the “cruising radius” of T. wilhelma (Nickel, 2006). Two timelapses of T. wilhelma rolling are provided below (Supplemental material: Nickel, 2006). It is not yet known whether many other Tethya species can locomote this way, but considering the similar morphological spherical shape and compact aquiferous system, T. robusta likely shares some locomotive behaviours.

Figure 12


Sexual Reproduction:
Tethya are oviparous sponges which produce ooctyes with a true primary envelope, a common feature to all of Eumetazoa (Ereskovsky, 2010) and released eggs have peripheral vacuoles to facilitate the attachment to the substrate and protect the egg (Ereskovsky, 2010). The synthesis of a collagen capsule by the lophocytes of the mesohyl is seen in Tethya citrina, and this is hypothesized to be a prototype of the Metazoan vitellaine membrane (Ereskovsky, 2010). Oocytes can be found throughout the choanosomal tissue of Tethya sp. Sexual reproduction includes; gametogenesis, fertilization, embryonic development and sexual differentiation. In the study of Corriero et al. (1996) on T.citrinaT.aurantium, no males were found in collection. This is either because of a short season of spermatogenesis or simply because males are scarce in these populations (Corriero et al. 1996). Scalera Liaci et al. (1971) only found 6 males in a population of 863 T.aurantium. More information is needed about the reproductive cycles of T.robusta. Unfortunately, no ooctyes were found in my specimen.

Sexual Strategies:

T.aurantium and T.citrina are two Tethya species that live sympatrically in the Mediterranean. Both are gonochoristic and partially coincide in oocyte production during the Summer months (Corriero et al., 1996). In the summer, T.citrina produces more ooctyes than T.aurantium. However, in the winter months when both bud asexually, T.aurantium produces many more asexual buds (Corriero et al., 1996). T.citrina is also adapted to colder, more northern waters and sexually reproduces earlier, perhaps suggesting that it outcompetes T.aurantium in sexual reproduction (Corriero et al., 1996). It is hypothesised by (Corriero et al., 1996) that these differences can be explained by differential reproductive resource allocation. 

Asexual Reproduction:
Asexual budding, provides an alternate method of self-replication, well-described in sponges. Bud shape and size within Tethya can be highly variable, for example, T.norvegica is a deep-water species that has notably fast budding, with buds that produce secondary budding (Corriero et al., 1996). According to Sarà (1992), T.robsuta has stolons to anchor its buds. According to observation of Tethya budding, stalks progressively constrict until the propagule breaks off and moves to grow on the substrate further away (Vacelet, 1959, Sánchez, 1984; Teixidó et al., 2006). Unfortunately, no stolons or asexual buds were observed in my collected specimen.

Cryptochrome Experiment:

Methods: In the interest of recording T.robusta locomotion, I used three Tethya found in a tank of UQ Lab Aquarium as subjects in an experiment, positioning each Tethya in the centre of their own container lined on one side with blue cellophane and the other with red (see figure 12A). The initial position of the sponge was recorded on the bottom of the container with permanent marker and recorded again the following week (t=7 days), over a period of two weeks (figure 12B). One of the sponges (T.robusta) was sacrificed from the experiment in the second week for dissection analysis (n=5). My experiment was based upon a previous experiment on Tethya sp., that used light and dark sides of a container to test for phototaxis (Star, 2015). The Tethya were fed with microalgal solution at the start of the second week and water was changed. Containers were rotated clockwise and swapped to another container position anti-clockwise to reduce environmental bias with randomization.

Aim: The experiment aimed to test for a bias in movement away from the blue-light side, because blue-light sensitive cryptochrome has been proposed as the major photoreceptive system for other demosponges such as Suberites domuncula (Müller et al.2010) and Amphimedon queenslandica (Rivera et al., 2012).
The key assumption of this experiment -that Tethya sensing blue-light will evade it in order to seek sheltered habitat -is perhaps flawed, because as noted in the literature (Sara, 1992), hard sponges, including T.robusta have been known to occupy open habitat (See Ecology: Sympatry & Ecological Niche, above).

Results: Minimal movement (<1cm) was observed for one of the sponges, with no obvious bias toward either side (figure 12B). Considering that the Tethya species were not removed from the rubble, as in Star (2015), there is the possibility that they might be less inclined to seek sheltered habitat. However, this experiment, low replication (n=5) and potential pseudoreplication aside, did not find evidence of blue-light sensitive negative-phototaxis, or a bias towards red light. 

With more Tethya subjects (n>10) of the same soft-bodied, sheltered-niche seeking species (T.viridis or T.microstella, for example; Sarà, 1992) this experiment could perhaps give more valuable insight into the presence of blue-light sensitive cryptochrome within demosponges, as a precursor to more complex photoreception (Gehring and Seimiya, 2010).

Figure 13

Anatomy and Physiology

The Aquiferous System:

The aquiferous system within Tethya species, as observed in T.wilhelma, consists of the following components, following the current of water through the sponge (figure 13):

ostia -> incurrent canals -> prosopyls -> choanocyte chambers -> apopyles -> excurrent canals -> oscules

Tethya, as a genera within demospongiae, have leucon-typecanal systems with complex canal regions and choanocyte chambers (Hammel & Nickel, 2014). The ostia, in T.robusta, are generally reduced compared to other species (Sarà, 1990). Microscopic sub-dermal lacunae act as incurrent openings to the incurrent canals (Sarà, 1990). From there, water is flushed between the prosopyle (incurrent opening of the choanocyte chamber) and apopyle (excurrent opening), through the choanocyte chambers where water velocity changes due to pressure drop (Figure 14 ). Small to large variations in apertures of the canal system contribute significantly to the pressure drop that occurs to optimize nutrient uptake and gas exchange within the system (Hammel & Nickel, 2014). While choanocyte chambers are necessary displacement pumps that create differentials in the aquiferous system, resistance is also necessary to increase particle rate capture, as can be seen in figure 14 (Hammer & Nickel, 2014). 

Figure 14
Figure 15

Functional Morphology & Fluid Dynamics:

As mostly, sessile filter feeders they are highly dependent on their aquiferous systems for fundamental physiological function (Hammel & Nickel, 2014). Therefore, the morphological and architectural complexities within the aquiferous system must reflect hydrodynamic constraints and fluid dynamics (Hammel & Nickel, 2014). The key effectors of contractions are endopinacocytes (Nickel et al., 2011). These cells cause changes in the diameters of canals, to increase resistance in the system (Nickel et al., 2011). Contractile waves, such as those observed in T.wilhelma, have also been shown to affect perfusion rates (Nickel et al, 2011). Interestingly, Hammer & Nickel (2014) discovered that choanocyte chambers in T.wilhelma have ‘reticuloapoplyoctyes’ which are involved in the regulation of water flow within choanocyte chambers. This cell type, as shown in figure 15 (4), is highly fenestrated to allow for the gradual influx of water through the apopylar opening, increasing the gauge inflation and resistance, without risk of self-inflicted damage (Hammer & Nickel, 2014). The number of choanocytes within a chamber was found to be dependent on the chamber and individual size of the Tethya (Hammer & Nickel, 2014). I was not able to find the choanocyte chambers in SEM, though one can observe similarities in the lacunae of T.robusta (figure 1 & 13). While T.wilhelma gives a good reference for T.robusta, more investigation of species-specific anatomy is needed within the Tethya genus.

Figure 16

Biogeographic Distribution

Tethya sponges are most often described as ‘cosmopolitan’ and have been found in shallow to deep waters in the Atlantic, Arctic, Pacific and Indian Oceans and Mediterranean Sea (Ribeiro & Muricy, 2011). Tethya robusta, specifically, has been found in the Pacific Ocean, including Australia, Papua New-Guinea, and the Solomon Islands; as well as some areas of the Indian Ocean (Sara et al., 1990). Interestingly, there is unusually high diversity within the genus Tethya in the South-west Pacific, around Australia and New Zealand’s temperate coasts (Sara & Sara, 2004). Australian Tethyidae genera characterise ~50% of known Tethyidae, globally (Sara, 2002). The highest diversity within the Tethya genus is also found in tropical habitats such as coral reefs (Sara & Sara, 2004). Sara & Sara (2004) hypothesize that the diversity of these regions is due to evolutionary radiations in reaction to Gondwanian-Cretaceous and early Cenozoic events, possibly from the origin of Tethya. Tethya robusta is also well known at Heron Island, on the Great Barrier Reef (Sarà et al., 1990).

Evolution and Systematics

Heim et al. (2007) used molecular and morphological characteristics of Tethya species to create a phylogeny of the genus. Unfortunately, this phylogeny does not include T. robusta. Despite this, nucleotide-based matrices uncovered that two major branches diverged early in the evolution of Tethya:

> an Indo-Pacific clade (including; T. seyshellensis, T. wilhelma & T. minuta)

> a Pan-European clade (including; T. citrina, T. norvegica, T. hibernica)

The results for the North American clade, including T. aurantium were inconclusive (Heim et al.,2007). Considering that T. robusta is primarily an Indo-Pacific species, appearing to originate from the early radiations suggested by Sara & Sara (2004), it likely pertains to the Indo-Pacific clade mentioned.

Tethya belongs to the G4 clade of Desmospongiae. G4 contain 75% of all known taxa of Desmospongia, but attempts to resolve the structure within this clade have been complicated by a lack of agreement between molecular and morphological phylogenies (Morrow et al.,2012). In sponges, there are relatively few morphological features upon which to base phylogenies, and the potential for secondary character loss, convergence and parallel evolution are high (Morrow et al.,2012). Recent molecular phylogenies of G4 Desmospongiae have attempted to resolve their own sensitivities to convergence by adding more taxa to their analysis (Morrow et al,2012). Figure 17 represents the relationships amongst G4 Desmospongiae obtained from mitochondrial CO1 sequences inclusive of 5 Tethya sp., unfortunately not T. robusta.  According to this phylogeny, the C3 clade to which Tethya belongs is a sister taxon to the clade C4 or the Clionaidae (Morrow et al.,2012). This is interesting because Clionaidae also has a worldwide distribution, and include bioeroding taxa that bore into calcium carbonate substrates on coral reefs, often with the assistance of solar-powered dinoflagellates (Symbiodinium sp.) that live endo-symbiotically within their cells and remain in light-exposed regions of the sponge (Achlatis et al.,2018). Compound microscope images of T. robusta, identified structures that are round in shape and of approximately the correct diameter to be Symbiodinium sp. (cf. Figure 18A and Figure 18B). Clearly, the evidence presented here is insufficient to conclusively identify these structures, or even suggest whether they are merely being digested by T. robusta, as opposed to fulling a role in this sponge similar to that fulfilled by Symbiodinium sp. in Cliona sp. It does, however, warrant further investigation given the observed preference for this hard-bodied Tethya for exposed locations in reef environments.

Figure 17
Figure 18

Conservation and Threats

The Value of Sponges:

Sponges are a critically important part of benthic ecosystems in polar, temperate and tropical seas.  In these settings, they provide important ecosystem services that range from the bio-erosion of calcium carbonate structures, formation of habitat for many other species and production and consolidation of sediments (Bell et al. 2008).  Demospongiasuch as Tethya sp. also provide important roles in cycling of silicon, which may be of global significance (Bell, 2008). One study on Caribbean coral reefs includes that sponges provide several, very crucial functional roles; primary production, nitrification, calcification, cementation, bioerosion, water filtering and chemical exhalatant properties (metabolites), among others (Diaz & Rützler,2001). Thus, losses of location-specific species are likely to have large-scale impacts on respective ecosystems.

While sponges, as a group, are not considered threatened in many parts of the world
(Bell et al. 2015) the destruction of habitat in tropical regions from activities such as seabed mining and declining water quality threaten many of the habitats that sponges depend on (Bell et al., 2015). A lack of data exists about how most species are affected by anthropomorphic activity and there is no comprehensive assessment of sponge conservation status, though efforts in the Mediterranean have received attention for threats resulting from the overharvesting of specific sponge species (Bell et al., 2015). Importantly, the impacts of ocean acidification and climate change need to be considered, as well. Warming related mass mortality (80-90%) in Mediterranean sponges was observed in 2011 (Cebrian et al., 2011).

Drug discovery potential:

Interestingly, there are species of Tethya, such as T.orphei, which are externally black or dark violet due to the filamentous cyanobacteria found in the outer layers of the cortex (Sarà,1990). Research also finds evidence that various microbes associated with sponges, including T.berquistae and C.nucula, produce a multitude of secondary metabolites (Mehbub et al., 2014); Nickel, 2006) that can be used for medicinal drug treatments (Anjum et al., 2016). In my observation under compound microscope of T.robusta's cortex, I found what could potentially be black cyanobacteria (figure 19).

 Research into nucleosides, such as ‘spongouridine’ in T.crypta, have been used for the synthesis of the anti-viral drug, ara-A, as well as the anticancer agent, ara-C (Anjum et al., 2016; Proksch et al., 2002), which is used to treat lymphoma and leukaemia. Derivatives are also being used for other cancers (Anjum et al., 2016). These examples among several others in the literature demonstrate the drug discovery potential associated with sponges, in particular, Tethya sp, such as T. robusta.

Figure 19


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Supplementary material by Hammer & Nickel(2014):