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Summary | |
Tethya are spherical, tuberculate sponges with the ability to locomote, despite an absence of muscles and nerves. The evolutionary, biogeographic and ecological success of Tethya is likely to be linked to this locomotive ability. Tethya have a cosmopolitan distribution with most species diversity represented in the temperate coastal regions of Australia and New Zealand (Biogeographic Distribution). Over one third of the species were described from these areas and subsequently are considered to be the centre of origin and radiation of Tethya (Evolution and Systematics). Tethya are mostly cryptic species, occupying crevices or coral rubble, however some prefer more exposed habitats. These interspecific differences in ecological requirements can be observed in the skeletal structure of the cortex (Morpho-functional Ecology).
My focus Tethya specimens were collected from the University of Queensland Aquariums, however, I have not yet identified them to the species level. They were observed inhabiting areas of low light intensity such as crevices and coral rubble undersides, thus, I speculated that light intensity could be an important cue in the directing and coordination of locomotion (Figure 1). I conducted an observational experiment to identify if light intensity is used as a cue to direct locomotion and if they have a preference for dark areas (Locomotion). I found that Tethya exhibit negative phototaxy, however, the small sample size limited the statistical power to detect an effect. Future studies should determine if the sensory capabilities of adult Tethya are derived from the same pigment ring eye as in larvae.
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| Figure 1 |
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Physical Description | |
The found Tethya specimens had a spherical or sub-spherical shape, a leuconoid design and a massive functional growth form. The choanosomes of all specimens were mustard-green and their cortices off-white/yellow. The surface of Tethya is tuberculate with three types of spiculated body extensions, each with different roles (Figure 2; Nickel & Brummer, 2004). Type I (budding extension) supports asexual buds (see 'Reproduction'), Type II (scout extension) is involved in sensing the environment and can attach to surrounding substrata and Type III (guide extension) leads during locomotion and communicates to basal cells to coordinate skeletal rotation (Figure 2; Nickel & Brummer, 2004).
There was intraspecific variation in external characteristics of found specimens, possibly giving an indication of their relative age. Specimen 1 was 10mm in diameter with irregular spiculated tubercles distributed over the surface and had many stalked buds attached to its pinacoderm (Figure 3b). Cyanobacteria growth was observed over one third of the sponge’s exterior, possibly representing a symbiotic relationship between the bacteria and the sponge (see 'Microbial Associations'). Specimen 2 was 9.4mm in diameter, had less distinct tubercles and no cyanobacterial growth, possibly indicating that it was less mature than Specimen 1 (Figure 3c). This specimen had many stalked buds of differing developmental stages. Specimen 3 was hemispherical, had a diameter of 4mm with no evidence of tubercles or budding (Figure 3d). This specimen was observed on a piece of coral rubble within 15cm of a conspecific of similar size and development. Thus, they could have been juveniles that were recently detached as buds from the same parent.
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| Figure 2 |
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| Figure 3 |
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Ecology |
Morpho-functional Ecology | |
Tethya are entirely marine with most species occupying intertidal and coral reef habitats (Bergquist, 1991). Tethya are observed in crevices, on the underside of boulders and coral rubble, on shallow subtidal rock platforms and on open rock platforms. The focus Tethya specimens were observed attached to coral rubble and in crevices (Figure 4). Tethya can be shallow or deep water species, with representatives being found within 0 - 805 m water depths (Heim et al., 2007). Most commonly they are observed as solitary individuals but there are cases of individuals existing in colonies (Bergquist, 1991). Further, they display differential tolerance to light and water current with some species preferring protected crevices and others more exposed rock platforms. These ecological requirements can often be reflected in the structure and plasticity of the cortex (Sará & Manara, 1991).
T. aurantium and T. citrina are sympatric species from the Mediterranean Sea whose cortical structure can be reflected in their different ecological niches (Sará & Manara, 1991). T. citrina occupy crevices and thus have a cortical structure adapted to calmer waters and greater sedimentation. The cortex is thin and highly plastic to enable increased pumping of the aquiferous system as required in areas with less water current (Sará & Manara, 1991). T. aurantium live in exposed areas and are more tolerant to light and high water currents. They can withstand these conditions due to a thick and rigid cortex, reinforced with a higher concentration of micrasters (Bavestrello et al., 2000). These differences in skeletal structure could have been the result of interspecific competition for resources imposing selective pressure on these species. Adaptive radiation of the cortical structure enabled the species to exploit slightly different niches in the same habitat, thus reducing resource competition.
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| Figure 4 |
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Microbial Associations | |
Many sponge species have mutualistic symbioses with microbes (Wulff, 2006). Sponge tissue is a nutrient rich environment for microbial growth and microbes often use sponge waste products such as nitrate and carbon dioxide to produce primary and secondary metabolites (Sipkema & Blanch, 2010; Wulff, 2006). In turn, microbes provide a wealth of benefits for sponges by providing defence, nutrients and energy products (Wulff, 2006). Bioactive secondary metabolites produced by microbes are involved in host chemical defense against predation and competition. (Sipkema & Blanch, 2010). Photosynthetic microbes such as cyanobacteria occupy the sponge surface while autotrophic and heterotrophic microbes are sheltered in the mesohyl matrix (Kim & Dewapriya, 2012).
Specimen 1 had cyanobacteria growing over approximately one third of the cortex (Figure 5a) and a nucleus staining fluorescent micrograph revealed colonisation of cyanobacteria in the sponges interior (Figure 5b). It is unknown if the cyanobacteria is a mutualistic symbiont or an invasive species in these cases, however, many Tethya species exhibit mutualism with algal species (Gaino et al., 2006; Gaino & Sará, 1994). T. orphei has a mutualistic symbiotic relationship with cyanobacteria (Oscillatoria spongeliae) embedded in the cortical layer (Gaino et al., 2006). The algae provides photosynthates to supplement the metabolism of T. orphei and produces amino acids involved in UV protection of the sponge (Gaino et al., 2006). T. seychellensis and green-algae (Ostreobium constrictum) have a mutualistic relationship (Gaino & Sará, 1994). Gaino & Sará (1994) observed growth of green-algae solely on the siliceous spicule bundles of T. seychellensis known as strongyloxeas. The spicule bundles of Tethya are radially arranged, originating in the core and extending to the external surface (see 'Skeletal Structure'). It was concluded that green-algae fixes light energy and uses the spicule to direct energy and nutrients to the core of the sponge (Gaino & Sará, 1994). The light energy from this process influences many biological aspects of Tethya, including the triggering of oogenesis (Gaino & Sará, 1994). Therefore, I infer that the observed cyanobacteria growing in and on (Figure 5) these Tethya specimens are mutualistic due to the wealth of benefits observed in other Tethya species.
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| Figure 5 |
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Life History and Behaviour |
Reproduction | |
Asexual Reproduction
Budding is the most frequent method of reproduction in Tethya as it enables higher production and dispersal of progeny with less energetic effort. The parent sponge produces spiculated stalks extending from the pinacoderm that support small cortical buds (Figure 6; Gaino et al., 2006). Buds are composed of archaeocytes and micro spicules with the outer cortex covered in pinacocytes (Gaino et al., 2006). Buds grow into functional adults while attached to the parent, however, they do not develop an aquiferous system until after detachment (Gaino et al., 2006). As the aquiferous system is vital for nutrition and respiration (see 'Aquiferous System'), buds rely on stored nutrients from the parent to supplement them during development (Gaino et al., 2006). Following detachment, buds are dispersed with water currents and settle, where they form the aquiferous system to become a fully functional adult.
Sexual Reproduction
Tethya are gonochoristic and oviparous species which undergo sexual reproduction in the summer months of the year (Corriero et al., 1996). Gametogenesis is initiated with increasing water temperatures and/ or shorter photoperiod (Usher et al., 2004). Poriferans have no specialised gonads; their oocytes derive from either archaeocytes or choanocytes and sperm from choanocytes. Males exhibit spermcast spawning which occurs in synchronicity with conspecifics to increase overall fertilisation rate.
Sperm enters through female ostia into the aquiferous system where choanocytes capture sperm and direct it to oocytes which line the choanoderm (Usher et al., 2004). Internal fertilisation occurs and fertilised eggs are released through the osculum. Sexual reproduction involves high energy expenditure, especially in oviparous females where oocytes line choanocyte chambers which decreases the efficiency of the aquiferous system (Usher et al., 2004). Eggs hatch externally into lecithotrophic parenchymella larvae (Bergquist, 1991) which use sensory capabilities to settle and metamorphose into the adult form. As filter feeders, their survival depends on the physical environment (Leys & Hill, 2006), therefore, the choice of settlement site is crucial to the future viability of the adult sponge. However, as Tethya exhibit locomotion as adults, this risk is reduced as they move in response to poor conditions and enhance their survivability (Moldonado & Uriz, 1999). Although, the cues and sensing involved in adult Tethya locomotion are largely unknown, I observed negative phototaxy in Tethya (see 'Locomotion') which could derive from a similar mechanism as in the larvae.
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| Figure 6 |
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Locomotion | |
Introduction:
Poriferans possess no muscles or nerves, however, species in the genus Tethya exhibit locomotive behaviour. The mechanisms involved in Tethya locomotion have been speculated (Nickel, 2006). Fishelson (1981) hypothesised that movement was enabled by the exertion of a pulling force by contracting of body extensions, however, this was disproved (Bond & Harris, 1998; Nickel & Brummer, 2004). Nickel (2006) conducted a study on T. wilhelma to investigate the mechanisms and factors involved in locomotive behaviour. It was concluded that movement occurs by skeletal rotation by utilising the spherical body morphology to ‘roll’ along a horizontal plane (Nickel, 2006). It has been observed that movement is triggered by changes in environmental factors such as water movement or sedimentation level , although, the full extent of the signals they use are unknown (Nickel, 2006). Further, it has been observed that Tethya locomotion is triggered by unfavourable environmental conditions, but generally do not use locomotion if conditions are favourable, suggesting that locomotion has some sort of energetic cost (Nickel, 2006).
Within Tethya, species have differential tolerances to light intensity, with some inhabiting crevices and some preferring exposed rock crests (Manara, 1991; Morpho-functional Ecology). The Tethya specimens I found were observed in crevices or on the underside of rocks or coral rubble, therefore, changes in light intensity could be an important cue used in triggering locomotion. Previous studies have investigated the cues and the signalling involved in inducing the locomotive behaviour, however, there is not yet an agreed explanation for these phenomena. Symbiotic cyanobacteria and/ or body extensions of Tethya are both have possible involvement in the sensing of signals (Nickel, 2006; Pronzato, 2004). Further, adult Tethya could have retained the larval capability to move by using the pigment ring eye to respond to light (Leys et al., 2002; Rivera et al., 2012).
This study aims to determine if this Tethya species uses light intensity as a cue to direct locomotion, and if they move towards light or dark areas. I hypothesised that these Tethya sp. will have a preference for dark areas as they are most commonly observed in crevices and on the underside of coral rubble.
Methods:
Three medium sized petri dishes were prepared by completely colouring in half of each dish (including the top and sides of the lid and the sides of the dish) using a black permanent marker (Figure 7). Three Tethya sp. of approximately 1 cm diameter were collected from the University of Queensland Aquaria and placed in petri dishes filled with seawater and covered with the lid. Individuals were placed in the centre of the petri dishes with their body halfway between the light and dark zone. Their position was marked with an ‘X’ and the petri dishes were left for 7 days in the same environmental conditions (with the water being changed every day). On the 7th day, it was recorded whether each moved towards the ‘light’ or ‘dark’ zone of the petri dish. This was repeated again two weeks later with 2 juvenile Tethya of the same species.
Observational Results:
In the first experiment, Individual 1 and 2 moved from the centre of the petri dish into the dark zone, with Individual 1 attached by its ‘scout extensions’ on to the edge of the ‘dark’ side of the petri dish (Figure 8). Individual 3 was found deceased. In the second experiment, no change in position of either individual was observed from the 1st to 7th day.
Discussion:
The results provide support for my hypothesis that Tethya sp. use light as a cue in locomotion and move towards darker areas, however, due to the small sample size limiting statistical power the null hypothesis cannot be accepted or rejected. Therefore, I present my results solely as observations of the natural history of Tethya sp. that could later be applied in a larger scale investigation on the effect of light intensity on Tethya locomotion.
Coordination of phototactic behaviour:
Most metazoans with visual capabilities use photosensitive opsin proteins to convey environmental information about light to the nervous system (Rivera et al., 2012). However, as sponges do not possess a Nervous system or opsin, the coordination of this of phototactic behaviour is difficult to determine (Rivera et al., 2012). Coordination of this phototactic behaviour has not yet been determined, however, I will propose three possible explanations. Firstly, Nickel (2006) proposed that two types of Tethya body extensions are involved in the coordination of movement (see 'Physical Description'). One of the Tethya in my study was observed with extensions facing towards the dark and attached to the substrate (Figure 8c), and I observed that Tethya with no extensions (juveniles) did not exhibit any locomotion. Secondly, symbiotic cyanobacteria have been identified as potential contributors in sponge movement, possibly having a role in light sensing or signalling (Nickel, 2006; Pronzato, 2004). Lastly, Tethya could have retained the larval capability to use light cues for settlement.
Light intensity is an important settlement cue in many species of sponge larvae such as the parenchymella larvae of Amphimedon queenslandica (Rivera et al., 2012). Parenchymella larvae have pigment ring eyes which are involved in phototactic swimming (Leys et al., 2002). Rivera et al. (2011) found that cryptochrome genes expressed in the A. queenslandica parenchymella pigment ring eyes are involved in detecting light intensity to mediate phototactic swimming. Although no studies have investigated the visual capabilities of Tethya larvae, the mechanism of larval settlement could have been retained in Tethya adults, using a pigment ring eye to detect light intensity.
Limitations of Experiment:
Environmental conditions could have been unfavourable for Tethya which may have affected the results. As they are filter feeders they require good water circulation, otherwise waste builds up and they cannot acquire sufficient nutrition. Water was only changed once daily, there was no current and there was a low water volume. When comparing the overall health and quality of the individuals on Day 1 and Day 7, there was a marked decrease in health for all specimens, with one individual found deceased (Figure 8). Individuals on Day 7 had dropped all their buds before they were fully developed, they exhibited discolouration and cyanobacteria had been expelled from the external tissue (Figure 8). Thus, the Tethya were probably under stress and this could have affected results. Previous studies have determined that Tethya initiate movement to ‘escape’ unfavourable conditions (Nickel, 2006). As there was a small sample size, I am unable to determine if Tethya exhibited negative phototaxy using light signals or moved randomly in an attempt to ‘escape’ these unfavourable conditions.
Future studies:
In future studies, a larger sample size is needed with more replicates in order to reduce error and increase the statistical power to test an effect of light on locomotive behaviour. To reduce error and environmental disturbance, experimental tanks should be used instead of petri dishes with a filter and constant water circulation. Furthermore, I would like to to determine if Tethya uses cryptochrome in phototactic behaviour. This could be tested by covering the petri dishes with a blue-light filter and carrying out the same experimental design as used above. As cryptochrome uses blue light to mediate phototaxy, I would expect Tethya to move randomly with no significant difference in preference for light or dark areas if cryptochrome is involved in coordination of Tethya locomotion.
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| Figure 7 |
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| Figure 8 |
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Anatomy and Physiology |
Cellular Organisation & Function | |
Poriferans do not possess true tissues, they are a loose aggregation of cells arranged into three layers: pinacoderm, choanoderm and mesohyl (Ruppert et al., 2004). Tethya are of leuconoid design, with the pinacoderm and choanoderm folded into an aquiferous system of choanocyte chambers and canals. Pinacocytes make up the pinacoderm, an epithelium-like layer which lines the outer surface and the aquiferous system canals (Figure 9). The pinacoderm has roles in contraction of ostia and physical defense (Ellwanger & Nickel, 2006; Ruppert et al., 2004). The choanoderm is an inner ciliated layer composed of choanocytes. Choanocytes are arranged into choanocyte chambers (Figure 10) which have roles in generating water flow through the aquiferous system and food capture. The mesohyl is the gelatinous matrix between the pinacoderm and choanoderm, composed of cells involved in skeletal strength, nutrient transport and development (Figure 9). Sclerocytes secrete siliceous spicules including megascleres (strongyloxeas) and microscleres (Figure 9, 10). Archaeocytes are totipotent cells with the ability to differentiate into any poriferan cell type and have roles in internal transport and digestion (Hayes, 2012).
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| Figure 9 |
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| Figure 10 |
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Skeletal Structure | |
Tethya have a spherical or subspherical body plan divided into a well developed external cortex and a dense inner choanosome (Figure 11; Bavestrello, 2000). The spherical shape of Tethya is a result of the skeletal structure composed of fan-shaped bundles of megascleres known as strongyloxeas radially arranged around the sponges’ central origin (Figure 11; Bavestrello, 2000; Sará, 2002; Sará & Manara, 1991). Microscleres include micrasters and megasters which are distributed in the mesohyl of the cortex (Figure 11).
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| Figure 11 |
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Aquiferous System | |
The aquiferous system is a phylum specific anatomical feature that has vital roles in almost all physiological aspects of poriferans. Respiration and nutrition occur through the extraction of oxygen and food particles, respectively, from water flowing through the aquiferous system. Incurrent water enters through openings in the pinacoderm known as ‘ostia’, where it flows into the lacunae and through the canals to the choanocyte chambers (Figure 12). Choanocytes create water flow through the beating of flagella and the microvilli collar secretes mucus to trap food particles (Ruppert et al., 2004). Archaeocytes phagocytose food particles and transports food and oxygen throughout the sponge body. Excretion also involves archaeocytes, these cells carry wastes to the osculum to be expelled. After nutrients and oxygen are extracted in the choanocyte chambers, water and waste flows through the excurrent canals and out through the osculum (Figure 12). As the sponge body is constantly ventilated by the aquiferous system, internal transport of wastes, nutrients and oxygen occurs by diffusion. As discussed previously, the aquiferous system is also vital in the reproduction of Tethya (see 'Reproduction').
Tethya do not have nerves or muscles, however, can respond to endogenous signals to coordinate contraction of the aquiferous system (Ellwanger & Nickel, 2006). Tethya contraction can be induced by neuroactive substances such as cAMP and nitric oxide which are involved in chemical messaging of metazoans with a Central Nervous System (Ellwanger & Nickel, 2006). These chemical messengers are thought to be spread through the aquiferous system and/or the mesohyl of the sponge (Ellwanger & Nickel, 2006). Thus, suggests that this chemical messenger based signalling system regulating contractile behaviour in sponges could be a precursor to the development of a CNS in metazoa.
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| Figure 12 |
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Evolution and Systematics | |
Phylogeny in Porifera traditionally uses spicule characters due to the plasticity of some skeletal and morphological characteristics. Tethya has siliceous (SiO2) spicules and spongin (Ruppert et al., 2004) with a radial arrangement of megascleres in the cortex, thus classifying it in the class Demospongiae and order Hadromerida respectively. Tethya is within the family Tethyidae, characterised by the possession of stylote megascleres, two types of microscleres (micrasters and megasters) and a tuberculate surface (Sará, 2002). Tethya is the most speciose genus of the family Tethyidae and is distinguished from other genera in its spherical body shape and well defined cortex (Gaino & Sará, 1994; Sará, 2002).
Traditionally, Tethyidae genera were thought to share a stalked, deep-water common ancestor. However, following phylogenetic reconstruction of the family, the ancestor is thought to be more similar to Tethya (Sará & Burlando, 1994). Sará and Burlando (1994) propose that Tethya radiated early and represents the most basal lineage of Tethyidae. Tethya is thought to form its own clade within Tethyidae, with a clade of encrusting and a clade of stalked genera forming following the divergence of Tethya (Figure 13; Sará & Burlando, 1994). There are 81 described species within Tethya and more than 40 have been observed but not yet described (Heim et al., 2007). Species are differentiated within Tethya based on morphology, spicule characters and skeletal structure (Bergquist & Kelly-Borges, 1991).
Australia and New Zealand temperate waters exhibit the most diversity in Tethya species, containing 30 of all described species (Sará & Sará, 2004). Due to this great diversity, it has been proposed that Australia and New Zealand represent the centre of origin and radiation of Tethya, during Cretaceous and early Cenzoic events following the breakup of Gondwana (Sará & Sará, 2004). Further, half of the genera within Tethyidae were described from Australia, with three new genera described in 2004, supporting the hypothesis of Tethya radiation occurring in these regions (Sará & Sará, 2002; Sará & Sará, 2004).
The found specimens were originally classified as Tethya robusta. T. robusta are found in the coastal temperate waters of eastern Australia, where these Tethya specimens were initially sourced and display similar morphological characteristics (Bergquist & Kelly-Borges, 1991; Sarà & Sarà, 2004). However, descriptions of the morphology and colour of T. robusta differ greatly across papers and I could not assume these specimens were T. robusta based on morphology alone. Therefore, spicular analysis should be used in order to accurately identify these specimens.
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| Figure 13 |
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Biogeographic Distribution | |
The genus Tethya has a cosmopolitan distribution from arctic to tropical areas, with the highest global diversity observed in coral reefs (Heim et al., 2007). Tethya are found in water depths of 0 to 805 metres, however are mostly shallow water dwellers (Heim et al., 2007). The main biogeographic regions of Tethya species are the Mediterranean Sea, North European seas, the Western Atlantic and the Indopacific (Heim et al., 2007). Indopacific species constituate 85% of the genus with over one third of species found in temperate coastal regions of Australia and New Zealand (Heim et al., 2007). These Tethya specimens were sourced from the Aquaria of University of Queensland, however were originally found at Heron Island, Great Barrier Reef.
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Conservation and Threats | |
There are no known threats or conservation issues regarding Tethya in the literature. However, overall little work has been done on the assessment of the global conservation status of poriferans, despite their ecological importance (Bell et al., 2015). Poriferans are vital in reef-building, water filtration and in nutrient and carbon cycling of coral reefs (Hutchings et al., 2007). Bell et al. (2015) carried out an assessment on the global conservation status of sponges and found they have a number of threats relating to a warming climate and other anthropogenic influences. As temperature directs timing of spawning, with rising temperatures, larval food availability could be affected if spawning is initiated early or late (Bell et al., 2015; Hutchings et al., 2007). Further, Corriero et al. (1989) found that temperature is a factor involved in the budding process of Tethya, whereby a decrease in water temperature results in increased budding. Therefore, as budding is favoured in lower water temperatures, there may be a reduction in the budding of Tethya with increasing water temperatures. Most sponge species are sessile, therefore, unfavourable environmental conditions are amplified as they cannot alter their position following settlement (Hutchings et al., 2007). The ability of Tethya to locomote, even at a microhabitat scale, may be a favourable adaptation that increases their chance of survival with environmental changes (Nickel, 2006). However, it is impossible to predict the resilience of Tethya in the face of environmental change without a greater understanding of the biology, distribution and evolution of these sponges. Further studies of the effect of temperature, turbidity, predators and introduced species on the distribution and abundance of Tethya must occur in order to identify and minimise potential threats.
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References | |
Bavestrello, G., Calcinai, B., Ceccati, L., Cerrano, C., & Sará, M. (2000). Skeletal development in two species of Tethya (Porifera, Demospongiae). Italian Journal of Zoology, 67(3), 241-244.
Bergquist, P. R. & Kelly-Borges, M. (1991). An evaluation of the genus Tethya (Porifera: Demospongiae: Hadromerjda) with descriptions of new species from the southwest Pacific. Beagle: Records of the Museums and Art Galleries of the Northern Territory, 8(1), 37-72.
Corriero, G., Sarà, M., & Vaccaro, P. (1996). Sexual and asexual reproduction in two species of Tethya (Porifera: Demospongiae) from a Mediterranean coastal lagoon. Marine Biology, 126(2), 175-181.
Ellwanger, K., & Nickel, M. (2006). Neuroactive substances specifically modulate rhythmic body contractions in the nerveless metazoon Tethya wilhelma (Demospongiae, Porifera). Frontiers in zoology, 3(7).
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Sarà, M., & Sarà, A. (2004). A revision of Australian and New Zealand Tethya (Porifera: Demospongiae) with a preliminary analysis of species-groupings. Invertebrate Systematics, 18(2), 117-156.
Sarà, M., & Manara, E. (1991). Cortical structure and adaptation in the genus Tethya (Porifera, Demospongiae). In J. Reitner, H. Keupp (Eds.) Fossil and recent sponges, (pp. 306-312). Berlin: Springer.
Sarà, M. (2002). Family Tethyidae Gray, 1848. In J. Hooper, R. W. M. van Soest (Eds.). Systema Porifera, (pp. 245-265). Springer US.
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Usher, K. M., Sutton, D. C., Toze, S., Kuo, J., & Fromont, J. (2004). Sexual reproduction in Chondrilla australiensis (Porifera: demospongiae). Marine and freshwater research, 55(2), 123-134.
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