Comprehensive Description
An example of a recorded specimen of Rhabdastrella globostellata found off South Africa, preserved in spirits and described as “smooth, soft, cheese-like in life, orange-brown outside, orange-yellow inside. The cortex is about 250μm thick; internal cavities are abundant, up to 4mm wide” (Pulitzer-Finali, 1993). Records concur in the live specimen description; tan to chocolate brown 200-250µm thick cortex with a bright yellow to orange choanosome; globular and subspherical shape; apical depressions with numerous small oscules in clumps (up to 40 oscules, 1-4mm wide) (Carter, 1883; Pulitzer-Finali, 1993; Kennedy, 2000; Uriz, 2002). Dry samples display a dull greyish beige-brown ectosome, with dirty yellow or beige-brown choanosome (Carter, 1883; Kennedy, 2000).
Spicule characteristics are key to currently identifying to species level. Microscleres (minute spicules that rigidify soft tissue) and megascleres (larger spicules that form the sponge "skeleton") are variable and a historical way of assigning specimens to species (Cardenas et al., 2011; Botting & Butterfield, 2005). The Rhabdastrella Thiele, 1903 genus is defined as Ancorinidae with euasters, among which large spherasters or sterrospherasters are abundant mainly in the cortex, triaenes may be reduced of absent in some species (Uriz, 2002). Spicule size ranges depending on location, and possible response to environmental factors. As shown by Kennedy (2000) there is a recognisable difference between specimens from Vanuatu and Torres Strait Australia, in both tissue structure and spiculation (below).
Figure: Rhabdastrella globostellata (Carter, 1883). A, whole wet specimen from Vanuatu; B, holotype of Stellettinopsis carteri Ridley, 1884 (wet: Torres Strait, Australia); C, holotype (dry: Sri Lanka); D,E, sections through peripheral skeletons showing differences in structure between low latitude (D, specimen from Vanuatu) and higher latitude (E, from Heron Island, GBR, Australia) material; F, F’ – I,I’, examples of spiculation differences between specimens (F-I, holotype of S. carteri Ridley, 1884 from Torres Strait, Australia; F’-I’, from Vanuatu) (F,F’, oxeas; G,G’, orthotriaenes; H,H’, oxyasters; I,I’, oxyspherasters). (Figure reproduced from Kennedy (2000) paper)
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Spicules forming the sponge skeleton vary in size between locations. Rhabdastrella globostellata spicules include oxeas, orthotriaenes, oxyspheraster eusters and oxyaster eusters as key identifiers. Detailed observations on the range of sizes for the microscleres (small spicules ranging 10-60µm in length and often scattered through sponge tissue (especially the ectosome), include the oxyspheraster euasters and oxyaster euasters) and megascleres (large spicules 60-2000µm in length, including oxeas and orthotriaenes) is given here (Kennedy, 2000). The various names given to each spicule form is based on the shape of the spicule, as listed;
Microscleres
Spherasters: spicule form with multiple rays radiating from a common spherical centre.
Oxyspherasters: a type of spheraster with multiple pointed rays radiating from a common centre (I,I' on Figure).
Euasters: star-shaped spicule with multiple rays radiating from a common centre.
Oxyaster euasters: a type of euaster, with thin pointed rays (H,H' on Figure).
Megascleres
Oxeas: spicule form with both ends ending in a point (F,F' on Figure).
Orthotriaenese: a long, pointed spicule with one end finishing in a tri-radial formation (G,G' on Figure).
While historically and currently spicules are used as a main form of identification, new technology is allowing genetic determination of phylogeny, and may overtake this identification system in the future (Herbert et al., 2003; Cardenas et al., 2011). There is still much debate over the new phylogenetic trees being published using the nucleotide data from cytochrome c oxidase genes and ribosomal genes as it remakes many of the family lineages determined using the morphology of spicules (discussed here) (Kennedy, 2000; Cardenas, 2010; Cardenas et al., 2011).
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