Dercitus xanthus, Sutcliffe, Patricia R., Hooper, John N. A. & Pitcher, Roland, 2010

Sutcliffe, Patricia R., Hooper, John N. A. & Pitcher, Roland, 2010, The most common sponges on the Great Barrier Reef seabed, Australia, include species new to science (Phylum Porifera), Zootaxa 2616, pp. 1-30 : 6-11

Dercitus xanthus sp. nov.

(Figures 4, 5, Tables 1 , 2)

Material examined. Holotype: QMG329976 (SBD513022), south of Rock Cod Shoal, off Gladstone, Great Barrier Reef, 23° 43΄ 30ʺ S 151° 39΄ 53ʺ E, 34 m depth, epibenthic sled, 20 ix. 2004, coll. FRV Lady Basten.

Paratypes: QMG329977 (SBD513042), East of Lady Musgrave Island, Great Barrier Reef, 23° 53΄ 0 6ʺ S 152° 0 6΄ 17ʺ E, 42 m depth, epibenthic sled, 21 ix. 2004, coll. FRV Lady Basten,. QMG32978 (SBD505424), South of Big Broadhurst Reef, off Townsville, Great Barrier Reef, 19° 24΄ 18ʺ S 147° 56΄ 0 6ʺ E, 42 m depth, epibenthic sled, 27 xi. 2003, coll. RV Lady Basten.

Other material: 163 specimens distributed from adjacent to Bligh Reef in the far northern GBR to the Swain Reefs in the south (Figure 4). Housed at the Queensland Museum, Brisbane, Australia.

Description. Shape. Massive, agglutinating sponge where a thin layer of tissue cements biogenic rubble such as worm tubes, small gastropods and bivalve remnants. Usually found in fist sized patches or smaller, and occasionally as massive conglomerations.

Colour. Ranging from red to yellow. The bright red colour was predominant in frozen specimens. The sponge emits a very distinctive, profuse and intense yellow dye which stains easily. This eosin-like dye stains the surface of the sponge as it defrosts, masking the original red colour. Upon preservation in ethanol, the sponge loses all colour and becomes white. The ethanol, however, retains a brilliant yellow colour which darkens as the specimen remains immersed in ethanol.

Oscules. Oscules or papillae were never observed in this sponge, however, no in situ observations were possible.

Texture and surface characteristics. The agglutinating nature of this sponge means that it is easily broken, but does not crumble finely due to the large size of the rubble agglutinated by the sponge. The surface layer is uneven due to the presence of the rubble.

Skeletal structure. There is no definable structure in either the ectosomal or choanosomal skeletons. Sanidasters are scattered throughout the sponge with no particular organisation, but in high concentrations.

Megascleres ( Table 1 ). Two size classes of three rayed calthrops (ie triods) (small: 22–26 μm (mean 25 μm) and large: 49–94 μm (mean 72 μm)) were seen in the paratype QMG329977 and in approximately 20% of other specimens examined, occurring in high densities within these specimens ( Figure 5 ).

Microscleres ( Table 1 ). Sanidasters were universally present in all specimens. They display wide variation in length and width (10–20 μm x 1 –2.5 μm), with the shorter, wider examples containing large spines protruding up to 1 μm from the main shaft. The shaft is straight, and can be completely obstructed from view by the density of the spines present. Spines also vary in structure from conical with straight sides, to bulbous. In more sparsely spined sanidasters, spination is not evenly spaced but nonetheless consistent along the length of the spicule ( Figure 5 ).

Habitat and distribution. Distributed throughout the Great Barrier Reef, from the southern extremities adjacent to Rockhampton and the Capricorn Bunker group, extending through to the northern Great Barrier Reef and Torres Strait (Figure 4). Found on sandy bottoms in areas with a high component of calcium carbonate rubble. Depths range from 16 to 86 m, with the highest concentration of specimens found in depths of 30 to 40 m.

Etymology. Named for the distinctive yellow dye which exudes profusely from the sponge, derived from the greek word xanthos, yellow.

Remarks. This new species is assigned to Dercitus based on the presence of distinctive sanidasters, triods and the cementing growth form. This species also fits the description currently given for Stoeba Sollas 1888 , with the only apparent character differentiating Dercitus and Stoeba being the presence of toxas in Dercitus . Toxas, however, are confined only to D. bucklandi Bowerbank, 1858 , and are absent in the only other species currently assigned to this genus, D. natalensis ( Burton, 1926) (Van Soest, 2009a). We assign this new species to Dercitus rather than Stoeba , being the older of the two genera, given the current uncertainty surrounding whether both genera will remain valid in light of recently published comments and a pending revision of the pachastrellids ( Maldonado, 2002; Moraes & Muricy, 2007; Van Soest, pers. comm.).

This species differs from the two described species of Dercitus in its spicule composition (Table 2). D. bucklandi contains toxas, calthrops and short shafted plagiotriaenes with blunt clads; D. natalensis has short shafted dichotriaenes. The triod calthrops observed in D. xanthus are clearly different from the megascleres in both these other species.

There are 12 currently described species of Stoeba (van Soest, 2009b). Eight of these species possess dichotriaenes ( S. dissimilis , S. extensa , S. loricata , S. occulta , S. pauper , S. plicata , S. reptans and S. simplex ). Three species also contain monactinal spicules such as oxeas ( S. dissimilis , S. exostotica and S. lesinensis ), which clearly differ from the spicule composition of this new species. Only two species, S. latex Moraes & Muricy, 2007 and S. syrmatitus (de Laubenfels, 1930), contain only calthrops and sanidasters and are thus most similar in spicule composition to D.xanthus . The major characteristics of all these species are compared in Table 2.

Stoeba latex from north east Brazil is a red/brown, thickly encrusting sponge (not in aggregating or filling small cavities as in most described species). It has four-rayed calthrops which can be much larger (42.5–212.5 μm) than the triods found in D. xanthus (22–94 μm), and sanidasters that are slightly smaller (10–15 μm) than in the present species (10–20 μm). Stoeba syrmatitus from California has a similar agglutinating growth form as does D. xanthus and has similar ranges of spicule sizes, with sanidasters ranging from 8–12 μm, and calthrops 25–80 μm (compared to sanidasters 10–20 μm, and triods 22–94 μm), but is drab in colour (compared to D. xanthus which is yellow to orange, with a prominent deep yellow dye). Megasclere morphology is also the most similar, including some of the four-rayed calthrops in S. syrmatitus reduced to triods (compared to all the calthrops in D. xanthus being 3-rayed), but their respective microscleres differ substantially. Sanidasters in S. syrmatitus were only tentatively referred to as sanidasters (“abundant, discasters (?) or sanidasters”) by de Laubenfels (1930), with the spination confined to two nodes on the shaft of the rhabd. De Laubenfels (1930) also noted that some microscleres were so irregularly spiny they resembled acanthomicrostrongyles, but most were more similar to discorhabds based on the localisation of the spines in two nodes (compared to consistently spined sanidasters, with examples of both large and small spines, in the new species).

This is the first record of a Dercitus or Stoeba species in the south western Pacific.

Predicted distributions and biophysical preferences. Dercitus xanthus is distributed along the entire length of the GBR at depths ranging from 16 to 85 m. Specimens were found at 134 sled sites (biomass 12.2 kg) and 23 trawl sites (biomass 3.5 kg) and were most highly concentrated in the Capricorn Bunker, Swain Reefs and Townsville regions (Figure 4). Biomass was calculated as the total wet weight, including all FIGURE 4. Model distribution map displaying predicted and actual distribution and biomass of Dercitus xanthus sp. nov. from the Great Barrier Reef seabed, displaying: OTU code for this species (TSQMSB.BRS203644), P-AUC: Area Under the Curve performance diagnostic for the Presence/Absence model; Dev. Ratio: Deviance Ratio (P-AUC) performance diagnostic for the Biomass model; Measuresled: a device factor accounting for the difference in sampling rates between devices; Measuretrawllog(AreaHa) and Measuresledlog(AreaHa): offsets accounting for the swept area sampled by the trawl and sled; and influential predictor variables including: SW_K_B_IRR: relative benthic irradiance, GA_MUD: mud grainsize fraction (%),CRS_S_SD: standard deviation of salinity (psu), CRS_NO3_SD: standard deviation of nitrate levels (μM), SW_CHLA_AV: mean cholorophyll-a (mg/m3) concentration.

biogenic rubble incorporated within the body of the sponge, as this was impossible to separate from the tissue. The most important physical covariate affecting the distribution patterns of this sponge using the presence / absence model was mud. This species was found in habitats with very low mud content present in the substrate, and where there was low variability in salinity and nitrate levels (Figure 4). This species occurs in benthic habitats with high levels of carbonate (biogenic rubble) and sand ( Figure 3 ), accounting for its characteristic habit of incorporating prolific detritus into the skeleton and cementing sand and biogenic rubble. Predicted distributions show overall biomass to be low, with higher predicted presence and biomass in proximity to Townsville and the Capricorn Bunker regions, and within a small section of the Swain Reefs.

Figures

FIGURE 3. Substrate composition plot, showing the relative composition of sand, carbonate, gravel and mud for four species (Dercitus xanthus sp. nov., Xenospongia patelliformis Gray, 1858, Coscinoderma nardorus (Lendenfeld, 1886) and Spheciospongia vagabunda (Ridley, 1884 )).

FIGURE 5. Dercitus xanthus sp. nov. (QMG 329976 (SBD 513022 )). A, Choanosomal structure displaying a high density of foreign material (scale bar 300 μm). B, Surface of foreign material within the choanosome covered in Sanidasters (scale bar 300 μm). C, External morphology (scale bar 1 mm). D, Holotype (scale bar 2.5 cm). E, Sanidaster (scale bar 10 μm). F, Sanidaster (scale bar 10 μm). G, QMG 329977 (SBD 513042). Triod calthrops (scale bar 50 μm). H, QMG 329977 (SBD 513042). Triod calthrops (scale bar 50 μm).

Tables

TABLE 1. Measurement of spicules for Dercitus xanthus sp. nov., as range (and mean) of length x width in μm, N = 30.

Specimen number	Sanidasters	Calthrops Type I	Calthrops Type II
QMG329976 (SBD513022)	10-17 x 1-2.5 (14 x 1.5)	-	-
QMG329977 (SBD513042)	9-17 x 1-2 (14 x 1.5)	22-26 (25)	49-94 (72)
QMG329978 (SBD505424)	10-20 x 1-2.5 (15 x 2)	-	-