Rowella simplicissima Lopes & Klautau 2023

ROWELLA SIMPLICISSIMA (BURTON, 1932)

(FIGS 13, 14; TABLE 7)

Synonyms: Leucettusa simplicissima – Burton, 1932: 261; Burton, 1963: 51. Leucettusa simplissima – Borojević et al., 1990: 257 (misspelling). Leucaltis nuda – Azevedo et al., 2009: 9; Leucettusa nuda – Klautau et al., 2013: 449; Fontana et al., 2018: 336 Lopes et al., 2018a: 59; Riesgo et al., 2018: 830.

Type specimen: Holotype (BMNH 1928.2.15.35).

Type locality: Off Sea Lion Island, East Falkland Island, South Atlantic (52°33ʹ S, 59°04ʹ W). Malvinas / Falklands and Araucanian MEOW ecoregion.

Description: Sponge tubular or ramified with cylindrical tubes (Fig. 13A). It grows parallel to the substrate and raises the tubes that bear oscula. Colour white in life and creamy-white to beige in ethanol (Fig. 13A, B). Consistency slightly compressible and friable to the touch. Outer surface smooth, while the atrial surface is hispid due to the apical actines of tetractines. Single, circular and naked oscula are present in each tube. Body wall overall thin. Atrial cavity wide and spacious, many excurrent canals are evident (Fig. 13B). Aquiferous system syconoid, with elongated choanocyte chambers (Fig. 13C). Reproductive elements (oocytes) are present in the MNRJ specimens.

Skeleton: Oscular margin has a transitional skeleton comprised of sagittal triactines and rare tetractines, gradually becoming regular as the body wall thickens. Cortical skeleton well developed and comprised of several layers of tangential triactines and fewer tetractines (Fig. 13D, E). Choanosomal skeleton with sparse pygmy triactines and tetractines, mostly present around the exhalant canals. The atrial skeleton is not well developed, often with pygmy tangential triactines and tetractines (Fig. 13F). Overall, the pygmy triactines are less abundant than the pygmy tetractines.

Cortical triactines (Fig. 14A). Regular. Variable sizes. Actines are mostly cylindrical, straight, with blunt to sharp tips. Sometimes there is a constriction near the tip. Size – 274.0 (± 69.0) μm length/19.0 (± 5.0) μm width.

Cortical tetractines (Fig. 14B). Regular. Variable sizes. Basal actines are cylindrical to conical, straight, with blunt to sharp tips. The apical actine is long,

cylindrical to slightly conical, straight, smooth, with blunt tips. Its tip can be straight, spiral or curved. Tetractines are less abundant and larger than the

718 M. V. LOPES and M. KLAUTAU triactines. Size – basal actines: 315.0 (± 137.0) μm length/35.0 (± 10.0) μm width; apical actine: 454.0 (± 132.0) μm length/36.0 (± 10.8) μm width.

Choanosomal and atrial triactines (Fig. 14C). Regular. Pygmy. Actines are conical, straight with blunt to sharp tips. Sometimes they become cylindrical and are thicker near the tip. They are less abundant than the tetractines. Size – 35.5 (± 5.2) μm length/7.5 (± 1.7) μm width.

Choanosomal and atrial tetractines (Fig. 14D). Regular. Pygmy. Basal actines are conical, straight with blunt tips. Sometimes they become cylindrical and are thicker near the tip. The apical actine is long, thick, slightly conical, smooth, with sharp tips and slightly curved at the distal part. Size – basal actines: 33.8 (± 4.9) μm length/6.9 (± 1.1) μm width; apical actine: 63.8 (± 9.3) μm length/8.1 (± 1.1) μm width.

Ecology: Coarse sand, shell and stone grounds. The specimens from Chile were found in vertical substrate. Depth range 22 to 75 m.

Geographical distribution (MEOW ecoregion): Malvinas /Falklands and Araucanian (Sea Lion Island, Falkland Islands, South Atlantic – Burton, 1932), Chiloense (Reñihue Fjord, Central-South Chile – Azevedo et al., 2009).

Remarks: Burton described Leucettusa simplicissima in 1932 based on a couple of specimens collected in the Falklands during the Discovery Expedition. He stated that the diagnostic character of this species was the underdeveloped atrial skeleton, with occasional pygmy triactines. Indeed, the atrial skeleton has scattered pygmy spicules, nonetheless, pygmy tetractines are also present and they are even more abundant than the pygmy triactines. Despite this, the pygmy tetractines were overlooked in previous works (Burton, 1932, 1963; Borojević et al., 1990, 2002). The authors also overlooked the cortical tetractines, which are numerous, although less abundant than the cortical triactines.

aTaken from Azevedo et al. (2009).

H, holotype.

The main character that separates R. simplicissima from other species of RoƜella is the syconoid aquiferous system, although two other species currently classified as Leucettusa present this kind of aquiferous system: L. Ʋera (now R. Ʋera) from Kerguelen and L. nuda from the Chilean fjords.

The aquiferous system of R. Ʋera can be easely differentiated from that of R. simplicissima because its elongated chambers are restricted only to the region underneath the cortex, while in the lower parts of the choanosome, closer to the atrium, chambers are spherical (Poléjaeff, 1883; Borojević & Grua, 1965). For Leucettusa nuda the story is different. That species was originally described as Leucaltis because of the organisation of the aquiferous system (Azevedo et al., 2009) and, later, with molecular data, it was transferred to Leucettusa (Klautau et al., 2013). Leucettusa nuda and R. simplicissima occur in the Magellanic Province (Spalding et al., 2007) and both have the same morphological characters: syconoid aquiferous system, triactines and tetractines in the cortical skeleton and pygmy triactines and tetractines in the choanosome and atrium. Moreover, the shape and size of the spicules are similar in both species (Table 7). Taking this into consideration, we conclude that L. nuda is a junior synonym of R. simplicissima.

In the original description of R. simplicissima, it was stated that the only difference between this species and R. haeckeliana is the presence of tetractines in the cortex of the latter (Burton, 1932). However, after re-analysing the holotype skeleton of R. simplicissima, we found cortical tetractines in it. In fact, the two species are morphologically similar, but they differ from each other by the aquiferous system, which is syconoid in R. simplicissima and leuconoid in R. haeckeliana, and by the shape of the pygmy spicules, with basal actines always straight in R. simplicissima and casually curved in the osculum in R. haeckeliana. Moreover, the pygmy spicules of R. haeckeliana are broadened at the distal portion of the actine. The cortical tetractines are abundant in R. simplicissima and rare in R. haeckeliana. Therefore, we consider them distinct species, although we have found that specimens from the Falkland Islands identified as ‘ Leucettusa ’ haeckeliana (Burton, 1932) are probably R. simplicissima (see Remarks under R. haeckeliana).