Amphoriscus elongatus Poléjaeff, 1883

Chagas, Cléslei & Cavalcanti, Fernanda F., 2021, Partial taxonomic revision of Amphoriscus Haeckel, 1870 (Porifera: Calcarea) with description of A. decennis sp. nov., Zootaxa 5061 (1), pp. 39-68 : 45-48

Amphoriscus elongatus Poléjaeff, 1883

Citations and synonymies: Amphoriscus elongatus Poléjaeff 1883: 48 , pl. iv, fig. 5, pl. v, fig. 4; Dendy & Row 1913: 782; Burton 1956: 117; Burton 1963: 538; Klautau et al. 2017: 106; Cóndor-Luján et al. 2019: 1825.

Type material: BMNH 1884.4 .22.27 (holotype). Station 145, off Prince Edward Islands (46° 40’ S – 37° 50’ E), 275 to 567 m depth, 27 December 1873 .

Type locality: off Prince Edward Islands , Indian Ocean .

Analysed material: BMNH 1884.4 .22.27 (holotype; specimen and one slide containing sections of the skeleton) . BMNH 1955.12 .13.10 (one slide containing sections of the skeleton) and BMNH 1955.12 .13.11 (one slide containing sections of the skeleton), both from Plymouth ; R.W.H. Row collection.

Morphology: The colour of the holotype in ethanol is white ( Fig. 4A ). It is a fragment with tubular shape and apical osculum, measuring 4 cm x 0.2 cm (length x width). The surface is slightly hispid. Syconoid aquiferous system ( Fig. 5 ).

Anatomy: The skeleton is typical of the genus Amphoriscus , and the inarticulation is evident ( Figs. 4D and 5D, E ). The cortical region is formed by giant tetractines and small sagittal triactines ( Figs. 4B, C ). The apical actine of these giant tetractines may reach the atrial cavity. All the analysed samples exhibited trichoxeas perforating the cortex without forming tufts ( Fig. 5C ). The subatrial region is formed by abundant triactines with long unpaired actine and rare tetractines ( Fig. 5D ). The latter are more frequent in the specimens BMNH 1955.12 .13.10 and BMNH 1955.12 .13.11 but also occur in the holotype. Additionally, in these specimens from Plymouth, we also observed a few modified subatrial triactines in which the actines that surround the atrial wall have different lengths and shapes ( Fig. 5E ). These spicules were found in the sections. Due to the occurrence of other spicules overlapping them, it was not clear if they were pseudosagittal. All the subatrial spicules point their unpaired actines to the cortex, in opposition to the apical actine of the cortical tetractines ( Figs. 4D and 5D, E ). The atrial region is comprised exclusively of tetractines ( Fig. 5F ) .

Spicules ( Table 1 ): A figure containing only the spicule categories, commonly shown in studies of the taxonomy of calcareous sponges, could not be prepared since only slides containing sections of the skeleton were available.

Cortical tetractines: Giant, conical with blunt tips. Paired actines are curved and long. The unpaired actine is curved and smaller or the same size as the paired actines. The apical actine is straight and long.

Cortical triactines: Slightly conical and sharp. Paired actines are straight to slightly curved. Unpaired actine is shorter and straight.

Subatrial triactines: Cylindrical to slightly conical with sharp tips. Paired actines are straight. Unpaired actine is straight and long, sometimes reaching the cortical region.

Subatrial tetractines: Rare, with cylindrical actines and sharp tips. Paired actines are slightly curved. Unpaired actine is straight and larger than the paired ones. Apical actine is short and curved.

Atrial tetractines: Vary in size. Cylindrical actines with sharp tips. Paired actines are long, thin, and straight. Unpaired actine is slightly curved. Apical actine is straight.

Remarks: The original description of Amphoriscus elongatus was based on a single specimen sampled during the Challenger expedition ( Poléjaeff 1883). It was tubular and elongated (hence the species name elongatus ). Close to the osculum, it was divided into two tubes, a feature that can no longer be recognised in the type ( Fig. 4A ). According to Poléjaeff (1883), A. elongatus has “an important anatomical peculiarity—the tendency of the radial tubes to meet in threes, fours, or in larger numbers around the same shallow invagination of the gastric cavity”. We could not clearly understand this organisation described by Poléjaeff (1883), even after analysing the holotype and additional specimens. Nevertheless, we are convinced that the aquiferous system of A. elongatus is not a typical syconoid in which choanocyte chambers run in parallel from the atrium to the cortex. Instead, choanocyte chambers of A. elongatus seem to branch, possibly causing, when sectioned, the holes observed in Figure 5 along the chambers’ length. More studies are needed to better describe this organisation, which depends on the discovery of fresh specimens for histological and electron microscopy analyses.

Our results indicate an important difference between our description of the holotype and that provided by Poléjaeff (1883), namely the occurrence of rare subatrial tetractines. We first observed this characteristic in the slides of the specimens from Plymouth and then, after a careful reanalysis, also in the holotype .

With respect to the geographical distribution, BMNH 1955.12 .13.10 and BMNH 1955.12 .13.11 are from Plymouth, United Kingdom ( Northern Atlantic Ocean ), while the holotype BMNH 1884.4 .22.27 was sampled “off Prince Edward Islands” ( Southern Indian Ocean ). In general, representatives of a species with such a disjunct distribution are viewed as questionable in the literature, mainly due to the low dispersal capability of the larvae of calcareous sponges ( Maldonado 2006). The only morphological difference between the three analysed samples is the putative presence of pseudosagittal triactines in the subatrial region of those from Plymouth. These spicules were not found in the holotype, and we assumed that the absence could be the result of plasticity. The analysis of a larger set of specimens, preferably using an integrative approach, is needed to elucidate this question .

The species that most closely resembles A. elongatus in its skeletal composition is A. pedunculatus (cortical triactines, tetractines and trichoxeas, subatrial triactines, and atrial tetractines). These species differ mainly in the presence of rare subatrial tetractines in the former and the size of the cortical tetractines, which are larger in A. elongatus (paired actines: 443.6±59.0/ 48.7±7.9μm; unpaired actine: 397.2±42.1/ 52.7±4.9 μm; apical actine: 453.8±60.3/48.2±6.4μm) than in A. pedunculatus (paired actines: 225.5±39.3/ 27.2 ±4.3 μm; unpaired actine: 182.9±7.2/ 27.2±3.8 μm; apical actine: 303.8±50.7/ 29.4±5.4 μm).

Distribution: Off Prince Edward Islands, South Africa ( Poléjaeff 1883) and Plymouth, United Kingdom (present study). Corresponding MEOW: Prince Edward Islands and Celtic Seas ( Spalding et al. 2007).

Figures

FIGURE 4. Amphoriscus elongatus (holotype BMNH 1884.4.22.27). A—Holotype fixed in ethanol. B—Tangential section of the cortex showing giant tetractines (black arrow) and small sagittal triactines (white arrow). C—General view of a transversal section. D—Detail of a transversal section showing cortical spicules and the inarticulated region.Abbreviations: at—atrium and ct—cortex.

FIGURE 5. Amphoriscus elongatus. A—Transversal section (BMNH 1955.12.13.10). B—Reproductive elements in the choanosome (black arrows; BMNH 1955.12.13.10). C—Cortical region with trichoxea and giant tetractines (BMNH 1955.12.13.11).D—Subatrial regionwith triactines and rare tetractines (arrow; BMNH 1955.12.13.11).E—Skeletal organization and a pseudo (?) subatrial triactine (arrow; BMNH 1955.12.13.11). F—Atrial region pierced by the apical actine of tetractines (BMNH 1955.12.13.10). Abbreviations: at—atrium and ct—cortex.