Arthuria tubuloreticulosa, Van Soest & De Voogd, 2015

Arthuria tubuloreticulosa View in CoL sp. nov.

Figures 11a–c View FIGURE 11 , 12a–b View FIGURE 12

Material examined. Holotype RMNH Por. 5547, Indonesia, Halmaheira , Ternate, Sulamadaha Bay, 0.8661°N 127.3316°E, depth 5–15 m, SCUBA, coll. N.J. de Voogd, #TER.05/261009/1, Ternate-Halmahera Expedition, 26 October 2009. GoogleMaps

Description. An orange flattened mass of short oscular tubes ( Fig. 11a View FIGURE 11 ), connected at the substratum by a basal tubular network, growing on an encrusting octocoral ( Briareum ). The erect tubes maybe divided into one or two side tubes. The preserved specimen ( Fig. 11b View FIGURE 11 ) is fragmented, but there are several larger pieces that show the short erect tubes forming the main basis of the specimen. Overall size estimated to be 5 x 3 cm, with small tubes about 2– 4 mm in diameter. Color beige in preserved condition.

Skeleton. ( Fig. 11c View FIGURE 11 ) The walls of the tubes are thin, approximately 15 µm in thickness. The spicules are dominated by triactines. The choanoderm lines all internal surfaces.

Spicules. ( Figs 12a–b View FIGURE 12 ) Triactines, tetractines.

Triactines ( Fig. 12a View FIGURE 12 ), equiangular, predominantly equiactinal, but a fair number are slightly parasagittal, with the longer actines often slightly wobbled; most actines are cylindrical with abruptly pointed ends, but small triactines may have their actines conical, size of actines 63– 112.1 –138 x 4– 5.3 –6.5 µm.

Tetractines ( Fig. 12b View FIGURE 12 ), similar to triactines in size and shape, actines of basal triadiate system, 62– 119.5 –156 x 4– 5.4 –6 µm; apical actines 39–132 x 3.5–6.5 µm.

Ecology. Shallow-water reef.

Distribution. Known only from the type locality, Ternate.

Etymology. The name reflects the combination of a network of tubes on the substratum and erect tubes protruding from it.

Remarks. The dominance of triactines over tetractines, and the normal-shaped apical actines of the tetractines makes this Clathrina -like species a likely member of the recently erected genus Arthuria . The genus has a few Indo-Pacific species, but none of them seem to be close to our new species: South Australian A. dubia ( Dendy, 1891) (as Leucosolenia ) has much thicker actines (averaging 15–16 µm) and the tetractines are very rare (they were not mentioned by Dendy, but subsequently reported by Klautau & Valentine, 2003: 26). Red Sea A. sueziana Klautau & Valentine, 2003 has a different habitus and also much thicker actines. The above-described A. tenuipilosa possesses diactines.

Haeckel’s (1872) description and figures of Ascaltis darwinii reminds rather strongly of the present species and at first we assumed that it could be conspecific. However, shapes and sizes of the tri- and tetractines presented by Haeckel consistently differ: actines are equiangular equiactinal, shorter (80–100 µm) but more robust and conical (10–12 µm thick) than those of our new species. Clathrina darwinii should be transferred to Arthuria (see below).

Genus Ernstia Klautau, Azevedo, Cóndor-Luján, Rapp, Collins & Russo, 2013

Clathrinidae with asconoid aquiferous system possessing both triactines and tetractines in approximately equal proportions or tetractines more frequently. The apical actine of the tetractines is long and thin (after Klautau et al. 2013).