Clathria (Microciona) acarnoides new species

Figs. 12a–i

Material examined. Holotype: RMNH Por. 9243, Caribbean Netherlands, Bonaire, Curoil Dock, (Dive 3), 12.137°N 68.286°W, on a coral rock wall at 152 m, coll. L.E. Becking & E. Meesters, field nr. BON2/ BDR0020, 30 May 2013.

Description. Orange hispid encrustation (Figs. 12 a 1–a 4) on a vermetid gastropod shell, lodged in a specimen of Spongosorites ruetzleri . The sponge specimen was detected only when on deck. The holotype consists of four fragments, on vermetids and on Spongosorites ruetzleri specimens. Size of fragments 1–2 cm 2.

Skeleton. Microcionid (i.e. choanosomal styles and echinating acanthostyles are grouped) or hymedesmioid (i.e. choanosomal styles and echinating acanthostyles are arranged singly erect with heads embedded in the spongin plate adhering to the substratum) (Figs. 12a, 12b). Ectosomal subtylostyles are strewn in the dermal membrane and also form bouquets.

Spicules. Choanosomal principal styles, ectosomal subtylostyles, acanthostyles/cladostyles, toxas in two shape categories, palmate isochelae.

Choanosomal styles (Figs. 12 c–d), smooth, slightly curved, apparently occurring in two size classes, (1) larger styles (Fig. 12c, 12c 1) with predominantly smooth heads, 550– 895 –1110 x 20– 22 – 25 µm and (2) smaller styles (Figs. 12d, 12d 2) predominantly with rugose heads (Figs. 12d 1, 12d 3), 150– 344 –640 x 10– 14 – 15 µm.

Ectosomal subtylostyles (Figs. 12e, 12e 1), with faint microspination on the head, 380– 512 –620 x 5.

Acanthostyles/cladostyles (Fig. 12f), unique spicule type shaped like a ‘normal’ echinating acanthostyle, but provided with four sharp hooks at the pointed end, 60– 70 –75 x 5 µm. The hooked ends are pointed outwards, the heads are embedded in the basal spongin plate.

Oxhorn toxa (Fig. 12g), 30– 77 –115 x 2.5– 4.7 – 5 µm.

Long thin shallow-curved toxa (Fig. 12h) 190– 331 – 450 µm.

Palmate isochelae (Fig. 12i), rather narrow, but normal-shaped, 15– 20.4 – 25 µm.

Ecology and distribution. Known only from the type locality, encrusting a mollusk, at a depth of 152 m off the SW coast of Bonaire.

Etymology. The name refers to the presence of acanthocladotylote-shaped acanthostyles, which so far were known only from members of the genus Acarnus Gray, 1867 (family Acarnidae Dendy, 1922).

Remarks. The new species is unique among Clathria species by the modification of its echinating acanthostyles to resemble the acanthocladotylotes of the genus Acarnus, more in particular those of the species Acarnus primigenius (Hiemstra & Hooper, 1991 (see also Van Soest et al. 1991). For that reason, we assigned the new species at first to Acarnus, but a major feature does not match: the presence of unmistakable ectosomal subtylostyles, in all its aspects thoroughly representative of the genus Clathria, whereas all Acarnus species, including A. primigenius have microspined tylotes as ectosomal spicules. Furthermore, most Acarnus species have two categories of cladotylotes, one of which appears derived of the principal styles (cf. Van Soest et al. 1991). An exception is A. primigeniu s, which has only the acanthocladotylote type, but significantly, this species also has normal echinating acanthostyles, without the hooks at the pointed end. We conclude that the combination of ectosomal subtylostyles with the acanthostyle-like cladotylotes, makes the new species morphologically more similar to many Clathria (Microciona) than to Acarnus and that this constitutes a remarkable case of parallel development in these two genera. There are more overlapping features between the two: several Clathria (Microciona) species are known to have the feature of (partial) modification of their ectosomal microspined subtylostyles to quasitylotes, often provided with microspines at both ends (see e.g. Van Soest et al. 2013). It indicates that the two groups ( Acarnus and Clathria) are likely closely related and their assignment to different families may be unjustified.