Plakina doudou Grenier & Ruiz & Lage & Perez 2020, sp. nov.

Plakina doudou sp. nov. (Figure 1).

Material examined. Holotype: Muséum National d’Histoire Naturelle de Paris, France, MNHN DJV200: Grotte Chauve-Souris at 12 m depth, Anse Noire, La Martinique (14°32.024’N, 61°05.278’W). Collector: Thierry Pérez, 26 March 2016. Sample code: 160326- MT6 -TP2. GenBank accession number: MN 175680.

Paratype: Muséum National d’Histoire Naturelle de Paris, France, MNHN DJV201: Grotte Chauve-Souris at 12 m depth, Anse Noire, La Martinique.

Collector: Thierry Pérez, 26 March 2016. Sample code: 160326- MT6 - TP3. GenBank accession number: MN 175681.

Comparative material examined. Plakina arletensis Ruiz et al. 2017. MNHN DJV 177, Grotte Chauve-Souris, Anse Noire, La Martinique (14°32.024’N, 61°05.278’W). Collector: Thierry Pérez.

Plakina elisa (Laubenfels, 1936). USMN 22237, Lemon Bay, Margarita Island, Fort Randolph, Panama Canal (9°22.817N, 79°55.683W). Collector: Max Walker de Laubenfels.

Etymology. The species name “doudou” comes from the Antillais dialect and refers to a beloved person. In French, “doudou” is the name given to a child’s cuddly toy, which is often a piece of soft fabric or a stuffed animal that the child holds for comfort in the darkness of the night. Our new species was first observed in photographs taken of the dark cave wall. Because it looks fairly similar to a piece of soft fabric, and because we could not bring ourselves to abandon the crusty little sponge in that dark cave—no more than a child would be able to abandon their teddy—we named the species “doudou”.

Diagnosis. Thinly encrusting Plakina with a microlobate surface and whitish color in vivo. Skeleton made of diods, triods, calthrops, monolophose triods and monolophose, dilophose and trilophose calthrops. Mesohyl with no specialized cells and a low abundance of prokaryotic symbionts.

Description. The new species is small and thinly encrusting, 2 mm to 10 mm large, and 2 mm to 3 mm thick. Its consistency is soft and fragile. Its color in vivo is whitish, becoming slightly cream after fixation in alcohol. It has rounded and irregular edges, and is without a marginal canal. The surface is irregular, microlobate, especially in the smallest individuals, and is densely punctuated by small inhalant openings. Oscules are circular, slightly elevated, up to 5 mm in diameter with a perioscular membrane.

Skeleton. The skeleton is dense with an alveolar arrangement formed by randomly distributed spicule types of all sorts. However, lophose calthrops tend to be concentrated at the boundaries with the external medium (base, surface and around aquiferous canals) (Figure 2 A–B).

Spicules (Table 1). Diods are abundant, regular, straight, smooth, with blunt ends and variable centrotylote-like central irregularity (Figure 2C); their total length/width: 31.7–39.7–59.4/3.2–3.6– 5.6 µm.

n—measured spicules in total. MNHN—Muséum National d’Histoire Naturelle. H—Holotype. P—Paratype.

Triods are abundant, regular, Y-shaped and uniplanar, smooth, with blunt ends (Figure 2D). Their actines measure 14.2–15.7–19.1/2.3–2.8–4.0 µm.

Calthrops are abundant, regular, and smooth with blunt ends, the fourth actine being reduced to a small button (Figure 2E); their actines measure 9.7–13.7–21/1.6–2.8– 5.1 µm.

Monolophose triods are rare, regular, with one lophose actine ramifying in 3–7 rays, sometimes with terminal spines. Basal (non-lophose) actines are regular with blunt ends (Figure 2F); their total length: 5.3–20.5– 31.7 µm.

Monolophose calthrops are common, regular, with lophose actines ramifying in 3–6 rays that may have terminal spines (ramification pattern 1m, 2d, ts; see Muricy & Díaz 2002). Non-lophose actines are regular with blunt or terminally spined tips (Figure 2G); their total length: 12–17.7– 24.6 µm.

Dilophose calthrops are abundant, regular, with lophose actines ramifying medially in 3–5 rays that may have terminal spines (ramification pattern 1m, 2d, ts). Non-lophose actines are regular with blunt or terminally spined tips (Figure 2H); their total length: 15.6–19.9– 28.3 µm.

Trilophose calthrops are rare, irregular, with two regular lophose actines ramifying medially in 3–4 rays that may have terminal spines (ramification pattern 1md, 2d, ts) and one lophose actine ramifying in 2 rays with blunt or terminally spined tips (ramification pattern 1m, ts). Basal (non-lophose) actines are regular with blunt or terminally spined tips (Figure 2I); their basal actines measure 5.8–7.8–10/1.5–2.3– 3.3 µm.

Internal anatomy (Figure 3). The aquiferous system is leuconoid, with irregularly positioned inhalant and exhalant canals. There are no ectosomal lacunae, but there is a well-developed system of basal cavities which can be up to 300 µm high. Choanocyte chambers are eurypylous, ovoid, 30 to 60 µm in diameter, with 16 to 33 choanocytes recorded per section. The ectosome is thin, only 0.3 to 3 µm in thickness. The mesohyl volume is approximately equal to the choanocyte chamber volume.

Cytology. Choanocytes are ovoid (5.3–7.2 µm long; 3.3–3.6 µm thick). The choanocyte nucleus is spherical and located in basal position (1.5–2.8 µm in diameter). The cytoplasm is dense, with numerous inclusions, mostly located in apical position. Choanocytes are flagellated (Figure 4 A–B).

Apopylar cells are rounded to ovoid (6.2–7.4 µm in diameter) with numerous rounded cytoplasmic inclusions (Figure 4C).

Exopinacocytes are fusiform (15.3–17 µm long; 0.4–2.4 µm thick), with a large nucleus and small cytoplasmic inclusions, apparently without flagella (Figure 4D).

Endopinacocytes are fusiform (12.4–13.8 µm long; 0.4–1.5 µm thick) and flagellated, with a large nucleus and some small cytoplasmic inclusions (Figure 4E).

Symbiotic prokaryotes. The new species is considered to be a low microbial abundance (LMA) sponge. TEM allowed to distinguish one dominant extracellular morphotype, a curved rod form (0.8–2 µm long; 0.2–0.25 µm thick) with a simple thin wall (0.03–0.05 µm thick), that was randomly dispersed in the mesohyl. The small spherical and ovoid forms were interpreted as transversal sections of this dominant morphotype (Figure 4F).

Reproduction. Embryos and pre-larvae were observed in both specimens prepared for cytology (Figs. 3A and 3 E–F). Their diameter measured between 60 and 97 µm. On semi-thin sections of the holotype, some premature oocytes were also observed (Figure 3B).

DNA analysis of studied species. For all species considered in this study, GenBank accession numbers are presented in Table 2. Two well-supported clades represent the two families of Homoscleromorpha. In this tree, most of the Plakinidae genera seem to be monophyletic. The only genus without a skeleton, Aspiculophora Ruiz et al., 2017 forms a well-supported clade with Tetralophophora Rützler et al., 2014. The Plakina group appears to be paraphyletic with three well-supported groups represented in the phylogenetic tree. The type species P. monolopha groups together with Corticium candelabrum Schmidt, 1862. A second group includes P. jani Muricy et al., 1998 and P. arletensis, while the third includes Plakina doudou sp. nov., P. trilopha Schulze, 1880 and P. crypta Muricy et al., 1998. The two individuals of the new species which were considered for this analysis present identical sequences.

Ecology. Plakina doudou sp. nov. is a sciaphilic species that forms tiny whitish crusts with a patchy distribution on the vertical walls of a shallow-water cave. In this cave, the hydrodynamics is always high, from the entrance to the back of the cave. This new species was found to live in syntopy with P. arletensis, from 10 to 12 m depth, in the darkest part of the cave. No signs of epibiosis or predation were observed.