Sycon magnapicale, Cóndor-Luján & Louzada & Hajdu & Klautau, 2018

SYCON MAGNAPICALE View in CoL SP. NOV.

( FIGS 28 View Figure 28 , 29 View Figure 29 ; TABLE 20)

Etimology: From the Latin magna (=large), for the long apical actine of the atrial tetractine.

Type locality: Tug Boat , Caracasbaai, Willemstadt, Curaçao .

Material examined: Holotype. UFRJPOR 6748, Tug Boat , Caracasbaai, Willemstadt, Curaçao (12°04′ 08.20″N, 68°51′ 44.40″W), 14.9 m depth, coll. B. Cóndor-Luján, 23 August 2011 GoogleMaps . Paratype. UFRJPOR 6763, Water Factory , Willemstadt, Curaçao (12°06′30.88″N, 68°57′13.53″W), 8.4 m depth, coll. E. Hajdu, 23 August 2011 GoogleMaps .

Diagnosis: Sycon with globular body composed of an oscular neck and a crown. Skeleton composed of diactines and triactines in the distal cones, tubar triactines, subatrial tetractines and triactines, and atrial tetractines. Tetractines with long apical actines projected into the atrium.

Colour: White to beige in life and yellowish in ethanol ( Fig. 28A, B View Figure 28 ).

Morphology and anatomy: This species has a globular body with an apical osculum ( Fig. 28A, B View Figure 28 ). The holotype measures 6.5 × 2.5 × 0.1 cm ( Fig. 28A View Figure 28 ). The consistency is hard, although compressible. The surface is very hispid with long diactines and trichoxeas protruding through the surface. The osculum has a neck with a delicate crown of trichoxeas. In the paratype, the crown is more conspicuous ( Fig. 28B View Figure 28 ). The radial tubes are fully coalescent. The atrial cavity is hispid and the aquiferous system is syconoid.

Skeleton: The osculum has a neck composed of T-shaped triactines and tetractines and a crown of trichoxeas ( Fig. 28C View Figure 28 ). The triactines (white arrow) and tetractines (black arrow) are arranged parallel to each other ( Fig. 28D View Figure 28 ). The skeleton of the body is typical of the genus ( Fig. 28E View Figure 28 ). The distal cones have diactines ( Fig. 28F View Figure 28 , black arrow), triactines ( Fig. 28F View Figure 28 , white arrow) and rare trichoxeas tangentially positioned. Some longer diactines cross the choanosome and occasionally reach the atrium. The tubar skeleton is articulated, exclusively formed by rows of triactines with the unpaired actine pointing to the distal cones ( Fig. 28G View Figure 28 ). The subatrial skeleton is composed of tetractines ( Fig. 28H View Figure 28 , white arrow) and rare triactines ( Fig. 28H View Figure 28 , black arrow) with the unpaired actine pointing to the surface. The atrial skeleton is formed by tetractines. The basal actines lie tangentially and the very long apical actine is projected into the atrium ( Fig. 28I View Figure 28 ).

Spicules: Trichoxeas. Straight with tips always broken. Thicker than usual. Size:>1690.2/1.4–5.4 µm. Diactines. Fusiform and straight. The proximal tip is sharp and the distal tip was always found broken ( Fig. 29A View Figure 29 ). Size:>2970.0/23.0–32.4 µm. Triactines of the distal cones. Sagittal. Actines are conical with sharp tips. Some paired actines can be slightly longer than the unpaired one ( Fig. 29B View Figure 29 ). Size: 54.0–153.9/5.4– 10.8 µm (paired actines) and 40.5–175.5/5.4–10.8 µm (unpaired actine). Tubar triactines. Sagittal. Actines are conical with sharp tips ( Fig. 29C View Figure 29 ). Some paired actines are curved. Size: 54.0–135.0/5.4–10.8 µm (paired actine) and 35.1–189.0/5.4–10.8 µm (unpaired actine). Subatrial triactines. Sagittal. Actines are conical with sharp tips. The unpaired actine is straight and longer than the paired ones ( Fig. 29D View Figure 29 ). The paired actines are inwardly curved. In some triactines, one paired actine can be longer than the other. Size: 67.5–126.9/2.7–6.8 µm (paired actine) and 67.5–245.7/4.1–8.1 µm (unpaired actine). Subatrial tetractines. Sagittal. Actines are conical with sharp tips. The unpaired actine is straight and is longer than the basal actines. It can be slightly conical. The paired actines are inwardly curved ( Fig. 29E View Figure 29 ). The apical actine is the smallest actine present in this species. Size: 40.0–143.1/4.1–8.1 µm (paired actines), 143.1–237.6/5.4–10.8 µm (unpaired actine) and 10.8– 75.6/2.7–6.8 µm (apical actine). Atrial tetractines. Sagittal. The basal actines are conical with sharp tips ( Fig. 29F View Figure 29 ). The apical actine is slightly conical to cylindrical, straight, smooth and very long. It can be straight or curved. Size: 110.7–237.6/5.4–13.5 µm (paired actines), 29.7–151.2/5.4–16.2 µm (unpaired actine) and 124.2–496.0/5.4–16.2 µm (apical actine).

Ecology: The specimens of this species were collected underneath coral boulders at 8 and 15 m depth. They were found partially covered with sediment. No associated organisms were observed.

Geographical distribution: Southern Caribbean (provisionally endemic to Curaçao, present study).

Molecular analysis: The two sequences of S. magnapicale sp. nov. (holotype UFRJPOR 6748 and paratype UFRJPOR 6763) grouped together with high support (pp = 1, b = 99, Fig. 15 View Figure 15 ) and 100% genetic similarity. The phylogenetic affinities between S. magnapicale sp. nov. and the other species were not conclusive. In the ML phylogenetic tree only (data not shown), Leucandra falakra ( Grantiidae ) appeared as the sister species of S. magnapicale sp. nov., but with low support (b = 48).

Taxonomic remarks: The difference between Grantia and Sycon is the presence of a ‘cortex composed of tangential triactines and/or tetractines, occasionally with small perpendicular diactines’ ( Borojevic et al., 2000) in Grantia , whereas in Sycon there is no such continuous cortex and the distal ends of the radial tubes form cones which are decorated by tufts of diactines ( Borojevic et al., 2002). The specimens from Curaçao do not perfectly match the diagnosis of either Grantia or Sycon as it was possible to distinguish some radial tubes covered by tangential triactines and perpendicular diactines and other radial tubes with distal cones decorated by tufts of diactines. Nonetheless, we decided to allocate the new species to Sycon due to the presence of distal cones. However, we would not be surprised if this species is reallocated to Grantia or to another genus in future studies.

The most remarkable characteristic of S. magnapicale sp. nov. is the presence of tetractines with long apical actines projected into the atrium. Other species of Sycon that also have this skeletal feature are S. defendens Borojevic, 1967 from South Africa, S. minutum Jenkin, 1908 from Zanzibar, S. plumosum Tanita, 1943 from Paulau (Carolinas Islands), S. setosum and S. villosum . Among them, the species that most resembles S. magnapicale sp. nov. is S. plumosum , as both species have the same skeletal composition. Sycon setosum and S. villosum differ from the new species by not having subatrial tetractines. Sycon defendens and S. minutum bear tubar tetractines which are absent in S. magnapicale sp. nov.

Although S. magnapicale sp. nov. and S. plumosum have similar skeletal composition, they greatly differ in spicule size. Spicule measurements of S. plumosum from its original description are provided here. Diactines: 800–3000/30–35 µm, triactines of the distal cone: 200–240/16 µm (paired actine) and 140–200/16 µm (unpaired actine), tubar triactines: 170–240/15–18 µm (paired actine) and 270–360/15–18 µm (unpaired actine), subatrial triactines: 120–180/8–10 µm (paired actine) and 250–360/8–10 µm (unpaired actine), subatrial tetractines: 120–180/8–10 µm (paired actine) and 250–360/8–10 µm (unpaired actine) and 70–100/8–10 µm (apical actine) and atrial tetractines: 170–200/12– 16 µm (paired actine), 200–280/12–16 µm (unpaired actine) and 130–350/12–16 µm (apical actine). Compared to S. plumosum , all the spicule categories but the diactines and atrial tetractines are smaller in S. magnapicale sp. nov. Additionally, S. plumosum lacks the oscular neck observed in both Curaçaoan specimens of S. magnapicale sp. nov.

In the phylogenetic tree, S. magnapicale sp. nov. did not cluster with any other Sycon with well-defined distal cones (namely S. ancora , S. carteri , Sycon cf. v illosum, S. raphanus and S. conulosum sp. nov.). Maybe a future detailed analysis of the presence or absence of well-defined distal cones in Sycon would guide us to an understanding of the polyphyletic position of this genus in molecular phylogenies ( Voigt et al., 2012; Voigt & Wörheide, 2016).