Sycon caissarum, Pereira & Azevedo & Hajdu & Cavalcanti & Klautau, 2025

Sycon caissarum sp. nov.

urn:lsid:zoobank.org:act:

( Figs. 45 View FIGURE 45 , 46 View FIGURE 46 ; Table 18)

Etymology: Dedicated to the traditional caiçara people of Southeastern Brazil, especially the community inhabiting Búzios Island, the type locality of this species.

Type locality: Parcel da Pedra Lisa , Búzios Island, Ilhabela, São Paulo State, Brazil .

Type material: Holotype —UFRJPOR7038, Parcel da Pedra Lisa , Búzios Island, Ilhabela, São Paulo State, Brazil, depth 7 m, coll. F. F. Cavalcanti, 01/XII/2008 . Paratypes —UFRJPOR7037, same as the holotype. UFRJPOR6922, Velasquez shipwreck, São Sebastião Island, Ilhabela, São Paulo State, Brazil, depth 13 m, coll. F. F. Cavalcanti, 30/XI/2008 .

Additional material examined: UFRJPOR6931, Sumítica Island , Ilhabela, São Paulo State, Brazil, depth 9 m, coll. F. F. Cavalcanti & V. Padula, 02/XII/2008 . UFRJPOR7043, Costa do Aquário , Búzios Island, Ilhabela, São Paulo State, Brazil, depth 12 m, coll. F. F. Cavalcanti, V. Padula & R. Berlinck, 05/XII/2008 . UFRJPOR7054, Coroa , Búzios Island, Ilhabela, São Paulo State, Brazil, depth 15 m, coll. F. F. Cavalcanti, V. Padula & L. Kremer, 05/XII/2008 .

Comparative material examined: Haeckel’s slides of Sycon ampulla ( Haeckel, 1870) , deposited in MNHN.

Diagnosis: Sycon with small body, bearing oscular membrane (neck) and a crown. Skeleton with diactines and trichoxeas in the distal cones, triactines in tubar and subatrial regions, and triactines and slightly fewer tetractines in the atrium. Tetractines with short apical actines, which are similar in thickness to the basal actines. Diactines notably thicker than the other spicules.

Colour: Beige in life and in ethanol ( Fig. 45A View FIGURE 45 ).

Morphology and anatomy: Small oval or vase-shaped Sycon , with soft consistency. The surface is minutely hispidated by the tufts of diactines projecting through the distal cones ( Fig. 45A View FIGURE 45 ). There is a single apical osculum ornamented by a crown of trichoxeas, which is supported by a membrane ( Fig. 45B View FIGURE 45 ). The crown may be very long, as in the specimen UFRJPOR6922, in which it corresponds to 40% of the body length. The atrium is central and slightly hispid. Aquiferous system syconoid, with fully coalescent and unbranched choanocyte chambers ( Fig. 45C View FIGURE 45 ).

The oscular margin comprises a ring of “T-shaped” triactines and tetractines ( Fig. 45B View FIGURE 45 ), supporting a long crown of trichoxeas. Between this ring and the choanocyte chambers, there is a membrane (or neck) formed by more sparsely distributed spicules, which are progressively less sagittal towards the base of the sponge. In addition, diactines similar to those found in the distal cones are present, projecting obliquely from the oscular membrane ( Fig. 45B View FIGURE 45 ). The skeleton of the distal cones is formed by triactines and tufts of diactines (about 10–15 per distal cone) and trichoxeas ( Fig. 45D View FIGURE 45 ). The tubar skeleton is articulate, composed of triactines organised in rows ( Fig. 45C, E View FIGURE 45 ). The subatrial skeleton is composed only of triactines ( Fig. 45F View FIGURE 45 ). The atrial skeleton is composed of triactines and tetractines, with triactines being slightly more abundant ( Fig. 45G View FIGURE 45 ).

Spicules ( Table 18):

Trichoxeas: Very thin, cylindrical and often broken.

Diactines: Variable in size, but usually large and slightly curved. They are thickest in the proximal half ( Fig. 46A View FIGURE 46 ). The distal tip is usually blunt and thicker than the proximal tip, which is sharp. Size: 328.4 (±106.1)/10.5 (±4.0) µm.

Triactines of the distal cones: Sagittal, variable in shape. Actines are slightly conical, with sharp tips. The unpaired actine is straight and shorter or longer than the paired ones, which are almost straight or curved towards the atrium, sometimes undulated and unequal in length ( Fig. 46B View FIGURE 46 ). Sometimes they are less sagittal and have thicker actines than the tubar triactines. Some of them have the unpaired actine strongly bent over the inhalant openings between the radial tubes ( Fig. 45D View FIGURE 45 , inset). Size: paired—79.7 (±12.6)/6.3 (±1.0) µm; unpaired—96.8 (±30.1)/6.0 (±1.0) µm.

Tubar triactines: Sagittal. Actines are slightly conical, with sharp tips. The unpaired actine is straight and longer than the paired ones, which are slightly curved and frequently unequal in length ( Fig. 46C View FIGURE 46 ). Size: paired— 93.3 (±10.6)/6.0 (±0.7) µm; unpaired—120.2 (±15.9)/5.8 (±0.6) µm.

Subatrial triactines: Strongly sagittal. Actines are cylindrical and sharp. The unpaired actine is straight and longer than the paired ones, which are curved towards the distal cones. They are slightly thinner than the tubar triactines ( Fig. 46D View FIGURE 46 ). Size: paired—64.8 (±13.6)/3.5 (±1.0) µm; unpaired—118.3 (±14.8)/4.4 (±0.8) µm.

Atrial triactines and tetractines: Slightly sagittal. Basal actines are cylindrical, with blunt tips. The paired actines are curved, sometimes unequal in size and undulated, and shorter than the unpaired one, which can be straight or undulated ( Fig. 46E, F View FIGURE 46 ). In the longest unpaired actines, there is usually a swelling near the tip. The apical actine of the tetractines is slightly conical, sharp, smooth and curved towards the osculum. Triactines size: paired—127.3 (±21.6)/4.6 (±0.8) µm; unpaired—172.2 (±33.1)/5.1 (±0.8) µm. Tetractines size: paired—117.4 (±24.7)/4.6 (±1.0) µm; unpaired—171.0 (±33.0)/5.3 (±0.4) µm; apical—46.5 (±10.8)/5.4 (±0.8) µm.

Ecology: Most of the specimens were collected in protected habitats, but some were found exposed to sunlight, among algae.

Geographic distribution: Southeastern Brazil ecoregion—São Sebastião, Búzios and Sumítica Islands (Ilhabela), São Paulo State (provisionally endemic), Brazil.

Remarks: The genus Sycon is speciose, currently comprising 84 valid species ( De Voogd et al. 2025). We compared our specimens with all valid species, but here we focus on the species morphologically most similar to S. caissarum sp. nov., i.e., species sharing the following features: similar spicule composition; osculum with membrane (neck) and a crown of trichoxeas; fully coalescent, unbranched choanocyte chambers; and atrial tetractines with relatively short apical actines (<100 µm). These include S. ampulla ( Haeckel, 1870) from Venezuela; S. barbadense ( Schuffner, 1877) from Barbados (Caribbean); S. boreale ( Schuffner, 1877) and S. schuffneri Dendy & Row, 1913 , both from southern Norway; and S. raphanus Schmidt, 1862 , from Croatia (Adriatic Sea).

Sycon ampulla was described by Haeckel (1872) based on two varieties: S. ampulla var. alopecurus , from Venezuela (Caribbean), and S. ampulla var. petiolata , from Brazil ( Rio de Janeiro and Santa Catarina).These varieties differ mainly by the presence of a peduncle only in var. petiolata , which also has shorter diactines (100–150/5 µm) in the distal cones compared to those of var. alopecurus (200–500/5 µm). Sycon caissarum sp. nov. is more similar to var. alopecurus due to the absence of a peduncle and similar diactine lengths [328.4 (±106.1) µm]. However, in var. alopecurus , the diactines are thinner, being similar in thickness to the tubar and atrial spicules, whereas in S. caissarum sp. nov., although variable in size, the diactines are notably thicker than the other spicules of the skeleton [diactines—10.5 (±4.0) µm; for example, tubar triactine—paired actines: 6.0 (±0.7) µm, unpaired actine: 5.8 (±0.6) µm]. Additionally, the atrial spicules of S. ampulla (both varieties) are smaller than those of S. caissarum sp. nov., with paired and unpaired actines measuring 60–80/5 µm, and the apical actines of the atrial tetractines sometimes reaching 100 µm, a length not observed in S. caissarum sp. nov. [atrial tetractines—paired actines: 117.4 (±24.7)/4.6 (±1.0) µm, unpaired actine: 171.0 (±33.0)/5.3 (±0.4) µm, apical actine: 24.3–46.5 (±10.8)–67.5 µm]. We examined Haeckel’s microscope slides of S. ampulla var. petiolata deposited in the MNHN (Paris) and the skeleton is consistent with the original description, confirming the aforementioned differences relative to the new species.

Our species is also similar to S. barbadense and S. boreale , but can be distinguished by differences in spicule shape and size. Both species possess atrial tetractines with apical actines considerably thicker (13 µm) than those of S. caissarum sp. nov. [5.4 (±0.8) µm]. In S. barbadense , the apical actines are longer (usually 80 µm), and the paired and unpaired actines of the atrial spicules are thicker (9 µm). In S. boreale , the tubar triactines are less sagittal than those of the new species. Moreover, in S. barbadense and S. boreale , the atrial spicules have straighter actines (see Schuffner 1877, p. 436, plate XXVI, figs. 13, 14).

Regarding S. schuffneri , the diactines of the distal cones are smaller and thinner (220/4 µm), while the atrial spicules have longer and thicker actines (paired actines: 200/9 µm, unpaired actine: 250/9 µm), when compared to our species [diactines—328.4 (±106.1)/10.5 (±4.0) µm; atrial triactine—paired actines: 127.3 (±21.6)/4.6 (±0.8) µm, unpaired actine: 172.2 (±33.1)/5.1 (±0.8) µm]. Lastly, S. caissarum sp. nov. can be readily distinguished from S. raphanus by the latter’s much larger diactines (1000–3000/20–24 µm), longer apical actines of the atrial tetractines (60–120 µm), and overall longer and thicker spicules (for example, tubar triactines—paired actines: 100–180/10–12 µm, unpaired actine: 150–250/10–12 µm). Therefore, we conclude that our specimens represent a new species to science.