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

Sycon crassapicale sp. nov.

urn:lsid:zoobank.org:act:

( Figs. 47–49 View FIGURE 47 View FIGURE 48 View FIGURE 49 ; Table 19)

Etymology: From the Latin “crassus” (= thick, stout), referring to the thick apical actines of the atrial tetractines.

Type locality: Costa do Aquário , Búzios Island, Ilhabela, São Paulo State, Brazil .

Type material: Holotype —UFRJPOR7046, Costa do Aquário , Búzios Island, Ilhabela, São Paulo State, Brazil, depth 12 m, coll. F. F. Cavalcanti, V. Padula & R. Berlinck, 05/XII/2008 . Paratypes —UFRJPOR6934, Sumítica Island , Ilhabela, São Paulo State, Brazil, depth 9 m, coll. F. F. Cavalcanti & V. Padula, 02/XII/2008 . MNRJ2976 View Materials , between Ponta do Baleeiro and Saco Grande, São Sebastião Channel, São Sebastião, São Paulo State, Brazil, depth 4 m, coll. E. Hajdu, 09/I/2000 .

Additional material examined: MNRJ30128 View Materials , Alcatrazes Archipelago , São Sebastião, São Paulo State, Brazil, depth 12 m, coll. M. Custódio & C. Santos , 03/ V /2002 . UFRJPOR6927, Parcel da Coroa , Búzios Island, Ilhabela, São Paulo State, Brazil, depth 6 m, coll. F. F. Cavalcanti & V. Padula , 02/XII/2008. UFRJPOR6939, Saquinho da Sumítica , Búzios Island, Ilhabela, São Paulo State, Brazil, depth 9 m, coll. F. F. Cavalcanti , 01/XII/2008. UFRJPOR6997, 7018, close to Dart shipwreck, São Sebastião Island, Ilhabela, São Paulo State, Brazil, depth 4 m, coll. F. F. Cavalcanti , 30/XI/2008. UFRJPOR7029, Serraria Islet , São Sebastião Island, Ilhabela, São Paulo State, Brazil, depth 9 m, coll. F. F. Cavalcanti , 04/XII/2008. UFRJPOR7053, Coroa , Búzios Island, Ilhabela, São Paulo State, Brazil, depth 15 m, coll. F. F. Cavalcanti & V. Padula , 05/XII/2008. UFRJPOR9069, 9075, Parcel da Pedra Lisa , Búzios Island, Ilhabela, São Paulo State, Brazil, depth 7 m, coll. F. F. Cavalcanti , 01/XII/2008. UFRJPOR9076, Saco do Poço , São Sebastião Island, Ilhabela, São Paulo State, Brazil, depth 13 m, coll. F. F. Cavalcanti , V. Padula & L. Kremer, 03/XII/2008. UFRJPOR9292, São Sebastião Island, Ilhabela, São Paulo State, Brazil, depth unknown, coll. F. Oricchio , date unknown.

Comparative material examined: Holotype of Sycon conulosum —UFRJPOR6707, Daai Booi , St.Willibrordus, Curaçao ( 12.21197, -69.08567), depth 3–5 m, coll. B. Cóndor-Luján, 18/VIII/2011. GoogleMaps

Diagnosis: Sycon of tubular body and conulose surface, with an oscular membrane but lacking a crown. The skeleton is composed of diactines, trichoxeas, triactines and few tetractines, the latter occurring exclusively in the atrium. The atrial triactines and tetractines are only slightly sagittal (with a reduced unpaired angle). Their unpaired actine is much longer than the paired ones, and often swollen at the tip. The apical actine of the tetractines is very short yet robust.

Colour: Slightly greenish (MNRJ2976) or white in vivo and white to beige in ethanol ( Fig. 47A, B View FIGURE 47 ).

Morphology and anatomy: Tubular to slightly oval body, with a single apical osculum, surrounded by a membrane ( Fig. 47A–C View FIGURE 47 ). The consistency is soft. The surface is conulose and delicately hispid due to tufts of diactines and trichoxeas protruding through the distal cones. The atrial cavity is central and only slightly hispid, as the atrial tetractines are few and have short apical actines. Aquiferous system syconoid, with fully coalescent and unbranched choanocyte chambers ( Fig. 47D View FIGURE 47 ).

The oscular membrane is supported by T-shaped triactines and tetractines ( Fig. 47C View FIGURE 47 , inset). Tetractines are common at the oscular margin but become rare towards the base of the membrane. Diactines similar to those of the distal cones and trichoxeas are present throughout the oscular membrane, projecting obliquely, but do not form a crown ( Fig. 47C View FIGURE 47 ). At the base of the membrane, transitional stages between oscular and atrial triactines and tetractines are found. The distal cones show tufts of diactines (about 10 to 15) and some trichoxeas, in addition to triactines ( Figs. 47E View FIGURE 47 ; 48A, B View FIGURE 48 ). The tubar skeleton is articulate, composed of successive layers of triactines, which point their unpaired actine to the external surface ( Figs. 47D View FIGURE 47 ; 48A View FIGURE 48 ). The subatrial skeleton has only triactines ( Fig. 47F View FIGURE 47 ). The atrial skeleton presents triactines and few tetractines, which project their apical actine into the atrial cavity, making it slightly hispid ( Figs. 47G View FIGURE 47 ; 48C, D View FIGURE 48 ). Rare fragments of trichoxeas were seen in the atrium.

Spicules ( Table 19):

Trichoxeas: Very thin, cylindrical, and always broken.

Diactines: Straight or slightly curved and fusiform, usually with both tips sharp. Rarely, the distal tip is lanceolate ( Fig. 49A View FIGURE 49 ). Size: 275.0 (±56.5)/9.2 (±2.1) µm.

Triactines of the distal cones: Slightly sagittal, with conical to slightly conical and sharp actines. The unpaired actine is straight and longer than the paired ones, which are curved outwardly ( Fig. 49B View FIGURE 49 ). Size: paired—67.5 (±7.6)/7.1 (±0.9) µm; unpaired—112.1 (±16.6)/6.8 (±1.1) µm.

Tubar triactines: Sagittal, variable in shape. Actines are slightly conical and sharp. The unpaired actine is straight and longer than the paired ones, which can be almost straight or curved and similar or unequal in length ( Fig. 49C View FIGURE 49 ). Size: paired—66.2 (±9.7)/6.1 (±1.1) µm; unpaired—108.6 (±20.7)/5.9 (±1.1) µm.

Subatrial triactines: Strongly sagittal, with cylindrical and sharp actines. The paired actines are thinner and curved towards the unpaired one, which is straight and longer ( Fig. 49D View FIGURE 49 ). Size: paired—73.4 (±9.4)/5.5 (±0.6) µm; unpaired—134.0 (±15.8)/6.9 (±0.9) µm.

Atrial triactines and tetractines: Slightly sagittal. Basal actines are slightly conical to cylindrical and sharp. The unpaired actine is straight and frequently much longer and slightly thinner than the paired ones, which are a little curved. Often, one of the paired actines is much shorter than the other. When the unpaired actine is particularly long, it tapers in the middle and then swells near the tip ( Fig. 49E, F View FIGURE 49 ). The apical actine of the tetractines is conical, short but stout (thick), sharply pointed and only slightly curved, sometimes with a constriction at the base ( Fig. 49G View FIGURE 49 ). Triactines size: paired—89.3 (±18.5)/6.6 (±0.9) µm; unpaired—171.8 (±30.6)/6.8 (±0.9) µm. Tetractines size: paired—91.9 (±12.5)/7.4 (±0.6) µm; unpaired—152.0 (±26.5)/7.6 (±0.8) µm; apical—34.8 (±4.1)/7.0 (±1.0) µm.

Ecology: Specimens were found exposed to sunlight in association with algae or protected from sunlight in burrows or on banks of calcareous algae. Some of them were partially covered with sediment.

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

Remarks: The spicular composition and skeletal organisation of S. crassapicale sp. nov. is common throughout the genus; however, a remarkable feature is the apical actine of the atrial tetractines, very short and thick. This feature is relatively rare in the genus, so we compared S. crassapicale sp. nov. more closely with four species whose atrial skeleton is composed of triactines and few tetractines, the latter with short apical actines (<50 µm). These species are: S. bellum Chagas & Cavalcanti, 2017 , from Bahia State ( Brazil), S. boreale ( Schuffner, 1877) , from southern Norway, S. conulosum Cóndor-Luján, Louzada, Hajdu & Klautau, 2018 , from Curaçao (Caribbean), and S. minutum Dendy, 1892 , from Australia.

Sycon boreale differs from S. crassapicale sp. nov. by the presence of an oscular membrane with a crown of trichoxeas, while a crown is not conspicuous in the new species. The spicule morphology also varies, as S. boreale has predominantly less sagittal tubar triactines compared to those in S. crassapicale sp. nov. Additionally, spicule dimensions differ, as in S. boreale the paired and unpaired actines of the atrial spicules are more similar in length (around 180 µm), and the apical actines of the atrial tetractines are slightly longer and considerably thicker (50/13 µm) compared to those of the new species [34.8 (±4.1)/7.0 (±1.0) µm].

Sycon minutum differs from S. crassapicale sp. nov. by presenting atrial spicules with paired and unpaired actines more similar in length (both actines around 120 µm) and much thinner (3.5 µm thick) when compared to those of S. crassapicale sp. nov. [atrial triactine—paired actines: 89.3 (±18.5)/6.6 (±0.9) µm, unpaired actine: 171.8 (±30.6)/6.8 (±0.9) µm]. Besides, in S. minutum the subatrial triactines are similar in shape to the tubar ones, while these categories can be easily distinguished in our new species.

As S. crassapicale sp. nov., S. bellum View in CoL has an osculum surrounded by a membrane and no crown of trichoxeas or diactines. Despite this shared feature and similar spicule composition, spicule shape and dimensions differ. The atrial spicules of S. bellum View in CoL are more sagittal (with a wider unpaired angle) (see Chagas & Cavalcanti 2017, p. 215, fig. 7E, F) compared to those of our specimens. Additionally, in S. bellum View in CoL , the unpaired actines of both the tubar and distal cone triactines, as well as the atrial triactines and tetractines, are very similar in length to the paired actines [e.g., triactines of the distal cones—paired actines: 67.6 (±12.3) µm, unpaired actine: 64.0 (±17.9) µm; atrial triactines—paired actines: 134.4 (±22.1) µm, unpaired actine: 142.7 (±27.4) µm]. In contrast, S. crassapicale sp. nov. has spicules with the unpaired actine notably longer than the paired actines, particularly the atrial ones ( Table 19). Lastly, the apical actines of the atrial tetractines in S. bellum View in CoL appear to be cylindrical (see Chagas & Cavalcanti 2017, p. 214, fig. 6F).

Sycon conulosum View in CoL is the most morphologically similar species to S. crassapicale sp. nov., which prompted us to examine the holotype of this species. We found relevant differences between them, once more related to the atrial spicules. The atrial triactines and tetractines are more sagittal in S. conulosum View in CoL (see Cóndor-Luján et al. 2018, p. 51, fig. 27E, F) than in the new species. In terms of spicule dimensions, the unpaired actine of the atrial triactines and tetractines of S. conulosum View in CoL is only slightly longer than the paired ones [atrial triactines—paired actines: 103.0 (±11.8) µm, unpaired actine: 126.6 (±23.7) µm]. In contrast, in S. crassapicale sp. nov., this difference is much more pronounced [atrial triactines—paired actines: 89.3 (±18.5) µm, unpaired actine: 171.8 (±30.6) µm]. Additionally, in S. crassapicale sp. nov., the unpaired actines are often swollen near the tip, a feature not observed in the holotype of S. conulosum View in CoL . Moreover, the atrial tetractines seem to be rarer in S. conulosum View in CoL than in the new species.

In our phylogenetic analysis, sequences from the holotype and one of the paratypes of S. crassapicale sp. nov. grouped with 100% similarity and high support (BS = 100%). Despite the morphological similarities between S. crassapicale sp. nov. and S. conulosum , these species were retrieved in distant clades in our tree ( Fig. 2 View FIGURE 2 ). Sycon conulosum appears as a sister species of the group formed by Paraleucilla perlucida and Leucilla antillana , while S. caissarum sp. nov. and S. crassapicale sp. nov. clustered with Sycon ciliatum from Norway, and Grantessa tumida from Curaçao ( Fig. 2 View FIGURE 2 ). Interestingly, the positions of S. crassapicale sp. nov. and S. conulosum in our phylogenetic tree suggests that morphological resemblance within the genus Sycon does not necessarily reflect close evolutionary relationships.