Falcidens Salvini-Plawen, 1968

Corrêa, Paulo Vinicius Ferraz, Fassina, Paola Visnardi & Passos, Flávio Dias, 2014, Falcidens targatus and F. acutargatus: two species of Caudofoveata (Mollusca, Aplacophora) new for Brazil, Journal of Natural History (J. Nat. Hist.) 48 (45 - 48), pp. 2947-2963 : 2949-2958

Genus Falcidens Salvini-Plawen, 1968

Subgenus Chiastofalcidens nom. nov. (pro Lepoderma Salvini-Plawen 1992 , nec Looss, 1899) Falcidens (Chiastofalcidens) targatus Salvini-Plawen, 1992

( Figures 1A–C , 2A–G , 3A, B , 4A, C) Falcidens (Chiastofalcidens) acutargatus Salvini-Plawen, 1992

( Figures 1D–F , 2H–J , 3C , 4B, D)

Chiastofalcidens is proposed to replace the species-group name (subgenus) of Lepoderma Salvini-Plawen, 1992 , which is a pre-occupied name: Lepoderma Looss, 1899 ( Trematoda). ‘Chiastos’, derived from Greek chiasma, means intercrossed; this new name refers to the crossed sculpture of the sclerites, present in Falcidens loveni ( Nierstrasz, 1902) , F. chiastos Scheltema, 1989 , F. nontargatus Salvini-Plawen, 1992 , F. targotegulatus Salvini-Plawen, 1992 , and F. acutargatus Salvini-Plawen, 1992 , all of them related to F. targatus , the type species designated by Salvini-Plawen (1992) for Lepoderma .

Falcidens (Chiastofalcidens) targatus and F. (Ch.) acutargatus have an elongated body shape with three distinct regions ( Figure 1 ): an anterior part, with an ‘anterium’ continuous to a neck; a trunk; and a posterium with a slender shank and a terminal knob. The collar separating the trunk from the neck is well marked in most individuals of both species. In F. (Ch.) targatus the neck is more prominent and the anterium can be inflated, or not ( Figures 1A–C , 2A, B ). Due to deposition of particles from the surrounding environment, the knob is red in almost all the individuals of this species ( Figure 1A ). In some animals the gills are partially exposed because the knob is very contracted ( Figure 2C ), while in others it is relaxed, showing the mantle cavity in an inflated condition ( Figure 2D ). In F. (Ch.) acutargatus , the knob is more distinct from the shank ( Figures 1D–F , 2J ).

The individuals of the material from the American Museum of Natural History labelled as ‘types’, which are considered the holotypes of both species ( Boyko and Sage III 1996) are quite worn ( Figure 1C, F ). The holotype of F. (Ch.) acutargatus corresponds exactly to the specimen of figure 8 of Salvini-Plawen (1992). Most specimens of the lot of the paratypes of F. (Ch.) targatus are well preserved and one of them is very similar to one individual from Brazil ( Figure 1A, B ). The fragments of the paratypes of F. (Ch.) acutargatus are not comparable. From external view, the animals of both species from Campos Basin are very similar to those illustrated in figures 7 and 8 of Salvini-Plawen (1992).

The sclerites extracted from Campos Basin specimens of both species and from one paratype of F. (Ch.) targatus are illustrated in Figures 2 and 3 . In general, they are smaller in F. (Ch.) targatus than in F. (Ch.) acutargatus , being similar in shape among these species, and differing only in the some details ( Figure 3 ). In the anterium, the sclerites are very small, with an elliptical shape and no ornamentation ( Figures 2E , 3A, C ). In the neck, trunk and shank they have a more slender shape in F. (Ch.) targatus ( Figure 3A, B ) than in F. (Ch.) acutargatus ( Figure 3C ); also, in the former species they have a tapered tip ( Figures 2F, G , 3A, B ), while in the latter they are more pointed distally ( Figures 2H–J , 3C ). In both species, the side of the sclerites facing the body is smooth (see the sclerites with an asterisk in Figure 3 ), while on the opposite surface they bear the typical ornamentation originally described for the subgenus: on the proximal half of the surface there are up to eight crossed ridges. On the distal half these ridges are parallel, a median one being stronger and forming a low keel, this being particularly more evident in F. (Ch.) acutargatus . In the neck they measure ~45 μm in F. (Ch.) targatus and ~120 μm in F. (Ch.) acutargatus ; in the trunk, ~100 and ~140 μm; and in the shank, ~70 and ~140 μm, respectively. Throughout the body, about 1/3 of the sclerites of F. (Ch.) targatus also have a basal deep notch. In F. (Ch.) acutargatus this notch is present in about 1/4 of the sclerites, and appears to be shallower. At the knob the sclerites are elongated, ~170 μm long in F. (Ch.) targatus and ~180 μm long in F. (Ch.) acutargatus , with the shape of long bats surrounding the mantle cavity. The extracted sclerites of the paratype of F. (Ch.) targatus ( Figure 3B ) are very similar to the ones obtained from Campos Basin’ s specimens ( Figure 3A ).

The radula of F. (Ch.) targatus ( Figure 4A) is similar to the one of F. (Ch.) acutargatus ( Figure 4B). A pair of sickle-shaped teeth is present in the distal end of a large cone-shaped support, and there is also a pair of lateral supports, which are approximately one third of the length of the central support in F. (Ch.) targatus and about a half of its length in F. (Ch.) acutargatus . A triangular plate is placed anteriorly, connected with the teeth by an apophysis of the plate.

The oral shield is U-shaped in both species ( Figure 4C, D).

From a total of about 2350 specimens of caudofoveates present in the samples collected by the Habitats Project, F. (Ch.) targatus and F. (Ch.) acutargatus are among the most common species, with 176 and 141 individuals, respectively ( Tables 1, 2). The former species was only collected on the shelf, represented by 17 samples from 30 to

HAB6-A07- R3 23°39 ′ 19.981 ″ S, 41°18 ′ 30.534 ″ W 732.9 25 June 2008 2

HAB3-G08- R2 22°7 ′ 19.663 ″ S, 39°52 ′ 22.613 ″ W 992.6 16 June 2008 1

HAB7-H06- R2 21°44 ′ 21.564 ″ S, 40°5 ′ 18.026 ″ W 402.7 07 July 2008 1

HAB7-H06- R3 21°44 ′ 21.493 ″ S, 40°5 ′ 18.066 ″ W 401.6 07 July 2008 1

HAB7-H07- R2 21°41 ′ 11.649 ″ S, 40°2 ′ 20.690 ″ W 699.4 07 July 2008 3

HAB4- CANG07 - R2 21°56 ′ 10.244 ″ S, 39°57 ′ 43.438 ″ W 709.7 28 May 2008 1

HAB6-B07- R1 23°13 ′ 1.396 ″ S, 40°57 ′ 36.705 ″ W 815.5 26 June 2008 1

HAB8-D06- R3 22°33 ′ 36.014 ″ S, 40°26 ′ 37.171 ″ W 383 31 January 2009 1

HAB8-E06- R3 22°25 ′ 59.389 ″ S, 40°17 ′ 33.343 ″ W 387.1 31 January 2009 2

HAB9-I07- R3 21°11 ′ 12.228 ″ S, 40°12 ′ 51.745 ″ W 683 04 February 2009 3

HAB9-H06- R1 21°44 ′ 21.608 ″ S, 40°4 ′ 59.614 ″ W 405 05 February 2009 1

HAB8-A07- R3 23°39 ′ 21.880 ″ S, 41°18 ′ 33.045 ″ W 692.7 28 January 2009 1

HAB8-A07- R1 23°39 ′ 19.742 ″ S, 41°18 ′ 28.369 ″ W 699 28 January 2009 2

HAB3-H09- R2 21°39 ′ 18.701 ″ S, 39°53 ′ 55.974 ″ W 1293.2 25 June 2008 1

HAB8-C08- R2 23°1 ′ 30.862 ″ S, 40°45 ′ 22.948 ″ W 964.8 16 January 2009 1 Mean depth 617.50 Total 156

147 m depth ( Figures 5 , 6 ). Most specimens and the richest samples of F. (Ch.) targatus were obtained in the south of the area of Campos Basin, namely in transects A and B ( Figure 5 ). Falcidens (Chiastofalcidens) acutargatus is more common on the continental slope, with 53 samples obtained from 376 to 1293 m depth; this species is homogenously distributed through all of the sampling area ( Figures 5 , 6 ).

The mean length of F. (Ch.) targatus is higher (5.34 mm), reaching up to 9.12 mm, while specimens of F. (Ch.) acutargatus have a mean length of 4.04 mm but are up to 10.44 mm long. The t -test revealed statistical differences (p <0.05) among species in relation to the NeL, TrL, TrW and ShW ( Table 3). Although the PoL was the only absolute measurement that can be considered similar among species (p = 0.45), the ratio PoL/TrL was not (p <0.05). In fact, the trunk is usually twice the length of the posterium (PoL/TrL = 0.42) in F. (Ch.) targatus , while in F. (Ch.) acutargatus the posterium is more distinct and longer when compared to the trunk (PoL/ TrL = 0.63). The PoL/TrL showed no correlation with the length of the body in F. (Ch.) targatus (r = −0.15) and in F. (Ch.) acutargatus (r = −0.21) ( Figure 7A ).

The ratio ToL/TrW was revealed to be distinct among these species (p <0.05). A Pearson product–moment coefficient showed a moderate correlation to the total length of the body in both F. (Ch.) targatus (r = 0.77) and F. (Ch.) acutargatus (r = 0.75). This relationship is illustrated in Figure 7B .

Figures

Figure 1. (A–C) Falcidens (Chiastofalcidens) targatus; and (D–F) Falcidens (Chiastofalcidens) acutargatus; photomicrographs of Campos Basin’s specimens from (A, D) the left and (E) right; (B) paratype and (C, F) holotypes are from the left. In A and D the different body regions are shown: anterium (a), neck (n), trunk (tr), shank (sh), and knob (k). The scale bar in C is the same for all photos, except E.

Figure 2. (A–G) Falcidens (Chiastofalcidens) targatus; and (H–J) Falcidens (Chiastofalcidens) acutargatus. Scanning electron micrographs of whole individuals (A, B, adults; J, juvenile), and of details of the body regions and their sclerites (C–I), all from the left side. In A and B the arrows show the collar separating the neck from the trunk. (C, D) Posterium; in C the knob is contracted, and the gills are partially exposed; in D, the sclerites have fallen off and the mantle cavity is shown in an inflated condition. (E) Anterium; (F) trunk; (H) anterium, neck and anterior part of the trunk; (I) posterior part of the trunk and posterium. g, gills; mc, mantle cavity; os, oral shield. The scale is the same in A and B, and in H and I.

Figure 3. (A, B) Falcidens (Chiastofalcidens) targatus and (C) Falcidens (Chiastofalcidens) acutargatus. Scanning electron micrographs of the sclerites isolated from different body regions of Campos Basin’s specimens (A, C) and holotype (B); the asterisks show the sclerites viewed from the side facing the body. a, anterium; k, knob; n, neck; sh, shank; tr, trunk. The scale bar is the same for all images.

Figure 5. Distribution of Falcidens (Chiastofalcidens) targatus and Falcidens (Chiastofalcidens) acutargatus in Campos Basin, off Rio de Janeiro (RJ) and Espírito Santo States (ES). The activities of the Habitats Project included sampling sites from 20 to 3000 m depths, with many of them made along the transects shown in ‘A’ to ‘I’.

Figure 6. Depth versus number of specimens of Falcidens (Chiastofalcidens) targatus and Falcidens (Chiastofalcidens) acutargatus.

Figure 7. Total length versus (A) posterium length/trunk length, and (B) total length/trunk width, for Falcidens (Chiastofalcidens) targatus and Falcidens (Chiastofalcidens) acutargatus. For abbreviations see text.