Dondersia festiva Hubrecht, 1888

Cobo, M. Carmen, Farris, William J. & Kocot, Kevin M., 2024, New data on the biodiversity of Solenogastres (Mollusca, Aplacophora) in the Mediterranean Sea: findings from the program “ Our Planet Reviewed ” Corsica 2019 - 2022, Zoosystema 46 (21), pp. 543-576 : 554-556

Dondersia festiva Hubrecht, 1888

Dondersia festiva Hubrecht, 1888: 324 .

MATERIAL EXAMINED. — Corsica (France) [19 specimens] • 2 specimens (used for sclerite preparation, DNA extraction and histology); CORSICABENTHOS 1, 3 ( Table 2 ); 15-200 m depth; MNHN-IM-2019-16169 (1 microscope slide with sclerites, 24 slides with 5 µm serial sections); MNHN-IM-2019-18277; GenBank: OR456222: OR458916 (1 microscope slide with sclerites, 1 SEM stub; 9 slides with 5 µm serial sections) • 16 specimens (preserved in 95%); CORSICABENTHOS 1, 3 ( Table 2 ); 15-200 m depth ethanol; MNHN-IM-2019-13916, MNHN-IM-2019-13917, MNHN-IM-2019-13918, MNHN-IM-2019-13919, MNHN-IM-2019-13920, MNHN-IM-2019-16171, MNHN-IM-2019-16172, MNHN-IM-2019-16173, MNHN-IM-2019-16174, MNHN-IM-2019-16175, MNHN-IM-2019-16181, MNHN-IM-2019-16182, MNHN-IM-2019-16183, MNHN-IM-2019-16184, MNHN-IM-2019-16185, MNHN-IM-2019-16186 • 1 specimen (mounted for SEM and preserved on the SEM stub); CORSICABENTHOS 1, 3 ( Table 2 ); 15-200 m depth; MNHN-IM-2019-16170 .

DESCRIPTION

Elongate animal (10-15 × 1-2 mm) with a posterior digitiform projection preceded by a bulbous lobe and a beak-like anterior end. Of bright purple color ( Fig. 3A ). Color fading after preservation in 95% ethanol but remaining light pink. Dorso-terminal sensory organs (two to four) externally visible as small, rounded holes in the cuticle ( Fig. 3 A’). Of the 19 specimens, clearly observed four dorso-terminal sensory organs in 12, in one specimen three and in three specimens observed two. In remaining three specimens, difficult to count, but at least one dorso-terminal sensory organ observed in each before preserving in ethanol. Pedal groove and opening of the pallial cavity marked externally ( Fig. 3A ). With characteristic scale-like sclerites of different types ( Fig. 6 ): entire body covered by a basal layer of small, oval-shaped scales (10- 18 × 5-10 µm) with a proximal rim and a pointed distal end ( Fig. 6B ), pallet-shaped (trowel-like) scales of four types and solid acicular sclerites are arranged between them ( Fig. 6A, B, F, I, J ). Pallet-shaped scales vary in their total length and the length of the stem: 1) shorter pallet-shaped scales with long stem (58-60 × 10-14 µm, where the stem is around 20 µm) ( Fig. 6A, C ); 2) shorter pallet-shaped scales with a short stem (38-50 × 12-18 µm, where the stem is around 8 µm) ( Fig. 6D, E ), less abundant; 3) longer pallet-shaped scales with long stem (80-120 × 8-10 µm, where the stem is around 20 µm) ( Fig. 6E, F, G, H ); and 4) longer pallet-shaped scales with short stem (75-100 × 10 µm, where the stem is around 5 µm) ( Fig. 6E, I ). In addition, one type of pallet-shaped scale (40- 50 × 10-14 µm) located just around the atrio-buccal cavity ( Fig. 6L ). Acicular sclerites (100-160 × 15-18 µm) are curved, striated and with rounded ends ( Fig. 6J, K ), mostly located at the mid-ventral areas of the body ( Fig. 6J ). Radula monoserial with eight to nine rows of small teeth (30-40 × 12 -14 µm) formed by a pair of middle denticles fused for most of their length and terminating in a thin distal tip ( Fig. 10A , A’) and with smaller, distally pointed, downwardly curved, lateral denticles. Ventrolateral foregut gland of type A ( García-Álvarez & Salvini-Plawen 2007) / Acanthomenia type ( Handl & Todt 2005) ( Fig. 10A ). With folded mantle cavity but without real respiratory folds ( Fig. 10D, E ). With copulatory stylets ( Fig. 10 B-D).

REMARKS

Dondersia festiva Hubrecht, 1888 has a characteristic coloration and body shape with a posterior projection that facilitated the identification of the Corsica specimens. Besides, the other diagnostic morphological characters (monoserial radula, copulatory stylets, dorso-terminal sensory organ(s) and absence of respiratory folds) were found in the examined animals. Particularly important for the identification of Dondersiidae species is the types of sclerites ( Scheltema et al. 2012; Cobo & Kocot 2021). The sclerites of the Corsica specimens ( Fig. 6 ) can be compared to those described previously for the species ( Hubrecht 1888: fig. 2a; Scheltema et al. 2012: figs 1-3). Nevertheless, the use of SEM allowed us to observe details not described before: 1) it was possible to determinate that the acicular sclerites are striated; 2) the exact shape and position of the oval-shaped scales, that are embedded in the cuticle by their rounded, rimmed proximal end ( Fig. 6A, B ); and 3) we confirmed that the pallet-shaped scales have pointed ends and not flat ends as described before ( Hubrecht 1888; Scheltema et al. 2012: figs 1-3a); the previously described flat-ended pallet-shaped sclerites correspond with broken sclerites ( Fig. 6G ). Previous works ( Nierstrasz 1902, 1908; van Lummel 1930; Nierstrasz & Stork 1940) stated the existence of two dorso-terminal sensory organs, not distinguished by Hubrecht (1888) or Scheltema et al. (2012). Observations of living animals revealed that D. festiva can have between two and four of these organs. We did not find any relationship between the size of the specimen and the number of dorso-terminal sensory organs, although we found that after fixation it was more difficult, or not possible, to find this organ externally and it was also challenging to distinguish it in the serial sections ( Fig. 10E ). The original description of D. festiva and subsequent descriptions (material and literature reviewed in Scheltema et al. 2012) state the absence of respiratory folds. The obtained sections of the Corsica specimens ( Fig. 10D, E ) show a strongly ciliated mantle cavity. In addition to the morphological data, we obtained DNA barcodes for this species for the first time. The characteristics of the copulatory stylets (the bag they are included in Figure 10B , and the accessory spicules, Figure 10C ), coincide with what was described and shown by Scheltema et al. (2012).

D. festiva was described from the Mediterranean Sea (Western Italy) from depths between 60 and 65 m. The specimens from Corsica were collected between 15 and 100 m extending not only the geographical but also the bathymetric distribution of the species. Of the type series, one specimen was collected on the hydrozoan Aglaophenia sp. and another specimen on the hydrozoan Lytocarpia myriophyllum (Linnaeus, 1758) , although the identification of this last specimen is doubtful ( Hubrecht 1888; Scheltema et al. 2012). Specimens from Corsica were not observed on cnidarians, likely due to the sampling methods used (dredge and manual suction pump sampler; Table 2 ).

Figures

FIG. 3. — Habitus of A, Dondersia festiva Hubrecht, 1888 (photo of the living animal – MNHN-IM-2019-18277); A’, Dondersia festiva (photo of the living animal – MNHN-IM-2019-16169); A’’, detail of the dorsoterminal sensory organs (MNHN-IM-2019-16169); B, Anamenia gorgonophila (Kowalevsky, 1880) (photo of the living animal on Paramuricea sp.): B’, Anamenia gorgonophila (photo of the preserved specimen – MNHN-IM-2019-18270); C, Nematomenia banyulensis (Pruvot, 1890) (photo of the preserved specimen – MNHN-IM-2019-16178); D, Dorymenia vagans (Kowalevsky & Marion,1887) (photo of the preserved specimen – MNHN- IM-2019-18274). Scale bars: 1 mm. Symbols: arrows, dorsoterminal sensory organs; stars, anterior end of the animals.

FIG. 6. — SEM images of the sclerites of Dondersia festiva (Pruvot, 1890) (MNHN-IM-2019-16170): A, layer of oval-shaped scales (many are broken) with palletshaped scales among them;B, oval-shaped scales;C, short pallet-shaped scales with a long stem;D, short pallet-shaped scales with a short stem;E-G, distal end of long pallet-shaped scales.H, long pallet-shaped scales with a long stem; I, long pallet-shaped scales with a short stem.; J, striated acicular sclerites; K, detail of the striated acicular sclerites; L, pallet-shaped scales of the anterior region. Scale bars: 20 µm. Symbols: arrows, oval-shaped scales from the basal layer.

FIG. 10. — Histological sections of Dondersia festiva (MNHN-IM-2019-18272): A, radular region; A’, detail of the radula and radular sac; B, posterior region, spawning duct, and bag of the copulatory stylets; C, posterior section with copulatory stylets (circled) (MNHN-IM-2019-16169); D, posterior region, mantle cavity; E, mantle cavity and dorsoterminal sensory organ. Abbreviations: see Material and methods. Scale bars: 100 µm.