Byrsophlebidae, Graff, 1905

Diez, Yander L., Monnens, Marlies, Wuyts, Arlien, Brendonck, Luc, Reygel, Patrick, Schmidt-Rhaesa, Andreas & Artois, Tom, 2023, Taxonomy and phylogeny of Dalytyphloplanida Willems et al., 2006 (Platyhelminthes: Rhabdocoela), with the description of a new family, a new genus, and sixteen new species from Cuba and Panama, Organisms Diversity & Evolution (New York, N. Y.) 23 (4), pp. 631-681 : 672

Byrsophlebidae

The position of Kaitalugia within Byrsophlebidae has implications for the taxonomic value of the number of gonopores as a synapomorphy. Previously, Byrsophlebidae was distinguished from Typhloplanidae Graff, 1905 (Limnotyphloplanida), by the presence of two gonopores in the former taxon. However, as species of Kaitalugia possess a single, common gonopore this is no longer a defining feature of Byrsophlebidae . Kaitalugia is recovered as the sister taxon to Maehrenthalia Graff, 1905 , but a clear morphological synapomorphy cannot be identified. The female system of Maehrenthalia includes a bursa with a seminal reservoir, whereas these structures are absent in species of Kaitalugia ( Willems et al., 2005a, b). The main features shared by members of Byrsophlebidae are the presence of a single ovary and a bursa with a single connection to the female duct.

The newly described species, Byrsophlebs thalassicola sp. n., can unambiguously be placed in Byrsophlebidae as it shows the above-mentioned characteristics of the family. The position of B. thalassicola sp. n. within Byrsophlebs was confirmed by the phylogenetic analysis. Byrsophlebs thalassicola sp. n. constitutes a monophyletic clade together with an unidentified species of Byrsophlebidae , Byrsophlebs sp. , and Byrsophlebs delamarei ( Ax, 1956) Karling, 1985 . In addition, the new species possesses a bursa, a feature present only in two genera of byrsophlebids: Byrsophlebs and Maehrenthalia . These genera can be distinguished from each other by the morphology of the female reproductive system: species of Maehrenthalia lack a seminal receptacle in the female duct and their bursa is bipartite, consisting of a uterus and a terminal resorptive vesicle; representatives of Byrsophlebs have a female duct with a seminal receptacle and a bursa without resorptive vesicle (see Karling, 1985). The Cuban specimens exhibit the latter morphology, corroborating their placement in Byrsophlebs . Byrsophlebs and Maehrenthalia also differ in the morphology of the male copulatory bulb, being globular-oviform with two layers of strong spiral muscle fibres in Byrsophlebs , and cylindrical with a single layer of strong spiral muscle fibres in Maehrenthalia . However, in B. thalassicola sp. n., the external muscle layer has a longitudinal orientation, a unique pattern of compared with all other species of Byrsophlebs .

All species of Byrsophlebs possess a simple, funnel-liketo-tubular stylet. In B. thalassicola sp. n., the distal tip of the stylet is hook shaped. This is the case in most species of this genus, except for B. dubia ( Ax, 1956) Karling, 1985 , and B. graffi Jensen, 1878 . Those species with a distal hook on the stylet differ from each other in the detailed morphology of the stylet and other atrial organs. In B. dubia , B. delamarei , and B. graffi , the stylet gradually tapers to a distal tip, as occurs in some of our specimens ( Fig. 9e ), whereas in other specimens of B. thalassicola sp. n., the stylet has a well-differentiated, proximal funnel-like and distal tubular part ( Fig. 9c, d ). A similar stylet morphology is seen in B. caligulachaena and B. lutheri ( Marcus, 1952) Karling, 1985 .

The stylet of B. thalassicola sp. n. is ~ 46 µm long, compared to a length of ~ 36 µm in B. caligulachaena ( Ehlers & Ehlers, 1981) and ~ 43 µm in B. lutheri ( Marcus, 1952) . While the stylet length of B. thalassicola sp. n. corresponds to that of B. lutheri , its stylet is more similar in shape to that of B. caligulachaena . The distal tip of the stylet of B. lutheri resembles a fishing hook, with the tip pointing forward; in B. thalassicola sp. n. and B. caligulachaena , the distal tip is directed sideways. In one specimen of B. thalassicola sp. n. ( Fig. 9e ), the distal tip of the stylet points forward. However, considering the delicate construction of the stylet, this situation is most likely a fixation artefact. Indeed, in the live specimen, the stylet had the same morphology as that of the other specimens. The distal tip of the stylet is pointed in B. thalassicola sp. n., while it is blunter in B. caligulachaena .

As mentioned before, a unique feature of B. thalassicola sp. n. is the longitudinal orientation of the external muscle fibres surrounding the prostate vesicle, whereas it is oblique in all other species of Byrsophlebs . Another diagnostic trait of B. thalassicola sp. n. is the fact that the bursa opens proximally into the female atrium, whereas this connection occurs more distally in its congeners. The presence of a bursal sphincter at the connection with the female duct in B. thalassicola sp. n. is uncommon in Byrsophlebs and has only been described for B. delamarei ( Ax, 1956) . The seminal receptacle as a differentiation of the proximal part of the female duct, apart from the new species, has only been reported in B. caligulachaena . The sphincter surrounding the distal part of the oviduct is a feature shared by B. thalassicola sp. n. and B. uncinata .

Figures

Fig. 9 Byrsophlebs thalassicola sp. n. a, b Body organisation (live animal). c–e Stylets. f Schematic reconstruction from sagittal sections of the pharynx and the genital systems from the right side. c From the holotype. a–e Structures oriented with the anterior end toward the top of the plate. f Structures oriented with the anterior end toward the right side of the plate