New species of Pisionidens (Sigalionidae, Annelida) from Akumal, México

Small worms of Sthenelanellinae, Pholoinae and Pisioninae (Annelida, Sigalionidae) from the Tropical Northwestern Atlantic

The family Sigalionidae is characterized, among other features, by including scale worms with large bodies. However, among sigalionids, the subfamilies Sthenelanellinae, Pholoinae, and Pisioninae stand out by their small representatives with few segments and fragile bodies. In the Tropical Northwestern Atlantic, which includes part of the Gulf of Mexico and the Caribbean Sea, these subfamilies have been rarely studied, with few species recorded, and questionable records. This contribution aims to improve the knowledge of sthenelanellins, pholoins, and pisionins in the region through a faunistic study based on material from two Mexican scientific collections: the Reference Collection of Laboratorio de Biodiversidad y Cambio Climático (BIOMARCCA) and the Reference Collection of Benthos (ECOSUR) of El Colegio de la Frontera Sur. Pisione wolfi is confirmed from the Tropical Northwestern Atlantic, and three new species are described: Sthenelanella pechi sp. nov., S. sarae sp. nov. and Taylorpholoe anabelae sp. nov. A key to all Sthenelanella and Taylorpholoe species is also included.

Phylogeny and biogeography of the scaleless scale worm Pisione (Sigalionidae, Annelida)

Pisione is a scaleless group of small scale worms inhabiting sandy bottoms in shallow marine waters. This group was once considered rare, but now 45 described species can be characterized, among others, by their paired, segmental copulatory organs (one to multiple external pairs), which display a complexity of various accessory structures. The evolutionary significance of these unique organs was suggested in the late 1960s, but has been heavily debated since the late 1990s and remains controversial. In the present paper, we study the internal relationships within Pisione, employing combined phylogenetic analyses of both molecular and morphological data from 16 terminals of Pisione, as well as two terminals of Pisionidens, and eight additional scale worms as outgroups. Our taxon sampling covers all geographical areas where the genus has been reported, as well as most of their morphological and copulatory variability, including representatives of the "africana," "remota," "crassa," and "papuensis" groups, established previously by Yamanishi. We hereby provide a first insight into the relationships of the genus, testing previously proposed hypotheses on the evolutionary significance of male copulatory structures within Pisione, while attempting to understand patterns of distribution. The phylogenetic analyses using maximum likelihood and Bayesian methods consistently recovered two large clades spanning the East Atlantic (including the Mediterranean) and the Indo-Pacific-West Atlantic, respectively. Character optimization on our trees revealed a high degree of homoplasy in both non-reproductive and sexual characters of Pisione, with buccal acicula found to be the sole apomorphy among the morphological features assessed herein, with none defining the biogeographical subclades within. Overall, our comparative analyses highlight the high degree of morphological variation in this widely distributed genus, rejecting previous assertions of an increasing number and complexity of copulatory structures across the genus.