Abstract
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.
Highlights
Since the mid-nineteenth century, the placement of the small and aberrant annelid genus Pisione Grube, 1857 has been one of the trials and tribulations
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
Ultrametric trees calculated with BEAST version 1.8.3 (Drummond, Suchard, Xie, & Rambaut, 2012) and all available terminals were used as a framework to evaluate the evolution of male copulatory structures in Pisione
Summary
Since the mid-nineteenth century, the placement of the small and aberrant annelid genus Pisione Grube, 1857 has been one of the trials and tribulations. Species of Pisione are unpigmented annelids, only a few millimeters in length, and with well over 50 segments They are commonly found in sandy bottoms of shallow marine waters (Rouse & Pleijel, 2001), with one exception in freshwater (San Martín, López, & Camacho, 1998). Due to immaturity or seasonality, penises may be lacking from the examined collected material, and Salcedo et al (2015) have suggested that other non-reproductive morphological characters may be systematically informative While these structures would include characters like neurochaetae and buccal and neuroacicula, to date, no detailed study across taxa has compared the significance of these characters, nor of the copulatory organs. We investigated the optimal distribution range for Pisione, testing for the presence of a latitudinal diversity gradient (LDG; Jablonski, Roy, & Valentine, 2006) and the preference of biogeographical hotspots (Bowen et al, 2013) based on all the records for the genus
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