Morphology and phylogeny of Priapulida

Abstract

Priapulida is a small phylum of marine worms with only 21 described species across seven genera. Although priapulidans are long-known, standardized documentations of their morphology are still lacking for many species. Some of the morphological structures are only superficially described, if at all, making comparisons among priapulidans difficult. Interestingly for Priapulida is the occurrence of two adult size classes, microscopic (<3.5 μm) and macroscopic (up to 40 cm), differing from related phyla (Kinorhyncha and Loricifera) that are exclusively of microscopic size. Due to the two size classes in Priapulida and contradicting reports of potential fossils among the three related phyla, the ancestral body size of Priapulida remains uncertain. Internal phylogenetic relationships of priapulidans have only been inferred from morphological data, as molecular data from priapulidans are scarce. Proposed phylogenies support both an ancestrally microscopic and macroscopic body size. Phylogenetic analyses based on molecular data may help to clarify the relationships in Priapulida and their ancestral body size. In this thesis, the following macroscopic species are examined and morphological structures are described in detail using light microscopy, scanning electron microscopy and, for some species, histology: Priapulus caudatus, Priapulus tuberculatospinosus, Priapulopsis bicaudatus, Priapulopsis australis, Halicryptus spinulosus and Halicryptus higginsi. Larval and postlarval stages, as well as adult specimens were collected during multiple field trips or obtained from museum collections. Additionally, molecular data (transcriptomic and genomic) was gathered from these species and analyzed together with publicly available data. Microscopic species were not available for morphological examinations, however, few Tubiluchus specimens could be used for molecular analyses. The morphological investigations conducted in this thesis describe almost all structures of these species in detail. Key findings include newly described structures on the circumoral fields of P. caudatus and P. tuberculatospinosus, as well as on the trunks of H. spinulosus and H. higginsi. Additionally, asymmetric growth of the caudal appendages and the bipartition process of first-ring teeth during postlarval development were observed in P. bicaudatus. The larval stages of P. bicaudatus were also described in greater detail compared to a previous study. While examining P. australis specimens, two individuals exhibit unique morphological characters, resulting in the description of a new macroscopic species, Priapulopsis papillatus. Furthermore, this thesis presents the first phylogenomic analysis of Priapulida, integrating transcriptomic data from seven species across four genera (including six newly assembled transcriptomes) and conserved genomic loci from ten species across five genera. The positions of rare genera Acanthopriapulus and Maccabeus, which lack molecular data, were inferred through morphological analysis. The topology of this combined phylogenetic tree corresponds almost entirely with those of previous studies that only analyzed morphological data. Key findings include microscopic Meiopriapulus as sister group to all remaining priapulidans, and microscopic Maccabeus that exhibits morphological characters of both size classes as sister group to all macroscopic species. Ancestral state reconstructions of morphological traits on the newly conducted phylogenetic tree suggest an ancestrally small body size and an internal fertilization in extant Priapulida. These traits are consistent with their microscopic sister groups, Kinorhyncha and Loricifera.