A synthesis of the external morphology of cypridiform larvae of Facetotecta (crustacea: Thecostraca) and the limits of the genus Hansenocaris

Abstract

Although the naupliar and cypridiform stages of the enigmatic y‐larvae of Facetotecta have been found in the marine plankton worldwide, they still represent the last significant group of crustaceans for which the adult forms are still unknown. From a number of y‐cyprids representing different taxa from different locations, we employ scanning electron microscopy to describe fine morphological details of all external structures of this unique larval form. We document different segmentation patterns of the abdomen and presence/absence of the labrum and structural differences in the antennules, labrum, paraocular process, thoracopods, and telson lend support for the erection of several new genera as opposed to the single Hansenocaris . The data presented here emphasize the morphological limits of the genus Hansenocaris and the “bauplan” of cyprydiform larvae of Facetotecta. Although the optimum pathway is a joint analysis of both molecular and morphological characters, we use the morphological characters of y‐cyprids to align them cladistically and determine the limits of the genus Hansenocaris s.s. and describe common characters for all y‐cyprids including six pairs of the lattice organs instead five pairs considered as a ground pattern for all Thecostraca. We also determine plesiomorphic and apomorphic characters of all known y‐cyprids and separate them from other thecostracan cypridiform larvae.

Novel molecular resources for single-larva barcoding of enigmatic crustacean y-larvae

“…We recommend mounting naupliar exuviae in glycerin jelly as it solidifies at room temperature, thus easing maintenance in museum collections and tropical research facilities. Some LSN exuvial specimens may also be imaged by confocal laser-scanning microscopy (CLSM), although we have not attempted this yet for DNA-extracted specimens (for information on mounting exuviae for SEM, see Grygier et al, 2019;Kolbasov et al, 2022).…”

“…Few studies have cast light on the systematics and evolution of facetotectans, despite their discovery more than a century ago and their global distribution at depths of 0-6000m (Hansen, 1899;Grygier, 1987;Dreyer et al, 2022;Kolbasov et al, 2022). Morphological studies based on wild-caught specimens have proven unsuccessful in delimiting species within geographically widespread "types" such as type IV (Hansen, 1899;Schram, 1972), "Pacific Type I" (Itô, 1986), and type VIII with its three subtypes VIII-a, -b, and -c (Itô, 1987), which are each practically identical wherever found.…”

“…Morphological studies based on wild-caught specimens have proven unsuccessful in delimiting species within geographically widespread "types" such as type IV (Hansen, 1899;Schram, 1972), "Pacific Type I" (Itô, 1986), and type VIII with its three subtypes VIII-a, -b, and -c (Itô, 1987), which are each practically identical wherever found. Formal cladistic analyses using morphological characters have not resolved facetotectan phylogeny (Pérez-Losada et al, 2009; Kolbasov et al, 2022), and the need for DNA sequences of live-imaged and properly vouchered material is inescapable . Phylogenetic analyses show that y-larvae form a distinct and monophyletic group (Grygier, 1987;Chan et al, 2021;Dreyer et al, 2022), but conflicting datasets currently place Facetotecta as sister either to Ascothoracida (parasitic "gall barnacles": Dreyer et al, 2022) or a monophyletic group composed of Ascothoracida and Cirripedia (the stalked, acorn, burrowing, and parasitic barnacles; see Pérez-Losada et al, 2009).…”

“…, 2002, 2009Kolbasov et al, 2022). To overcome the challenges inherent in inferring systematic relationships based on unvouchered, unphotographed specimens, as exemplified by Perez-Losada et al,(2002, 2009) and, we developed a cheap, fast, and voucher-preserving protocol capitalizing on the widely-used DNeasy extraction kit.…”

“…To overcome the challenges inherent in inferring systematic relationships based on unvouchered, unphotographed specimens, as exemplified by Perez-Losada et al,(2002, 2009) and, we developed a cheap, fast, and voucher-preserving protocol capitalizing on the widely-used DNeasy extraction kit. Our aim was to develop a rearing and extraction protocol that maximizes morphological and molecular information for single y-larval specimens spanning what is emerging as a complex and wide phylogenetic rangeKolbasov et al, 2022; Dreyer et al, submitted; data herein). By relying on a suite of light microscopic images of live larvae and by developing primers targeting both nuclear and mitochondrial genomic markers, we demonstrate a way to infer phylogenetic relationships based on larval instars of a crustacean taxon with a complicated and partly unknown lifecycle.…”