Microscopic priapulid larvae from Antarctica

Abstract

We report 24 larvae of macroscopic priapulids from north of the Antarctic Peninsula and from Potter Bay, King George Island. We assign the larvae to four groups according to their lorica pattern, which consists of platelets created by ridges. In larvae of the first group the platelets in the middle region of the lorica interdigitate in a zig-zag way, this has been described only for Priapulopsis australis. Larvae of groups 2 and 3 have more rectangular platelets and differ in the presence and abundance of paired versus unpaired platelets. The position of the larval tubuli corresponds to reported larvae of Priapulus tuberculatospinosus, but the lorica pattern has not been described in detail for this species. It is assumed to resemble the one in Priapulus caudatus, from which our larvae differ. Therefore the assignment to P. tuberculatospinosus is a bit vague. Larvae of group 4 are small and have little lorica structure. They may represent very young larval stages, probably of the co-occurring P. australis.

Introduction

Today, 22 extant species of priapulids are known (Schmidt-Rhaesa 2013; Schmidt-Rhaesa et al. 2017). Despite their small species number, they show a diversity of body sizes, life styles and reproductive patterns. Nine species (in the genera Priapulus, Priapulopsis, Acanthopriapulus and Halicryptus) are macroscopic, eleven (in the genera Meiopriapulus and Tubiluchus) are meiofaunal and two from the genus Maccabeus are small, sessile or hemisessile forms. The macroscopic species have external fertilization, while in the meiofaunal ones internal fertilization is likely. Meiopriapulus fijiensis Morse, 1981 develops directly, without free living larval forms and by vivipary (Higgins & Storch 1991), whereas in all other priapulids larvae hatch from the eggs.

Priapulid larvae typically have a lorica, which means that the integument of the trunk forms several stiff protective plates, which are connected by thinner regions that provide mobility of the plates. Such mobility is needed when the anterior body region, the introvert, is withdrawn into the trunk. Recently it was discovered that at least in the two species Priapulus caudatus Lamarck, 1816 and Halicryptus spinulosus Von Siebold, 1849 a larva without a lorica hatches from the egg (Wennberg et al. 2009; Janssen et al. 2009).

There are published observations on the loricate larvae of 11 species (see Table 1). In most cases, larvae are assigned to a certain species, because they co-occur with adults in a certain region. Often only single or few larvae are known. In Priapulus caudatus and H. spinulosus more data are available, but even in these species the complete series of larval stages is unknown. Priapulids moult during growth, but it is unknown whether there is a fixed number of moults.

Loricae of priapulid larvae belong to two types. In Tubiluchus, loricae are roundish in cross section (Adrianov & Malakhov 1996). In all other species, loricae are elongate and flat, they are composed of large dorsal and ventral plates and smaller lateral and sublateral plates. Most loricae are sculptured. Sensory structures, the tubuli, occur on the loricae of all priapulids. Several tubuli in different positions are present on loricae of Tubiluchus larvae, they occur at two levels in the Maccabeus larva and at one level in the loricae of macroscopic species (see, e.g. Adrianov & Malakhov 1996).

We report here the finding of 24 loricate priapulid larvae from Antarctic waters belonging to the lorica type of macroscopic priapulids and try to assign them to known species. There are only two species, from which adults were reported from southern cold waters, Priapulus tuberculatospinosus Baird, 1968 and Priapulopsis australis (De Guerne, 1886; Van der Land, 1970). Larvae of these species were documented by only few occasions. Lang (1951) and Van der Land (1970) compare larvae of P. tuberculatospinosus to those of P. caudatus and Adrianov & Malakhov (1996) figured younger and older larvae of P. australis.