Extreme secondary sexual dimorphism in the genus Florarctus (Heterotardigrada: Halechiniscidae)

Piotr Gąsiorek,Reinhardt Møbjerg Kristensen,David Møbjerg Kristensen

doi:10.1007/s12526-021-01183-y

Abstract

Secondary sexual dimorphism in florarctin tardigrades is a well-known phenomenon. Males are usually smaller than females, and primary clavae are relatively longer in the former. A new species Florarctus bellahelenae, collected from subtidal coralline sand just behind the reef fringe of Long Island, Chesterfield Reefs (Pacific Ocean), exhibits extreme secondary dimorphism. Males have developed primary clavae that are much thicker and three times longer than those present in females. Furthermore, the male primary clavae have an accordion-like outer structure, whereas primary clavae are smooth in females. Other species of Florarctus Delamare-Deboutteville & Renaud-Mornant, 1965 inhabiting the Pacific Ocean were investigated. Males are typically smaller than females, but males of Florarctus heimi Delamare-Deboutteville & Renaud-Mornant, 1965 and females of Florarctus cervinus Renaud-Mornant, 1987 have never been recorded. The Renaud-Mornant collection was re-examined, and type series were analysed. Florarctus heimi and F. cervinus were always found together in the coralline sand of Heron Island (Great Barrier Reef). The animals were kept alive and surveyed in the laboratory of the Queensland Museum. All studied individuals of the larger F. heimi (up to ca. 400 μm) were females, and all adults of the smaller F. cervinus (about 170 μm) were males. Males of F. cervinus were observed mating with females of F. heimi. Following those morphological and behavioural lines of evidence, we propose that F. cervinus is a junior synonym of F. heimi. Based on the discovery of dimorphism in F. bellahelenae sp. nov. and the strong sex-related morphological disparities in F. heimi, we suggest that extreme secondary dimorphism may be present in other florarctin arthrotardigrades.

Highlights

Meiofauna comprises microscopic animals (Schmidt-Rhaesa 2020), some of which were often referred to as “smaller neglected phyla” (e.g. Kershaw 1983; Blaxter et al 2004)
Mating, copulation and related sexual reproduction behaviours remain generally poorly investigated for most meiofaunal organisms
Cephalic sense organs without tertiary clavae, strong secondary dimorphism in the primary clavae, secondary clavae transformed to domeshaped papillae or H-shaped flat sacs

Summary

Introduction

Meiofauna comprises microscopic animals (Schmidt-Rhaesa 2020), some of which were often referred to as “smaller neglected phyla” (e.g. Kershaw 1983; Blaxter et al 2004). Marine Biodiversity (2021) 51: 52 monogonont Rotifera (Gilbert 1963; Rico-Martínez and Snell 1997; Velázquez-Rojas et al 2002; Rico-Martínez and Walsh 2013) and Tardigrada, restricted to limno-terrestrial species (Bingemer et al 2016; Sugiura et al 2019; Bartel and Hohberg 2020). Mating-associated behaviours in microscopic metazoans are a tabula rasa, despite the anatomy of reproductive system, and especially of parts presumed to be involved in copulation being an important character used in the systematics of some phyla, such as Gnathostomulida (Sterrer 1972) or Tardigrada (Degma and Guidetti 2018). Species identification in marine heterotardigrades relies on genital anatomy (Kristensen and Hallas 1980; D’Addabbo Gallo et al 1989; Kristensen and Higgins 1989; Pollock 1995; Bartels et al 2018). Given that the genital traits have been extensively utilised in systematics, unravelling the ethological background of courtship and reproduction may likewise provide an insight into the classification of animals (e.g. Thiel and Duffy 2007)

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Conclusion