Sexual discrimination at work: Spinicaudatan ‘Clam Shrimp’ (Crustacea: Branchiopoda) as a model organism for the study of sexual system evolution

Abstract

Biological interactions are rarely preserved in the fossil record and where they do occur, are often difficult to discern. Therefore, the evolution of sexual systems over geologic time in animals has been difficult to investigate. The reproductively labile Spinicaudata ('clam shrimp') are a model clade for the study of sexual systems, containing dioecious (males and females), androdioecious (males and hermaphrodites) and self-fertilizing hermaphrodites. Herein we present a methodology in which mating systems can be inferred by the quantification of carapace shape differences attributable to sexual dimorphism in fossil specimens. We develop our methodology by comparing the carapaces of six species from two families of extant Spinicaudatans using eigenshape analyses. Sexual dimorphism was successfully quantified using morphometric techniques combined with discriminant analyses, correctly identifying males and females/hermaphrodites 92% of the time in extant taxa. Thirty-four specimens of the Jurassic clam shrimp Carapacetheria disgragaris were analyzed utilizing the methods developed with extant species. From these fossil data, we were able to detect two distinct carapace shapes and assign 100% of individuals to either shape. The mean carapace shapes of the fossil specimens fit well with the average outlines for males and females in the extant species, enabling us to calculate a sex ratio of 51:49 males:females and thereby assign the sexual system of dioecy. This study begins to successfully utilize the fossil record of the Spinicaudata to elucidate ancient sexual systems, which will likely have far reaching implications for our understanding of the evolutionary dynamics of sexual systems over geologic timescales.

Sex determination of the Early Cretaceous clam shrimp Eosestheria middendorfii (Yixian Formation, China)

“…In addition, the Cyzicidae exhibit only subtle differences in sexual morphotypes, as expressed in the modern cyzicid Cyzicus mexicanus (Astrop et al . ).…”

“…Astrop et al . () report sexual dimorphism for limnadiid specimens with a much more convex dorsal margin in females that is associated with a brood chamber. Sexual dimorphism in the Cyzicidae, exemplified by Cyzicus mexicanus , is more subtle with a slightly more elongate female that displays a flattened posteroventral margin.…”

“…Astrop et al . () pioneered the identification of sexually dimorphic carapace shapes in extant species, for which the sex of individuals was known a priori . While morphometric separation was excellent for some species, methods were not able to adequately separate sexual morphotypes in all of the extant species studied.…”

“…This diversity in breeding systems makes the Spinicaudata a model clade for the study of reproductive system evolution (Astrop et al . ). Unisexuality has arisen from an ancestral condition of obligate sexuality.…”

Triassic Spinicaudatan Fauna from the Cerro de Las Cabras Formation (Cuyo Basin), Mendoza Province (Argentina): Description of New Species and Revision of Previous Records

“…Measurement abbreviations follow the scheme of Tasch (1987). All suggested sexual dimorphism referred to in this paper is based on different outlines found in each population and on valve height/ length ratio of the specimens following the prepositions of Astrop et al (2012) and references therein mentioned to assign them to a male or female shape.…”