The incubatory chamber of marsupial carditids (Bivalvia: Carditidae: Thecaliinae) as an exaptation

Abstract

Marsupial carditids of the subfamily Thecaliinae are characterized by the presence of an “incubatory chamber” in female shells, where the eggs hatch and develop during their first stages. According to recent phylogenetic studies, Thecaliinae are closely related to Carditinae, a group that has a byssal gape. This structure occurs in the same area as the incubatory chamber, and both structures could be evolutionarily related. Using the newest phylogenetic context for the subfamilies, we test whether the incubatory chamber of Thecaliinae is related to characters present in Carditinae. We also provided a more precise definition of the implied structures. Two distinct morphologies for the incubatory chamber are described: one with an exteriorly opened pouch (present in the genera Powellina and Milneria) and the other with a completely internal funicular infold (present in Thecalia). The byssal gape is present in the Cardiobyssata clade (Carditamerinae + (Carditinae + Thecaliinae)), and we discuss whether the incubatory chamber could be the result of an exaptation event, and the possible evolutionary pathways implied. According to the present evidence, we propose a co-optation of the byssal gape into a new function (brooding of larvae) at some point during the transition from the Carditinae to the Thecaliinae lineages, thus determining an exaptation. Adaptative processes probably modified this structure into the incubatory chamber (an external pouch first, and a funicular infold later). We discuss alternative scenarios and implications on phylogenetic studies and the importance of considering non-adaptative evolutionary scenarios in the study of evolutionary narratives.

Phylogenetic relationships of Malarossia Berezovsky (Eocene, Ukraine) and trends in the size evolution of the carditids (Bivalvia, Carditidae)

“…Carditamera arata (Conrad, 1838) or Beguina semiorbiculata (Linnaeus, 1758)). Nevertheless, the smallest Cardiobyssata, the thecalins, are living today (Pérez & Soto 2021).…”

“…On the one hand, the Carditamerinae includes the small‐sized Glans and Centrocardita (10–24 mm) (Beu 2006), and large‐sized Carditamera and Byssomera (26–62 mm) (González & Giribet 2013). On the other hand, the Carditinae comprise the large‐sized Beguina and Cardita (between 20 and 70 mm) and small‐sized Birkelundita (up to 9 mm) (Heinberg 1993) and thecalins ( Thecalia , Powellina , Milneria with sizes between 4 and 13 mm) (Pérez & Soto 2021). Thecalins live attached to mollusc shell surfaces, and their small size could be an adaptation to this mode of life.…”

The earliest example of sexual dimorphism in bivalves—evidence from the astartid Nicaniella (Lower Jurassic, southern Germany)

“…In the gonochoristic unionids Epioblasma capsaeformis (Lea, 1834) and Epioblasma brevidens (Lea, 1831), the postero-ventral part of the females differs in size and shape due to modifications of the marsupial gills (Mackie, 1984). In a few living carditid species (e.g., Thecalia concamera [Gmelin, 1791], Milneria kelseyi Dall, 1916), the females form a pouch in the shell that serves as marsupium for the larvae (Stanley, 1988;Pérez and Soto, 2021). Differences in shell convexity and ornamentation pattern in some Eocene-Oligocene alticostate Venericardia species (family Carditidae) and in two Upper Cretaceous morphotypes of Sphenoceramus were interpreted as sexual dimorphism by Heaslip (1969) (see Stanley in Heaslip, 1969, for an alternative view).…”