Larval development of the brachiopod Coptothyris grayi (Davidson, 1852) (Terebratulida: Rhynchonelliformea) and the evolution of brachiopod life cycles

Abstract

Larval development of the brachiopod Coptothyris grayi (Davidson, 1852) (Terebratulida: Rhynchonelliformea) is investigated with scanning electron microscopy and confocal laser scanning microscopy. At the early larval stage, the mantle rudiment forms obliquely to the anteroposterior axis of the larva: its dorsal side is located closer to the larval apical lobe than its ventral side. Young three-lobed larvae exhibit three pairs of coelomic pouches: one pair in the apical lobe and two pairs in the mantle lobe. The number of coelomic pouches in the mantle lobe of the larvae corresponds to the number of bundles of setae. At the early larval stage, the dorsal pair of bundles of setae is located closer to the larval apical lobe than the lateral pair, whereas in full-grown larvae, both pairs of bundles of setae are at the same level. This means that the dorsal side of the mantle grows faster at the late larval stage. We analyze the data on the development of extant brachiopods and propose a hypothesis on the evolution of brachiopod life cycles. The ancestral brachiopod had a pelago-benthic life cycle with a ciliary planktotrophic larva (not to be confused with the shelled planktotrophic juvenile of recent lingulids) that folded during metamorphosis. The next evolutionary stage was the transition from planktotrophy to lecithotrophy, but the lecithotrophic larvae of recent brachiopods are not homologous. Recent craniiforms retain the true unfolded lecithotrophic larvae. The full-grown lecithotrophic larva of recent rhynchonelliforms corresponds to the folded juvenile that ascends into the water column. The pelagic life cycle phase of recent discinids comprises the lecithotrophic larva that corresponds to the folded stage and the shelled planktotrophic stage that is essentially the juvenile ascended into the water column. As for recent lingulids, their life cycle is characterized by the embryonization of all developmental stages except the shelled planktotrophic juvenile. Accordingly, the lingulid life cycle is the most modified among all recent brachiopods.