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