New data on the development of the brachiopod Hemithiris psittacea (Rhynchonelliformea: Rhynchonellida)

Abstract

: Brachiopods are marine invertebrates with a pelago-benthic life cycle. The life cycle and structure of planktonic stages vary among brachiopod taxa. Many rhynchonelliform brachiopods, including those in the order Rhynchonellida, brood three-lobed larvae (i.e., the cephalulae) in the mantle cavity. However, no cephalulae were found in the mantle cavity of the rhynchonellid Hemithiris psittacea (Gmelin, 1791) from the White Sea. In this research, artificial insemination and induced spawning were used to study the ontogeny of H. psittacea from the White Sea Biological Station of Moscow State University (Kandalak-shskii Bay of the White Sea). After fertilization, a thick egg envelope forms around the zygote and adheres to the substrate. Cleavage, gastrulation, and formation of the three-lobed cephalula occur within the egg envelope. A worm-like competent cephalula hatches from the egg envelope and swims along the substrate rather than in the water column. The cephalula of H. psittacea differs in structure from the terebratulid larva, which actively swims in the water column. In contrast to terebratulid larva, the cephalula of H. psittacea lacks the eyespots and has shorter and less abundant setae. Development of H. psittacea from the White Sea proceeds in the external environment rather in the lophophore of the female. These results are inconsistent with the published data on H. psittacea from the San Juan Channel (Washington, USA), whose larvae develop outside the egg envelope in the lophophore of the female. A possible explanation is that Hemithiris from the San Juan Channel and from the White Sea represent separate species. The current report for H. psittacea from the White Sea is the first to describe embryonization of larval development for a rhynchonelliform.