Laboratory study of Fritillaria lifecycle reveals key morphogenetic events leading to genus-specific anatomy.

A fascinating variety of adult body plans can be found in the Tunicates, the closest existing relatives of vertebrates. A distinctive feature of the larvacean class of pelagic tunicates is the presence of a highly specialized surface epithelium that produces a cellulose test, the "larvacean house". While substantial differences exist between the anatomy of larvacean families, most of the ontogeny is derived from the observations of a single genus, Oikopleura. We present the first study of Fritillaria development based on the observation of individuals reproduced in the laboratory. Like the other small epipelagic species Oikopleura dioica, the larvae of Fritillaria borealis grow rapidly in the laboratory, and they acquire the adult form within a day. We could show that major morphological differences exhibited by Fritillaria and Oikopleura adults originate from a key developmental stage during larval organogenesis. Here, the surface epithelium progressively retracts from the posterior digestive organs of Fritillaria larvae, and it establishes house-producing territories around the pharynx. Our results show that the divergence between larvacean genera was associated with a profound rearrangement of the mechanisms controlling the differentiation of the larval ectoderm.

Increased fitness of a key appendicularian zooplankton species under warmer, acidified seawater conditions.

Ocean warming and acidification (OA) may alter the fitness of species in marine pelagic ecosystems through community effects or direct physiological impacts. We used the zooplanktonic appendicularian, Oikopleura dioica, to assess temperature and pH effects at mesocosm and microcosm scales. In mesocosms, both OA and warming positively impacted O. dioica abundance over successive generations. In microcosms, the positive impact of OA, was observed to result from increased fecundity. In contrast, increased pH, observed for example during phytoplankton blooms, reduced fecundity. Oocyte fertility and juvenile development were equivalent under all pH conditions, indicating that the positive effect of lower pH on O. dioica abundance was principally due to increased egg number. This effect was influenced by food quantity and quality, supporting possible improved digestion and assimilation at lowered pH. Higher temperature resulted in more rapid growth, faster maturation and earlier reproduction. Thus, increased temperature and reduced pH had significant positive impacts on O. dioica fitness through increased fecundity and shortened generation time, suggesting that predicted future ocean conditions may favour this zooplankton species.