Genetic barcodes of arctic medusae and meiobenthic cnidarians have uncovered a fortuitous connection between the medusa Plotocnide borealis Wagner, 1885 and the minute, mud-dwelling polyp Boreohydra simplex Westblad, 1937. Little to no sequence differences exist among independently collected samples identified as Boreohydra simplex and Plotocnide borealis, showing that the two different forms represent a single species that is henceforth known by the older name Plotocnide borealis Wagner, 1885. The polyp form has been observed to produce bulges previously hypothesized to be gonophores, and the results here are consistent with that view. Interestingly, the polyp has also been reported to produce egg cells in the epiderm, a surprising phenomenon that we document here for only the second time. Thus, P. borealis produces eggs in two different life stages, polyp and medusa. This is the first documented case of a metagenetic medusozoan species being able to produce gametes in both the medusa and polyp stage. It remains unclear what environmental/ecological conditions modulate the production of eggs and/or medusa buds in the polyp stage. Similarly, sperm production, fertilization and development are unknown, warranting further studies.

Development of Leptoecia vivipara, a brooding deep-sea onuphid polychaete with a circum-Antarctic distribution, was studied using light microscopy, scanning and transmission electron microscopy, and histology. All specimens examined were brooding viviparous females or juveniles; no male gametes were detected. Anterior segments of juvenile and adult worms bore paired compact ovaries with clusters of vitellogenic oocytes. In adults, the mid-body region formed a chamber containing up to 12 offspring at different stages of development, from oocyte to 13 chaetigers. Mature oocytes freely floated in the coelomic fluid, while embryos and juveniles were enclosed in peritoneal envelopes. Chaetal replacement in juveniles and the morphology of the provisional maxillae are described. Leptoecia vivipara is argued to be a progenetic species with juvenile-like external morphology and accelerated sexual maturation. These traits may have arisen as adaptations to epibenthic life in a high-latitude deep-sea environment affected by seasonal pulses of organic matter.Additional key words: Antarctica, brooding, chaetal replacement, Polychaeta, progenesis Invertebrate Biology 133(3): 242-260.

The simplified text-book view holds that hydroids' soft body is composed of a branched double-layered tube, whose wall consists of two epithelial layers (the inner gastrodermis and the outer epidermis) separated by the mesoglea. Some hydroids are characterized by large, complex colonies and likely an even more complicated inner organization. By using three species from the thecate hydroids of the family Sertulariidae we investigated the soft body structure of such hydroids. The anatomical study revealed some new features of colonial hydroids. The double layered coenosarc fills the perisarc (outer skeleton) tube only at the endings of the branched colony. More proximally, the coenosarc tube becomes narrower and a thin epidermal lining covers the inner surface of the perisarc tube. In some species the soft tissues of the shoots form a network of anastomosing canals. The canals are formed by the gastrodermal epithelium and they are embedded in epidermal tissue. In the upper part of the shoot, these canals are located at the periphery, along the inner surface of the perisarc. In more proximal regions of the stem, the whole lumen of the perisarc tube can be occupied by gastrodermal canals; the canals are enclosed in a parenchyma-like epidermal tissue. The organization of the soft tissue in these thecate hydroids is a striking example of structural complexity that does not contravene the limits of the ground plan of the phylum.

A new location for Protohydra leuckarti is reported near Plymouth at Millbrook Lake (Tamar Estuary). To place this finding in context, notes follow on a familiar habitat of this species in the White Sea, and on the general ecology and distribution of Protohydra and its enigmatic phylogeny.