Metamysidopsis macaensis is reported for the first time in the south of Brazil, and a comparative analysis of morphological characteristics between the original description of the species and a female described here is made. The occurrence of the species in the south of Brazil and exclusively in the Brazilian shelf suggests this species has a wide distribution along the Brazilian shelf. The morphological differences between the holotype and the female described here are significant and suggest that the original description was probably made based on an immature female.

We revised the zoogeographical distribution of mysids in the south-west Atlantic, from northern Brazil to Argentina (0° to 40°S), providing body length, and temperature and salinity range of occurrence for each species. All information concerning mysids from the south-west Atlantic was obtained from published/unpublished studies. Of currently 1128 mysids species described worldwide, only 31 were recorded in the south-west Atlantic which are distributed in 14 genera, 6 subfamilies and one family. This extremely low number of recorded species highlights the need for further studies focusing on mysids in the south-west Atlantic.

An inventory of Antarctic and Subantarctic mysid fauna is presented, together with a summary of the present state of knowledge of species and their taxonomic diversity, geographic and bathymetric distribution patterns. Fifty nine species of Mysidacea (Crustacea, Peracarida) are now known. Of these, 37 were reported for the Antarctic region and 31 for the Magellan region; six species occur further north in the Southern Ocean, but south of 40°S. 51% of the Antarctic Mysidacea are endemic, and the figure for the Magellan region is 48%. Most of the species live hyperbenthically, but some also occur bathy- or mesopelagically. Mysidetes has the most species in the Southern Ocean, and Eucopia australis is the species with the widest bathymetric distribution (600–6000 m depth). It is concluded that an emergence of species onto the Antarctic shelf in the Neogene was quite unlikely, because none of the mysid species is a true deepsea species, and most species occur on the shelf or at the shelf break. It is more probable that present day species colonized the Southern Ocean via shallower waters. The examples of the distribution of different genera suggest that the Mysidacea of the Southern Ocean probably had various geographical origins.

An analysis of swimming in living crustaceans is presented in order to elucidate the range of ways this function has been achieved, and to reveal the principles which constrain it. The study focuses on Gnathophausia ingens, a primitive, bathypelagic malacostracan that swims with thoracic exopods and pleopods. These structures consist of a muscular peduncle and one or two flagella that are fringed with setulate setae. The basic motion is rowing with the limb and setal fan extended on the power stroke and flexed on recovery.

A survey of other crustaceans shows that rowing with this type of swimming structure dominates throughout, although paddles often replace the flagella. Particularly pervasive is the large relative area of setae, whose effectiveness must stem from the ability to extend and flex passively and from the high drag generated on the power stroke by the setules at low Reynolds numbers.

A review of reconstructions of Palaeozoic trilobites and marrellomorphs reveals the likelihood that if swimming was the function of the exites, they operated inefficiently or were employed in other methods as well. Sculling and drag reduction on the recovery stroke through feathering rather than flexion are possible alternatives. The more common occurrence of paddle-like limb shafts and blade-like marginal structures in other Palaeozoic arthropods is also noted.