Sexes are separate in amphipods.  After fertilisation, which takes place as the eggs pass through sperm packets, or spermatophores, attached to the gonopores, the eggs are incubated in brood chambers on the ventral body surface.  The juveniles are released from these when development is complete.  Brood sizes tend to be small and dispersion is limited to crawling, swimming, and “rafting”. 

The brood chamber is formed by several flat, broad, overlapping appendages known as oöstegites that develop on either side of the ventral thoracic region.  The chamber provides protection from drying and mechanical injury during the weeks of development from egg to juvenile.  The unique “sideplate” morphology of the thorax in amphipods creates a moisture-containing groove that maintains 100% humidity for egg development in females and for gas-exchange in both sexes.  This feature is considered to be an important feature enabling land colonisation in amphipods.  Friend & Richardson 1986 Ann Rev Entomol 31: 25.

Preserved and stained amphipod Gammarus sp. showing
developmental stages in the brood-chamber area, as
well as a reasonably full gut. The chamber contents
appear to have been disrupted during preparation 12X

Ovigerous females of terrestrial talitrid species keep their brood chambers moist, partly by capillarity during tail-dipping into seawater, rain puddles, and dew-drops, and partly by the shunting of urine along cuticular channels ventrally into the chamber.  By selectively shunting their urine in this way, the brooding female is able to regulate the osmotic concentration of the fluid in its brood-chamber.  It is not known what sensory mechanisms are present in the marsupium to enable this regulation. Talitrids are capable of maintaining their hemolymph osmotic concentration at about 500-600 mOsm ^. kg ^-1, where 100% seawater is equal to 1000 mOsm ^. kg ^-1, when exposed to dilute salinities (see graph). The urine is isosmotic with their hemolymph, indicating that ion regulation takes place elsewhere, probably via the gills. Morritt & Richardson 1998 Functional Ecol 12: 351.

NOTE  this study is done on a fully terrestrial talitrid species Mysticotalitrus cryptus in Tasmania. It is included in the ODYSSEY for the purpose only of stimulating possible similar research on our local west-coast talitrids

NOTE  this experiment involves keeping adult amphipods in Petri dishes on filter paper moistened with differen dilutions of seawaterfor 5d, then measuring their hemolymph concentrations