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 dispersal is limited to crawling, swimming, and “rafting”.  No studies specifically on early development in west-coast amphipods appear to have been done.

Egg-bearing females of Traskorchestia traskiana in the Strait of Georgia, Washington appear in February and young are released in April.  Some of these young-of-the-year individuals become reproductive in mid-summer and this extends the period of recruitment from April into October.  Most individuals die later in the year or early the following year.  Thus, 2 overlapping generations complete their life cycles in 7-11mo.  Sex ratios are skewed in the population (63% females: 37% males).  Brood sizes range from 20 for the largest females at 13mm body length to 12 for the smallest at 10mm.  Koch 1990 Crustaceana 59: 35. Photograph courtesy Mary Jo Adams and BEACH WATCHERS.

Semiterrestrial amphipod Traskorchestia
traskiana crawling on an algae-covered rock 3X