An extra embryonic phase in the true freshwater crab Sinopotamon yangtsekiense Bott, 1967 (Decapoda, Potamidae)

The true freshwater crabs (Crustacea, Decapoda, Brachyura) are highly adapted to life in freshwater and complete their life cycle entirely independently of sea water. All true freshwater crabs exhibit direct development and lack the free-living larval forms (zoea and megalopa) typical of most other brachyurans. After a prolonged embryonic period (during which they pass through the typical brachyuran larval forms embryologically) the eggs of true freshwater crabs hatch to produce juvenile (hatchling) crabs. We provide here the first report and description of the continuous record of embryonic development from egg-laying up to hatching in the Chinese true freshwater crab Sinopotamon yangtsekiense Bott, 1967 (Potamoidea, Potamidae). Direct development (complete secondary embryonization) in S. yangtsekiense was observed to take 77 days and to include an additional embryonic phase (termed here the egg-juvenile-crab) that occurs in the embryo between the imprisoned megalopa and the newly-emerged juvenile (hatchling) crab. This is significant because the only other freshwater crab whose embryonic development has been studied in detail is Potamon fluviatilis (Potamidae) which takes 45-47 days and involves only nine embryonic stages.