To my knowledge, this is the first visual documentation of mating behavior in this species. Footage taken late June 2008.
As with other species of Macrobrachium and Caridean shrimp in general, successful copulation can take place only between a post-nuptial-molt female and hard-shelled (intermolt) male, which places a spermatophore below the female thorax. Following copulation, eggs are fertilized by the spermatophore by being “extruded through the gonopores and guided by the ovipositing setae (stiff hairs), which are at the base of the walking legs, into the brood chamber” — that is, amongst the pleopods or swimmerets (FAO, 2002). Here they will remain, aerated by fluttering movements of the swimmerets, until the larvae hatch.
The female crenulatum had been temporarily confined to a plastic container within the aquarium (10 US gallons, around 40 liters) where a conspecific male was being housed. I had offered pellet food (Wardley) to all of my shrimp the evening of their arrival from Puerto Rico; this female was the only one to leave its food untouched, which I attributed to the stress of transport. At around 1 PM the following afternoon, however, I saw a cast shell lying beside the newly molted female. Checking on it 12 hours later, I observed that the exuvium remained uneaten, and that the male housed with it was probing the isolation container with its first chelipeds. I opened its lid, and my suspicions were confirmed.
Immediately after copulation, the male began to consume the female exuvium. Granted that I had little epxerience with M. crenulatum, the cast shell seemed to have remained untouched for an uncharacteristically long time, vaguely reminding me of the nuptial gift phenomenon in some predaceous insects. Opportunism, in retrospect, seems the likelier possibility.
Thousands upon thousands of miniscule eggs descended into the female’s pleopods around 30 minutes later.
I was surprised that the female cuticle remained soft enough for spermatophore placement so long after molting. Perhaps anti-hardening agents are released into the post-ecdysis exoskeleton by reproductively receptive females, since the window of opportunity for mating would seem to be otherwise quite narrow if population density is low or males remain concentrated beyond the female’s pool.
M. crenulatum, like all of its congenerics in Puerto Rico and many others elsewhere in the world, is characterized by a diadromous lifecycle termed amphidromy (Fièvet, 2000; sensu McDowall, 1998). That is, though adults inhabit and reproduce in freshwater, their larvae are swept downstream into estuarine or marine conditions, where they live as plankton until becoming benthic post-larvae. Exact residence time of these bottom-dwelling juveniles in saline water is in most cases unknown, but they ultimately make their way back upstream, sometimes surmounting waterfalls and crawling overland, to complete the cycle.
With this female crenulatum, eggs were retained in the pleopods for over a month before the floating zoeal larvae were released. I netted around a dozen with a fine-meshed aquarium net of the sort used to handle brine shrimp and placed them in a separate aquarium (5 US gallons, around 19 liters) containing aerated water at 17.5 ppt salinity. All disappeared over the course of a few days (some may have been splashed out of the water by the airstone). The female in question has since become ovigerous again, but larvae were released much sooner than I’d expected (within a few weeks) and became casualties of the filter intake.
I haven’t given up on rearing them.