The question nobody finished

12 November 2026

Part 6 of 17 · A history of the giant freshwater prawn

In 1981, a researcher named N. R. Howe put fifty young prawns into fifty separate chambers that shared one supply of water, and checked them every day, for up to fifty days, to see which of them had moulted.[1]

The moults were not random. They came in waves. Within each set of chambers, they bunched along the paths the water took from one chamber to the next. And the longer the water went unchanged, the more prawns moulted; change the water, and moulting dropped.[1] Howe measured moulting, not growth, but he put a name to what he thought he was seeing: chemical communication. Something released into the water, perhaps by the moulting prawns themselves, seemed to reach neighbours they never touched.[1]

That sat awkwardly beside the other finding of those years, that a boss slows a junior’s growth only when the two can meet. And the puzzle went deeper than the bosses. At the Hebrew University of Jerusalem, Ziva Ra’anan and Dan Cohen raised young prawns together and alone, in identical nine-litre aquaria. Together, they split into two classes that never appeared among the prawns raised alone: exceptionally fast growers, which the researchers called jumpers, and laggards, suppressed in the jumpers’ presence.[2] Suppression by competition was already known. What puzzled them was the other half: growth that speeds up in a group and not alone was, they wrote, “still a puzzling biological phenomenon”.[2] Prawns too young for claws or rank were already pushing each other’s growth up and down. Through the water was one possible answer.

Two studies of the 1980s pointed that way. In one, by Moore, Malecha and colleagues, isolated young prawns given water from a tank of thirty interacting adults developed, over nine months, the same lopsided spread of sizes as prawns living together; given clean water, their sizes spread evenly.[3] In the other, young prawns further down a line of compartments sharing one current grew less, and the authors put it down to chemicals excreted by the prawns upstream, a substance they never isolated or measured.[4] A 2010 review reported the same pattern for prawns: water that had passed over prawns interacting freely made separately housed juveniles vary more in size. Across farmed crustaceans generally, though, it found that no consistent conclusion had emerged that led to practical solutions.[5]

Nobody disproved it. In 2005, reviewing the field, Ilan Karplus wrote that the Moore study “opened a new line of research that should be pursued”.[3] As far as I can tell, it stalled anyway, for duller reasons. Farms found ways to manage uneven growth without explaining it. The researchers who had asked the question moved on to others. Research money went to breeding, disease and feed. And nobody yet had the tools to find one unknown substance in a tank of water. A question that could be neither answered nor applied attracted neither grants nor industry.

It is coming back. In June 2026, a Mexican team reported what complete isolation does to young male prawns. Kept alone in containers of 406.9 millilitres, they reached an average of 1.13 grams in 75 days, against 3.03 grams for prawns raised in communal tanks, and fewer of them survived. Ten isolated survivors were later moved into a shared tank; after 30 days their average weight had roughly doubled, though only six were still alive. The authors are careful to say that their isolated prawns also lived in far smaller containers, so the results show isolation and set-up combined, not isolation alone.[6] And the tools now exist. A 2017 study of the female prawn’s antennal gland found one of its proteins in the water around the prawns.[7] It was about mating, not growth, but it showed that molecules prawns release into the water can now be found and named.

While the question sat parked, the prawn found a new market. India began farming it for export, freezing most of the harvest for buyers in Europe and beyond, under a name I grew up with on Italian menus: scampi.[8]

Sources

1. Howe, N. R. (1981), Partial molting synchrony in the giant Malaysian prawn, Macrobrachium rosenbergii: a chemical communication hypothesis. Journal of Chemical Ecology 7: 487–500. link.springer.com
2. Ra’anan, Z., Cohen, D. (1984), The effect of group interactions on the development of size distribution in Macrobrachium rosenbergii (De Man) juvenile populations. Biological Bulletin 166: 22–31. Hebrew University of Jerusalem. archive.org
3. Moore, L. B., Malecha, S. R., MacMichael, E., Masuno, S. (1985), Evidence for the presence of a water borne factor affecting heterogeneous individual growth in the freshwater prawn, Macrobrachium rosenbergii. Conference paper, 16th Annual Meeting of the World Mariculture Society, Orlando, 13–17 January 1985. Described, and quoted here, from Karplus, I. (2005), Social control of growth in Macrobrachium rosenbergii (De Man): a review and prospects for future research. Aquaculture Research 36: 238–254 (p. 245). wiley.com
4. Juarez, L. M., Holtschmit, K.-H., Salmerón, J. J., Smith, M. K. (1987), The effects of chemical and visual communication, space availability, and substratum color on growth of the juvenile freshwater prawn Macrobrachium rosenbergii (De Man). Journal of Experimental Marine Biology and Ecology 110(3): 285–295.
5. Barki, A., Jones, C., Karplus, I. (2010), Chemical communication and aquaculture of decapod crustaceans: needs, problems, and possible solutions. In Breithaupt and Thiel (eds), Chemical Communication in Crustaceans. Springer, pp. 485–506. link.springer.com
6. Meya-Jiménez, V., Pérez-Rostro, C. I., Hernández-Vergara, M. P., Gallardo-Collí, A., Rosas-Vázquez, C. (2026), Potential effects of social isolation on growth and morphotype development in juvenile male Macrobrachium rosenbergii. Aquaculture International 34: 180. link.springer.com
7. Bose, U., et al. (2017), Biomolecular changes that occur in the antennal gland of the giant freshwater prawn. PLoS ONE 12(6): e0177064. University of the Sunshine Coast, The University of Queensland, Mahidol University and Burapha University. doi.org
8. CIFT, Growth and instability in Indian frozen scampi export (1995–2009). drs.cift.res.in

Part 6 of 17 · A history of the giant freshwater prawn · Previous: Part 5, Even the pond has office politics · Next: Part 7, Scampi for Europe · SHRMP.bio’s strategy behind it: shrmp.bio/strategy.html