Even the pond has office politics

11 November 2026

Part 5 of 17 · A history of the giant freshwater prawn

Taiwanese diàoxiāchǎng owners were not the only ones interested in getting bigger prawns, faster. But raised together, from the same hatch, in the same water, male prawns do not grow up equal. In one Israeli study, about half of the adult males stayed small, about four in ten grew fast, and about one in ten became a large male with long blue claws, after which he barely grew at all.[1]

The researchers gave the two extremes farmyard names: the big blue-clawed males were bulls, the stunted small ones runts, and the ones in between went by their orange claws.[2] In office terms, it is a few bosses, an ambitious middle, and a junior rank that stays small.

Much of what science knows about this hierarchy came out of Israel, which is yet another remarkable turn in the prawn’s journey. Israel is a dry country, far from any river the prawn calls home, and many Israelis would not eat it at all: prawns are not kosher.[3] So why farm them at all? I assume for export, but I don’t know for certain. What is on record is that from 1979, Israel steered most of its aquaculture research money towards products it could export, and the freshwater prawn was one of three new crops on the list, alongside eels and ornamental fish.[4] Israeli farmers had raised carp and tilapia in ponds since the 1930s and were experts at it,[5] and in trials, prawns stocked into those same fish ponds did not cost the fish any growth.[6]

The challenge was the summer. Or rather, the winter. In Israel’s first pilot trials, around 1980, prawns could be grown through only six or seven warm months, and nothing under 30 grams was worth selling.[7] In a season that short, a pond where most males stall is a failed harvest. And with that, Israel had more reason than anyone so far to find out why they stalled.

The first answer was rank, and rank goes with the claws, not the body. A blue-claw male with bigger claws outranks an orange-claw male even when the orange-claw male is the much bigger prawn.[8] Size matters, but the blue claw matters more. When a blue-claw male’s claws were removed, or their tips bound with rubber bands, the small males’ growth suppression disappeared: they grew as fast as small males raised alone.[9]

The second answer was how the bosses keep the juniors small. In experiments in the early 1990s, small males were raised for 112 days either next to a blue-claw male or alone.[9] Next to the boss, a small male still moulted as often as before, but each moult added less weight, and it needed more food for every gram it grew. But take the blue-claw male away, and that reversed.[9][10] Take the bosses out of a pond, and the promotions start: some small males grow fast, turn into orange-claw males, and in time grow blue claws of their own.[8]

What it takes is closeness. To find out how the suppression travels, researchers kept pairs apart so that the small male shared the blue-claw male’s water, or could see him, but could not reach him. Neither slowed the small male’s growth: kept apart, in the same water and in plain sight of each other, the suppression was not shown.[8] Sight and smell, it seemed, were not enough. What mattered was being close enough for the two to meet. The boss had to be in the room.

Ilan Karplus, Spencer Malecha and Amir Sagi later gathered the Israeli and Hawaiian work into one review. Their conclusion: in this prawn, growth is regulated mainly by social interactions. Remove the largest prawns, and the smaller ones catch up fast.[11]

Not every study from those years agreed on the power of the blue claw alone. A handful suggested that something in the water itself might carry part of the effect, but what it was remained elusive.[12]

Sources

1. Ra’anan, Z., Sagi, A., Wax, Y., Karplus, I., Hulata, G., Kuris, A. (1991), Growth, size rank, and maturation of the freshwater prawn, Macrobrachium rosenbergii. Biological Bulletin 181: 379–386. Proportions of male forms; blue-claw males grow slowly if at all. archive.org
2. Naga, The ICLARM Quarterly 25(2) (2002), Heterogeneous individual growth of Macrobrachium rosenbergii male morphotypes. “Bull” and “runt” (after Karplus et al., 1992). worldfishcenter.org
3. Leviticus 11:9–12. Of what lives in the water, only creatures with fins and scales may be eaten.
4. Mires, D., Chairman, National Board of Aquaculture, Israel, country report to FAO (mid-1980s). Research and development funds directed to exportable products since 1979; freshwater prawns, eels and ornamental fish as the three new cultures. fao.org
5. Hulata, G., An overview on desert aquaculture in Israel. FAO. Carp introduced in the early 1930s; the first ponds at Kibbutz Tel Amal (Nir David). fao.org
6. Polyculture of the freshwater prawn Macrobrachium rosenbergii in intensively manured ponds, and the effect of stocking rate of prawns and fish on their production characteristics, Aquaculture. Fish growth not affected by prawn stocking rates. sciencedirect.com
7. Yield characteristics of the prawn Macrobrachium rosenbergii in temperate zone aquaculture (1980), Aquaculture. The first pilot growth trial in Israel; six or seven summer months; 30 g minimum commercial size. sciencedirect.com
8. New, M. B. (2002), Farming freshwater prawns, FAO Fisheries Technical Paper 428, Annex 8, Size management (derived from Karplus, Malecha and Sagi, 2000). Claws and rank; removing blue-claw males; small-male mating; closeness, not sight or smell. fao.org
9. Karplus, I., Hulata, G., Ovadia, D., Jaffe, R. (1992), Social control of growth in Macrobrachium rosenbergii. III. The role of claws in bull–runt interactions. Aquaculture 105: 281–296. The 112-day experiment; claws removed or bound with rubber bands; suppression removed, growth equal to isolated small males (p. 295); smaller increments per moult at the same moulting rate. sciencedirect.com
10. Karplus, I., Hulata, G., Zafrir, S. (1992), Social control of growth in Macrobrachium rosenbergii. IV. The mechanism of growth suppression in runts. Aquaculture 106: 275–283. Food conversion; reversible when the blue-claw male is removed. scienceon.kisti.re.kr
11. Karplus, I., Malecha, S. R., Sagi, A. (2000), The biology and management of size variation. In New and Valenti (eds), Freshwater Prawn Culture. Blackwell. Growth regulated mainly by social interactions; compensatory growth when the largest prawns are removed. cris.bgu.ac.il
12. Barki, A., Jones, C., Karplus, I. (2010), Chemical communication and aquaculture of decapod crustaceans. Springer. link.springer.com

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