Why the biggest males stop growing

18 November 2026

Part 10 of 18 · A history of the giant freshwater prawn

In cages in Israel at the start of the 1990s, researchers watched nine young males turn into blue-claw bosses while older bosses were still there. In eight of the nine, the new boss ended up bigger than every boss already in the cage, on average by 45 per cent. They called it leapfrog growth.[1]

Then, like the bosses before them, they stopped. In another study, the first orange-claw male to become a blue claw did so at 35 grams, and his growth rate fell by about 95 per cent.[2] Becoming the boss is the end of growing. For a farmer, it is a strange arrangement: the biggest prawn in the pond is the one that has stopped growing, and the ones still growing are the ones the boss keeps small. Once the blue claws appear, they slow the growth of the small males around them.[3]

So the question that matters is when the switch comes, and what decides it. The first clue came from ponds. In Israeli ponds stocked at three densities, blue-claw males averaged 29 grams in the most crowded pond, 61 in the middle one and 112 in the sparsest, and the orange-claw males 26, 34 and 72 grams.[4] The switch came in all three ponds. What the crowding changed was the size at which it came: a crowded pond produced small bosses, and smaller males all round.

The second clue came from company. In one experiment, males lived either in large cages, ten males and ten females together, or as couples: one male and one female to a cell, divided from the next cell only by mesh, so each male could see and smell other males but never reach them. By week 12, 62 per cent of the coupled males had become blue claws, against 22 per cent of those in the large cages.[1] Seeing and smelling other males through mesh did not delay the switch. Living among them, with bigger blue claws around, did.

Whatever the social cue is, it has to work through something in the prawn’s body. The search for that something led to a small gland, the androgenic gland, which makes a male a male. Removed from small males, it let them become orange claws but never blue claws. Removed from orange-claw males, it did not stop the switch, though it slowed their growth.[5] Sagi suggested that by the orange-claw stage, a male has passed a point of no return.[6] It is the same gland whose study, twenty years on, produced the all-male seed and the 2013 prize, after Amir Sagi’s laboratory identified its hormone, the insulin-like androgenic hormone.[7]

In 2017, a team in India turned that hormone’s gene down in orange-claw males living in groups. None of the treated males became blue claws, against six of the untreated ones: in a group, at least, there was no point of no return, and the switch could be delayed from inside. Kept apart, each in its own perforated basket inside a shared tank, treated and untreated males switched equally. The knockdown, strong on day four, was gone soon after, though the treatment was still in their bodies; the authors suspected that isolation itself had restored the hormone. And in every group tank of untreated males, the first to switch died or lost his claws in fights. The second survived, and each tank ended with one blue claw.[6]

Put together, the archive explains a great deal: a male stops growing once he becomes a blue claw; crowding decides how big he is when that happens; and the androgenic gland and its hormone drive the change. What it does not explain is the trigger: what tells one male, on one day, to flip while a blue-claw boss is still beside him. Contact, sight, or something in the water? The few studies that tried to pull those apart did not settle it.[1] For anyone who wants to grow big males on purpose, rather than harvest whatever a pond produces, the trigger is the missing piece.

It became the second question the field parked. The first, told in Part 6, was how one prawn slows another’s growth, and whether something in the water carries it. The second is what decides when a male stops growing.

And there was one sense none of those studies had tested. Until 2021, as far as I can find, nobody had published what this prawn sounds like.[8]

Sources

1. Karplus, I., Barki, A., Israel, Y., Cohen, S. (1991), Social control of growth in Macrobrachium rosenbergii. II. The “leapfrog” growth pattern. Aquaculture 96(3–4): 353–365. Nine switches with blue claws present, eight new blue claws larger than all existing ones (by 45% on average). Cells of 0.5 m², one male and one female each, divided by 5 mm mesh, against large cages of ten males and ten females: 62% against 22% blue claws by week 12. doi.org
2. 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. First switch at 35 g; growth rate 1.22 in orange claws, 0.07 in blue claws. archive.org
3. New, M. B. (2002), Farming freshwater prawns, FAO Fisheries Technical Paper 428, Annex 8, Size management. Blue-claw males suppress the growth of small males. fao.org
4. Cohen, D., Ra’anan, Z., Brody, T. (1981), Population profile development and morphotypic differentiation in the giant freshwater prawn Macrobrachium rosenbergii (de Man). Blue claws averaging 29, 61 and 112 g, orange claws 26, 34 and 72 g, at three densities. doi.org
5. Sagi, A., Cohen, D., Milner, Y. (1990), Effect of androgenic gland ablation on morphotypic differentiation and sexual characteristics of male freshwater prawns, Macrobrachium rosenbergii. General and Comparative Endocrinology 77(1): 15–22. doi.org Removed from small males: orange claw permitted, blue claw blocked. Removed from orange-claw males: the switch still happened, but growth slowed and male characters regressed.
6. Priyadarshi, H., Das, R., Pavan-Kumar, A., Gireesh-Babu, P., et al. (2017), Silencing and augmentation of IAG hormone transcripts in adult Macrobrachium rosenbergii males affects morphotype transformation. Journal of Experimental Biology 220: 4101–4108. ICAR-Central Institute of Fisheries Education, Mumbai. doi.org In groups, no treated male switched (six untreated did). Isolated males each in a 450 cm² perforated cage, several per shared tank, switched equally (3 and 4, days 18–25); the knockdown (4.2-fold on day 4) recovered though the construct persisted; the authors suggest a social control mechanism, and discuss Sagi’s proposed commitment point. In each control tank the first new blue claw died or lost its claws in a territorial fight; the second survived (p. 4105).
7. Global Aquaculture Alliance (2013), Ben Gurion University professor wins inaugural aquaculture innovation award. Sagi’s research culminated in the discovery of the insulin-like androgenic hormone in decapod crustaceans. globalseafood.org
8. Hamilton, S., Silva, J. F., Filho, F. C., Peixoto, S. (2021), Sound emission of Macrobrachium rosenbergii during feeding activity. Aquaculture Research. doi.org

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