The prawn's night shift: turn off the lights and the party starts
20 November 2026
Part 12 of 18 · A history of the giant freshwater prawn
The number one frustration of our students last year: “You turn the lights off, and the party starts. If only we had night vision on the tank.” And it was true. Between the moment the stream went dark and the dawn that brought back some greyish outlines, mysteries happened: invitations to study further that we never got round to. One prawn even disappeared altogether. We were not set up to document their night behaviour properly. But others were.
In a 2018 laboratory study, giant freshwater prawns did most of their fighting, crawling and swimming in the dark, and the activity peaked at 18:00, the moment the lights went off. By day, they sheltered.[1]
The pond’s office politics of Part 5, it turns out, are mostly a night shift. That matters for everything in this series so far. Most of what researchers measured about how prawns treat each other was measured by day, by people who could see the tanks. The prawns were busiest when nobody was looking.
The disappearing prawn should not have surprised us. A prawn that can walk over wet stones and up small weirs as a youngster (Part 1) does not stay put as an adult, either.
But what does a prawn see in the dark? Or in general? In 2014, researchers measured how the adult eye responds to light. It is most sensitive in the yellow-green, at 563 nanometres, and less than half as sensitive above 621 nanometres, in the deep red. Its light receptors are about 1.75 degrees apart, which limits how finely it can see. Whether adults see colour has never been tested by watching what they do.[2] It raises a practical question nobody has answered: could you watch the night shift under deep red light without changing it?
Light itself has been tested on young prawns, and the results disagree. One study found that constant darkness grew the largest juveniles, though its own survival figures do not fully support that summary.[3] Another found no difference in growth, and better survival under constant light.[4] Even the colour of a tank seems to matter. Dark tanks produced darker whiteleg shrimp, with more of the pigment that turns red in the pan, and a 2024 study reported the same in this prawn.[5] Our own observations pointed the same way: the prawns’ shell colour seemed to adapt to their surroundings.
Put the last two parts together, and a pattern appears. Light, sound, the colour of the background, what is in the water: each is a channel through which one prawn could affect another, and each is something an indoor system can set and measure. The pond hid them all.
Almost everything in this series so far was learned on farmed prawns from Asia and Israel. But the giant prawn also lives in the rivers of northern Australia. And it has its own yarns to tell.
Sources
- dos Santos, D. B., de Almeida, L. M., de Moura, E. E. S., Arruda, M. D. F., et al. (2018), Behavior activities of Macrobrachium rosenbergii (De Man, 1879) in monosex and mixed populations, in the laboratory. Fighting, crawling and swimming mostly in the dark phase, peaking at 18:00; sheltering by day. doi.org
- Matsuda, K., Wilder, M. N. (2014), Eye structure and function in the giant freshwater prawn Macrobrachium rosenbergii. Fisheries Science. Peak sensitivity 563 nm; less than half above 621 nm; receptors about 1.75° apart. doi.org
- Withyachumnarnkul, B., Poolsanguan, B., Poolsanguan, W. (1990), Continuous darkness stimulates body growth of the juvenile giant freshwater prawn, Macrobrachium rosenbergii de Man. Chronobiology International. doi.org
- Tidwell, J. H., Coyle, S. D., Vanarnum, A., Bright, L. A., et al. (2001), The effect of photoperiod on growth and survival of juvenile freshwater prawn, Macrobrachium rosenbergii, in nursery tanks. Journal of Applied Aquaculture. doi.org
- Liu, Y., Li, Y., Zhao, Q., Luan, G., et al. (2026), Substrate-induced alterations of astaxanthin and crustacyanin mediate body color changes in Litopenaeus vannamei. Aquaculture. Dark substrate darkens shrimp and raises astaxanthin; a 2024 study cited for the same in M. rosenbergii. doi.org
Part 12 of 18 · A history of the giant freshwater prawn · Previous: Part 11, Do prawns hear each other? · Next: Part 13, The prawn on the other side of the line · SHRMP.bio’s strategy behind it: shrmp.bio/strategy.html