Cherry Shrimp (Neocaridina davidi)

Red cherry shrimp are the beginner's invertebrate — a hardy, colourful, endlessly breeding little decapod crustacean that has become the most popular shrimp in the planted-aquarium hobby. At a centimetre and a half to three centimetres as adults they are the largest-bodied consumer in the model, carrying a couple of milligrams of nitrogen per animal where a copepod or a Daphnia holds a thousandth as much, and they sit at the top of the small-tank invertebrate world. They earn their keep as the cleanup crew: benthic scraper-detritivores that walk the substrate and surfaces all day, raking biofilm off glass and leaves with specialised feeding appendages and picking through settled debris. They are no threat to fish or snails, though gut-content studies show they do incidentally swallow a fair number of the tiny animals living in the biofilm they graze.

A versatile grazer of almost everything

Cherry shrimp are generalists with a clear pecking order of preferences, drawn from studies of what is actually in their guts (Barroso-Lopez et al. 2014; Wester 2013). Their staples are surface periphyton — the biofilm of algae and microbes coating every surface — and the settled detritus on the bottom, which turns up in nearly every gut examined. They readily scrape up the bacteria and cyanobacteria embedded in that biofilm alongside the algae, and they take suspended detritus, drifting planktonic algae, and small biofilm animals like ciliates and rotifers as lesser, incidental fare.

What makes them such effective cleaners is reach. Their jointed walking legs, tipped with grasping spines, let them grip and graze glass, gravel, sand, porous ceramic, and even strands of filamentous algae with near-equal ease. They work smooth glass about as well as a bladder snail does, and they beat snails handily on gravel, because their legs can probe into crevices that a snail's muscular foot simply cannot enter.

Breeding is the first thing to stop

The trait that defines cherry shrimp in the model is how they respond to bad water — and it mirrors exactly what keepers see in real tanks. Under poor conditions the adults do not sicken or die; they simply stop breeding. They keep feeding and behaving normally, but all their energy gain is diverted into staying alive rather than making young, so the population stalls instead of crashing. This is the well-known hobbyist rule that cherry shrimp "go on strike" in a tank that is chemically off — healthy-looking, active, but not producing berried females (Cheng et al. 2002; Wester 2013).

Several stressors can throw that switch, and breeding is governed by whichever is worst at the time. Elevated nitrate is the classic culprit — the one that catches keepers out, because the shrimp look fine while the colony quietly fails to grow — but pH outside a comfortable band, temperatures too warm or too cool, and even a trace of toxic unionized ammonia will each suppress reproduction. The energy that would have gone into offspring is written off to the detritus pool, representing aborted eggs and wasted reproductive effort. The exact onset and shut-off thresholds for each stressor are tabulated in the Parameter Reference.

Moulting and the calcium question

Like all crustaceans, cherry shrimp grow by moulting — shedding the old shell and hardening a new one — every few weeks, and in the model they are the main calcium sink among the crustaceans. Each moult pulls calcium out of the water to stiffen the fresh exoskeleton, but it is not a permanent loss: the cast-off old shell (exuviae) carries an equal amount of calcium and is returned to the substrate's calcium-carbonate store, only slowly redissolving later. There is a brief soft-shell window of about half a day after each moult when the animal is vulnerable and more likely to die. Moulting speeds up in warmer water and slows in the cold, the usual temperature dependence of crustacean metabolism.

Calcium availability is where this becomes a practical keeping issue. In soft water — low general hardness — shrimp struggle to harden a new shell: they delay moulting and suffer markedly higher post-moult mortality, the model's version of the well-documented failures of moulting in calcium-poor water. It is why cherry-shrimp keepers in soft-water regions remineralise specifically for the shrimp.

A long-lived, sit-tight population

Cherry shrimp are slow and steady where the tiny grazers are fast and flighty. Their baseline death rate is low, befitting an animal that lives around a year and a half, and even at their best — ample food, ideal water — the colony's effective doubling takes the better part of three weeks. Their numbers are held in check by crowding: because they are large-bodied and live on the bottom, crowding is gauged by how densely packed they are over the available floor and surface area rather than by their concentration in the water. Being fully benthic, they are also essentially immune to water changes — a siphon passes over animals clinging to the substrate — so unlike the planktonic grazers they are never flushed out when you change the water.

Their waste keeps to the bottom: the great majority of their feces, and of their bodies when they die, settles rather than staying suspended. The feces are moderately nitrogen-depleted relative to the protein-rich shrimp body, the normal enrichment of crustacean fecal pellets.

Key references

Barroso-Lopez, V. et al. (2014). Gut content analysis and feeding behaviour of ornamental freshwater shrimp. Journal of Crustacean Biology 34, 1–8.
Cheng, W. et al. (2002). Effect of water quality on the growth of Neocaridina denticulata sinensis. Aquaculture 205, 141–152.
Liang, Y.J. & Huang, J.T. (1994). Freshwater Shrimp Culture. National Taiwan Ocean University Press.
Wester, J. (2013). Keeping and Breeding Freshwater Shrimp. Vivarium Press.