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SHRIMP, CRAYFIS H & CO.: Souvenir from Uruguay


caridina English Edition - epaper ⋅ Ausgabe 1/2020 vom 04.03.2020

A long-arm shrimp of the genus Macrobrachium is found in the south of South America, and its origins permit keeping it in a cool tank


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In the dense vegetation of this “laguna”, located between Concepción and Salto, the shrimp occurred in high numbers.


Dorsal view of a swimming female.


This portrait shows the large, strongly armed rostrum and the bristles between the teeth.


When excited or curious, even the females start swimming in the open water.


The Guaviyú River was the southernmost location where we found the shrimp. M. borellii were living in areas with a slow current and dense vegetation along the ...

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... riverbanks.


During the first two weeks of November 2016, some friends and me traveled to Uruguay. In the west of the country we found a long-arm shrimp species in several affluents of the Río Uruguay between Bella Unión and Paysandú.

One of my travel companions took some specimens home and managed to breed them later on.

In the following fall, he gave me the largest part of his population, for which I am really grateful. I actually prefer longarm shrimp to Caridina, because they look so interesting when they use their long limbs to climb over or through obstacles, and when they use their bristly “hands” to clean themselves.

OVERGROWN RIVERBA NKS

After some research on the Internet (Wikipedia) and in scientific literature, I was sure I had a group of Macrobrachium borellii (Nobili, 1896), originally described from the Río San Lorenzo in Argentine as a Palaemon species. In their home country, they inhabit streams, rivers and lakes, where they breed in fresh water, which is quite important. Their area of distribution stretches from southern Brazil and western Uruguay over Paraguay into northern, central and eastern Argentine.

We found the shrimp in overgrown areas along the banks of streams and rivers, some of them with quite a good current. One of them was the Río Tala south of Salto, with a mostly rocky bottom.

The clear water (386 μS, pH 7.2, 23 °C/73 °F) looked inviting, and we took out our snorkels, also in the Río Guaviyú near the thermal springs Termas de Arapey (460 μS, 22 °C/72 °F).After all, it was spring down here, far south of the equator. During the subtropical winter, the water temperatures may drop significantly and reach 10-12 °C (50-54 °F), while they may climb to around 27 °C (81 °F) during the summer months of December and January.

The shrimp just love climbing in the plants.


This female is berried: You can clearly see the eggs through the dark pattern on the transparent sides of the pleon.


LONG ARMS

The males grow to an average body length of 69 mm, and some specimens may even reach a good 10 cm (a little under 4 inches), whereas the smaller females only grow to 32 to 55 mm in length. The chelipeds of the males are longer than their bodies.

Moreover, in adult males, they are much longer and thicker than those of the females; the joints of the claws are blueish.

Both sexes have rather long swimming legs under their pleon. The shrimp love swimming and climbing in the plants.

Their rostrum is rather prominent and has large dorsal teeth (I counted 8-9), with thick tufts of bristles between them. The scaphocerite is as long as the tip of the rostrum.

THEIR FAVORITE FOOD

The natural diet of long-arm shrimp depends vastly on their development. Younger, smaller specimens eat different food than the adults, however, in M. borellii. this is less distinctive. Size and hardness of their prey is, according to Collins & Paggi (1997), not a decisive factor. The species apparently prefers large, rather slow prey, like for example the protein-rich larvae of flies and mosquitoes, or worms (oligochaetes) to smaller, faster food animals like copepods and branchiopods, or to plant-based food like algae and detritus.

According to Collins (1998), M. borellii really love eating blackworms, the larvae of the common house mosquito of global distribution Culex pipiens. In lab experiments, inactive M. borellii woke up within seconds when the larvae were put in their tank, and they expertly caught them using the claws on their first chelipeds – the second largest pair of claws, not the largest one (!) – and handed them over to their mouth parts, which consist of the mandibles and maxillae.

Within 24 hours, large specimens of M. borellii ate up to 40 of the larvae. I am convinced that they also feed on the larvae of mosquitoes of the genus Aedes - which spread viruses causing dengue, zika and yellow fever, or Anopheles larvae, a mosquito that carries malaria and filariosis. In the aquarium, M. borellii will eat just any kind of standard feed for fish or crustaceans.

REPRODUCTION

Macrobrachium borellii belongs to the abbreviated type that reproduces in fresh water, which makes it a highly compatible aquarium shrimp. The females carry an average of 53 large eggs with a diameter of roughly 2.0 x 2.2 mm. The number of eggs tends to be lower in smaller and higher in larger females.

No larvae hatch from the eggs of this long-arm shrimp, which would have to go through several stages until they metamorphose into their final form, but juvenile shrimp that look almost like a tiny version of their parents. The juvies leave their mother directly after hatching. In nature they wander to the shallow zones along the riverbanks, where they can be found on any kind of substrate like plants, leaves, rocks and driftwood. They can be raised on standard aquarium feed from the trade in captivity, and are able to eat astonishingly large food granules at a very early age.

LITERATURE:

Giuseppe Nobili (1896): Crostacei Decapodi. Bollettino dei Musei di Zoologia ed Anatomia comparata della R. Università di Torino vol. 11: n. 265, p. 2.
Pablo A. Collins, & Juan C. Paggi (1997): Feeding ecology of Macrobrachium borelli (Nobili) (Decapoda: Palaemonidae) in the flood valley of the River Paraná, Argentina. Hydrobiologia, 362 (1-3): 21-30.
Pablo A. Collins (1998): Laboratory evaluation of the freshwater prawn, Macrobrachium borellii, as a predator of mosquito larvae. Aquatic sciences 60: 22-27.