Australian Atyidae Shrimps
The Atyidae is an ancient family of shrimps with a very wide distribution on all continents. Australian atyids are relatively small (<35 mm, except the genus Australatya, with species reaching ~60 mm). Most are exclusively found in freshwater although some are occasionally found in estuaries. A few have larval development that require saltwater to survive to the postlarvae stage. The hatching larvae are swept downstream into the estuaries, where they undergo larval development, then as juveniles they make the long migration back upstream. The inflow of both seawater and freshwater provide high levels of nutrients in both the water column and sediment, making estuaries among the most productive natural habitats in the world. Despite the salinity tolerance of some species, there are no known fully marine atyids, in contrast to the other family of common freshwater shrimps in Australia, Palaemonidae (e.g. the genus Macrobrachium), which has many marine representatives.
There are a number of different species of shrimps that are found in freshwater environments all around Australia. Most, if not all, are suitable for keeping in aquariums. They are kept primarily as algae and detritus consumers in planted aquariums. However, they can be an interesting companion for the rainbowfish aquarium or maintained on their own. Several species are now available in the aquarium hobby, some of which are being specifically cultured for this purpose - see Aquagreen
Atyid shrimps have a prominent rostrum that usually extends beyond the eyes, and an abdomen bearing a tail fan. The eyes are on stalks and the carapace completely covers the dorsal and lateral thorax. The rostrum is most commonly long, sharp and serrated. The legs are long and thin, and without strikingly conspicuous chelae. The first two pairs of legs are similar in appearance and bear small chelae tipped with tufts of long setae. They are mostly small, transparent (sometimes invisible except for their black eyes), quick-moving animals that are found in many types of habitats but prefer comparatively still waters where they congregate under banks, around large submerged boulders and amongst aquatic vegetation. They do not have heavy armour like a crayfish for defence and generally rely on their invisibility for defence.
Atyid shrimps also display a large range of different egg-related life history strategies: (1) small-sized eggs and planktonic larvae with high salinity tolerance, often estuarine; (2) medium eggs, intermediate number of larval stages, and intermediate salinity tolerance, often lowland; and (3) large eggs, direct (abbreviated) development, and low salinity tolerance, often upland. Egg size differences are evident not only between recognised species, but also between different forms of both Australian Paratya and Caridina.
Atyid shrimp are generally filter feeders, detritivores and scrapers that feed by scraping food particles off rocks and plants using the brush-like setae on the tips of the claws of their first and second legs. Mostly they feed on detritus, fine decomposing vegetation, bacteria and algal particles. Some are consumers of living planktonic algae and other suspended particles. At least one species, Pycnisia raptor, is believed to be a predator. Examination of gut contents revealed an amorphous mass of gray-green material along with minute alga cells, inorganic particles, and fragments of higher plants. It was also reported that the gut fills and empties rapidly, suggesting that the animals rely on a constant abundance of food. Much of the organic matter ingested is loose and flocculent and is released in a partially digested condition or in compact, elongated faecal pellets.
Spawning of Atyid shrimp occurs after the female moults, the male mates with the female (facing each other) depositing his sperm at the opening of the female genital opening. Shortly after, the eggs move from the female's ovaries through her genital opening to the abdomen. As the eggs pass through the female genital opening, they are fertilised by the sperm that had been deposited there by the male earlier.
Females of the families Atyidae and Kakaducarididae attach eggs under their abdomen. The pleopods (swimming legs) are used to keep fresh oxygenated water flowing over the eggs. Most freshwater species take 3-4 weeks to hatch depending on temperature. The young hatch directly from among the pleopods. The hatchlings are tiny, barely visible and only a few millimetres long. Freshwater species do not have long planktonic juvenile stages like the marine decapods. The young start life as smaller versions of the parents with claws and strong legs. Some species may need brackish water to survive to the postlarval stage, at which point they can be acclimated to freshwater.
There are currently eight genera of Atyidae recognised in Australia consisting of Australatya, Caridina, Caridinides, Paratya, Parisia, Pycneus, Pycnisia and Stygiocaris. They are distributed in every State but are most common in the eastern half of the continent. Paratya and Caridina species appear to be the most common. Paratya australiensis is the most widespread species in eastern Australia. A new (undescribed) species of atyid shrimp, representing a new genus, is known from a small spring in the upper catchment of Nourlangie Creek in the Northern Territory. The last intensive study of atyid shrimps in Australia was done about fifty years ago. Since then, several new species have been discovered and the family is in need of taxonomic revision.
The freshwater shrimp fauna of New Guinea is poorly known, with only a few old reports and descriptions available. The majority of these reports are from the northern, coastal region, with the mountain ranges and the southern lowlands remaining unstudied. Amongst these are several endemic species. Giuseppe Nobili (1905) provided the first record of an atyid shrimp from New Guinea. He described Caridina weberi var. papuana and discussed some other specimens, which he identified as Caridina wyckii. Later, J. Roux (1911) described two endemic species, Caridina fecunda and Caridina demani. In 1915, two more endemic species were described by De Man: Caridina cognata and Caridina rouxi. Some widely distributed species, such as Caridina nilotica and Caridina gracilipes were also recorded. The New Guinea fauna currently comprises around 25+ species, with many of them still to be described. Atyopsis spinipes and Atyoida pilipes have been reported from New Guinea and are found more commonly on hard substrates in riffle areas, while species in the genus Caridina can be seen more frequently in deeper areas with calmer waters. This habitat differentiation corresponds with morphological differences in the shrimp. Atyoida possess longer setae appropriate for filter-feeding in faster moving water while Caridina have both short and long setae appropriate for filtering or scraping in a wider range of habitats.
Just like any other aquarium inhabitant, freshwater shrimp should get regular water changes. About 25-40% of the water should be changed every week depending on population numbers. Shrimp are highly sensitive to chloramines so may sure you use a chloramine neutraliser when doing water changes. Be careful using copper-based medications in an aquarium containing freshwater shrimp. Freshwater shrimp are extremely sensitive to copper, dying at levels of only 2 µg Cu/L. The most suitable filters for a shrimp tank are sponge filters. These are usually air driven and cannot suck adult or juvenile shrimp into the filter, which can occur with other types of filters. Also the shrimp love to pick on the detritus and algae that accumulate in and around the sponge.
In one breeding report the larvae and postlarvae of Caridina gracilirostris was being raised on a phytoplankton mixture (PhycoPure), and at the stocking density of 25 shrimp per litre in brackish water (15 ppt). Under these conditions, the newly hatched larvae developed, through six zoeal stages, into postlarvae in 15 days, with a survival rate of 90 ± 8%. The postlarvae were acclimated to freshwater when they reached 69 days post-hatch.
PhycoPure contains 7 different types of microalgae as well as a dinoflagellate, zooxanthellae. It is grown in natural seawater that has been ozonated, charcoal filtered and UV treated. It offers a wide range of particle sizes and nutritional content that, individually, have been proven very effective in various aquaculture projects.
Adrian R. Tappin