Caridina typus

Preserved Caridina typus specimens with tagmata separated.[8]

The adult and larval morphologies of C. typus are similar to those of other Caridina. As an adult, it measures 2.0-2.5 cm in total length, though adult individuals can be smaller or larger than this range.[9] It has two well-developed compound eyes that rest on stalks.[10] It has a variable rostrum length and variable rostrum features.[10] Adults are observed to be armed with anywhere between one and six teeth on their rostrum.[3] However, in a study of C. typus in Pohnpei, adults were observed to lack teeth on the ventral margin of their rostrum.[3]

The carapace length of ovigerous females ranges between 3.9 and 4.2 mm.[3] Adults have a cephalothorax with cephalic appendages, six pleomeres with the sixth being 0.40 times as long as the carapace, a telson 2.8 times as long as wide, 4-5 pairs of dorsal spicules, a uropodal diaeresis with 19-24 spicules, a preanal carina lacking a spine, a slender antennular peduncle 0.7 times as long as the carapace, and similar mouthparts to C. grandirostris.[10]

It possesses five pereiopods, the first four having epipods.[10] The first pereiopod is short with claws present on both fingers, reaching the end of the basal segment of the antennular peduncle; the second pereiopod reaches the end of the second segment of the antennular peduncle, the third pereiopod is long and slender with a strong claw at the end of the dactylus, reaching the end of the antennular peduncle and the fifth pereiopod reaching the end of the scaphocerite.[10]

There are five pleopods.[10] The endopod of a male's first pleopod extends to half the length of the exopod.[10] The second pleopod of a male is slender.[10]

Preserved Caridina typus specimens.[11]

Caridina typus larvae can eat a variety of phytoplankton, zooplankton, and fine detritus.[12] Early larvae ingest phytoplankton by means of suspension feeding.[12] The fatty acid content and composition of live food affects the larval survival, growth, and development of C. typus.[12] A captive study observed that C. typus larvae could survive and develop into juveniles when fed only cultured phytoplankton Tetraselmis sp.[12] When fed Tetraselmis mixed with zooplankton rotifers, the survival of larvae improved and larval development velocity increased as compared to a Tetraselmis-only diet.[12] However, a diet with rotifers alone could not sustain larvae, suggesting that early C. typus larvae cannot ingest rotifers, but larvae in middle to late larval stages can.[12] Development of endopods on the thoracic appendages in older C. typus larvae allows the capture and ingestion of lumps of food.[12] A diverse diet and one including rotifers greatly benefits the development of C. typus larvae by increasing survival and the rate of development. If starved, the larvae will not molt and therefore stop developing.[12]

Juveniles and adults will eat primarily algae, leaf litter, and detritus.[13] They are considered scavengers. Adults possess clawed fingers on their first pereiopod, which help the organism grab and feed on discrete particles or invertebrates such as juvenile insects and oligochaetes.[13] Their diet contrasts with other Caridina species lacking clawed fingers, which feed on detritus or periphyton instead.[13] Like other atyid shrimp, C. typus possess hairs and setae on their chelae used for scraping, filtering, and collecting food.[5]

Caridina typus uses chemoreception to avoid heterospecific predators and find conspecifics.[9] They can detect chemical cues from conspecifics and heterospecifics and distinguish whether heterospecifics are predatory or non-predatory.[9] They avoid chemical cues from predatory heterospecifics such as the redclaw crayfish Cherax quadricarinatus and barramundi Lates calcarife.[9] C. typus shows no response to non-predatory heterospecific cues such as from Nerite snail Nerita atramentosa, and prefers conspecific cues.[9] Conspecific cues are useful in low visibility conditions to help find mates or to defend a territory from potential competitors.[9]

Females can carry large broods of small-sized oval-shaped eggs, each which measure approximately 0.45-0.48 mm x 0.23-0.26 mm in diameter.[10] Broods of over 3000 eggs on average, up to a record 3700, are carried amongst the pleopods of ovigerous females.[15] Brood size varies with body size. Smaller ovigerous females carry fewer eggs. One study observed an ovigerous female 11.7 mm in total orbital length carrying only 126 eggs, but another ovigerous female 34.3 mm in total orbital length carried 3542 eggs.[5]

Between hatching from eggs and becoming an adult, C. typus goes through larval and juvenile stages, all of which are pelagic.[12] Females migrate downstream (to the mouth of a river or stream) or into brackish waters to spawn, using the upstream headwaters (the smallest part of a river or stream) only as a nursery.[5] This has been confirmed observation of ovigerous females in headwaters carrying non-eyed eggs, but ovigerous females downstream having eyed eggs.[12] C. typus larvae recruit to the mouth of a coastal river or stream, emerging after sunset and increasing as darkness progresses.[12] Significantly large numbers of larvae can be found downstream versus upstream.

Caridina typus has nine zoeal instars or stages: a relatively high number of stages compared to other Caridina, which usually have around four zoeal stages.[16] The zoea develop in brackish water, and the first few zoeal stages have a relatively high optimal salinity of 25.5 ppm.[16] Food is taken in during the first zoeal stage.[17]

An adult Caridina typus.

After the larval stages are complete, C. typus will metamorphose into a juvenile stage, which will begin to migrate back to its parental freshwater habitat.[18] Upstream migration of juveniles has been reported for other Caridina, and has been confirmed by observation of smaller C. typus juveniles downstream and larger sized juveniles upstream.[12] Juveniles will enter their adult stage in freshwater and begin recruiting in August.[5] Since juveniles were commonly found during the spawning season and the post-spawning season between August and February, this indicates continuous recruitment.[5]

Males and females will sexually reproduce in freshwater. After mating, females will migrate downstream for spawning in estuarine, brackish environments.[18]

Holtodrilus truncatus is a branchiobdellidan ectosymbiotic parasite found on atyid shrimps, both in captivity and the wild.[20] C. typus is a host shrimp, and H. truncatus can be found inside the carapace, on the external surface of the antennule, rostrum, and abdomen of its host.[20]

Macrobrachium japonicum and M. lar are Palaemonidae shrimp that have similar diadromous life cycles to C. typus.[6] All three shrimp live in sympatry, as observed in Japan.[6] Despite their similar life cycles and similar range, they do not compete for food nor space as one would predict. They can freely migrate along the same river continuum without competitive effects.

Various anthropogenic effects have affected C. typus in some way. One effect of humans on C. typus has been the alteration of their habitat and distribution using artificial structures such as dams and floodgates.[21] Dams and floodgates affect the amount of flow in rivers. Since C. typus occurs in rivers with smaller flows, they are less abundant in rivers possessing floodgates. Dams and floodgates may also prevent migration routes. Interestingly, C. typus are abundant in rivers with artificial riverbeds or shore projections compared to rivers with only natural vegetation.[21] Not only do human structures impact the shrimp directly such as by intercepting migration routes, but indirect effects such as predation and competition increase can affect the keystone species, leading to ecological complications over time.[21]

Caridina typus can be consumed by humans. Historically, Pohnpei islanders utilized shrimp such as C. typus as a food source when technology for harvesting marine resources was not advanced or readily available.[3] However, freshwater shrimps are not currently as an important part of the Pohnpei islanders' diet.[3]

Amano shrimp such as C. typus can be kept as pets in the aquarium hobby and are referred to as "Australian amano shrimp" despite the species having a much wider distribution in the wild.