Long-armed River Prawn, Macrobrachium tenellum

Long-armed River Prawn, Macrobrachium tenellum. Collected out of the San José del Cabo Rio, Baja California Sur, with a cast net, August 2011. Length: 7.0 cm (2.8 inches).

Phylogeny:  Long-armed River Prawns, Macrobrachium tenellum (Smith, 1871), are members of the  family of Palaemonid Shrimp. The genus Macrobrachium is a very large genus in a very large family with one hundred and seventy-three genera and two hundred seventy-nine species in the Macrobachium genus. They are also known as the Long-clawed River Prawn and in Mexico as chacalín, langostino, manopalito and manocarrizo.

Morphology:  Long-armed River Prawns have a cylindrical carapace that is somewhat laterally, or dorso-ventrally, compressed. They have a cephalothorax  and 6 abdominal segments. The head includes a pair of antennae, a pair of antennules, and mouth parts. These animals have a two chambered stomach. River Prawns have five pairs of appendages that function as legs (pereiopods), three pairs that function as mouth parts (maxillipeds), and one pair is modified into enlarged pincers (chalae). They have six pairs of biramous (dividing to form two branches) appendages along their abdomen. The first five pairs function as swimmerets (pleopods). The last pair is flattened to form a tail fan. Long-armed River Prawn has a right claw arm that is about half again as long as the left claw arm. The right claw is almost always closed and the left one is usually open. The body is yellowish brown, becoming dusky on the top surface. They reach a maximum of 9.0 cm (3.5 inches) in length.

Habitat and Distribution:  Long-armed River Prawns are found in rivers and estuaries in either flowing water or in stagnant pools. This species is amphidromous-living in fresh water, but returning to brackish water to spawn. They are usually found at depths less than than 3 m (10 feet). Long-armed River Prawns are an Eastern Pacific species. In Mexican waters they range from the village of Marecuaco, Baja California Sur south to Guatemala. They are found in the southern half of the Gulf of California, from Mulegé to the Cape region of Baja California Sur.

Diet: Long-armed River Prawns are omnivorous scavengers, feeding on algae, detritus, small invertebrates, and insect larvae.

Predators: Long-armed River Prawns are preyed upon by shore birds, fish, and reptiles.

Reproduction:  Long-armed River Prawns are gonochoric (male or female for life). They reproduce sexually by external fertilization. The males attach spermatophores to the female’s abdomen. The female later releases eggs into the water where they are fertilized by the male’s sperm outside of her body; this occurs within a brood chamber on the female’s abdomen where the eggs are carried until hatching. The eggs hatch into a larval stage.

Ecosystem Interactions: Long-armed River Prawns can be parasitized by ectoparasitic isopods from the Bopyridae family, including Probopyrus pacificensis. Long-armed River Prawns play an important role in recycling and distributing organ matter within an ecosystem.

Human Interactions: Long-armed River Prawns have long been used as a food source by the local indigenous people. They are still currently the target of artisanal fisheries being caught primarily with cast nets. They are also considered a good aquaculture specimen because of their non-aggression, and tolerance for varying salinity, temperature, and oxygen levels. The commercial viability of this species is limited by their small size. From a conservation perspective is currently considered to be of Least Concern with stable, widely distributed populations.

Synonyms:  Palaemon longipes and Palaemon tenellus.