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 a member of the Palaemonidae Family of Palaemonid Shrimp. The Macrobrachium genus is one of one hundred and seventy-three genera in this large family. There are two hundred seventy-nine species in the Macrobachium genus. This species is also known as the Long-clawed River Prawn and in Mexico as Chacalín, Langostino, Manocarrizo and Manopalito. The genus name Macrobranchium comes from the Greek words meaning “large arm” or “long arm”. The species name tenellum comes from Latin and means “very tender”, “very delicate”, or “slender”. Both names refer to this species long, slender, arms.
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 that are found in all Mexican waters of the Pacific Ocean with the exception that they are absent from north of San Juanico, Baja California, along the central an northwest coasts of the Baja Peninsula and north of Mulegé, Baja California Sur, in the Gulf of California.
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 organic 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.