Oriental Shrimp

Male individuals are typically 9.7 to 13.5 mm (0.38 to 0.53 in) in length. Ovigerous females, meaning that they bear eggs, are approximately 16.5 to 19 mm (0.65 to 0.75 in) in length. These females carry approximately 150-2000 eggs. Females that do not bear eggs are typically 11.8 to 14.1 mm (0.46 to 0.56 in) in length. Females tend to be larger than males and are pigmented with red spots on the body surface with white longitudinal stripes going down the back. The body of males are transparent with a reddish hue on the tail fan and antennary area. Due to the similarities in morphology, it is not simple to distinguish between the local species and the alien, meaning introduced, species. Embryos are able to develop in vitro in conditions similar to adults, but the full development of these embryos are more successful in high salinity conditions.

Palaemon macrodactylus is native to Japan, Korea, and Northern China. It is, additionally, native to the Far East region of Russia, or the Northwest area of the Pacific Ocean. In 1957, the first non-native P. macrodactylus was accidentally introduced to San Francisco Bay, USA. Nowadays, it is officially considered as an invasive species in California, USA, and very abundant. In Northern America, P. macrodactylus can also be found in the US states of Washington, Oregon, Virginia, Connecticut, and New York. In the early 2000s, P. macrodactylus made its first appearance in Australia and Argentina. It is thought that its introduction was accidental via discharge of ballast water from the US to Mar De Plata harbor in Argentina. This was first reported in the southwestern Atlantic coast. Along the Atlantic coasts of Europe, it can be found at different localities such as Southwestern Spain.

Palaemon macrodactylus typically eats plants, mysids such as Neomysis, amphipods, and crabs. Particularly, its primary natural food sources are recorded to be Corophium (so called "mud shrimp") or Rhithropanopeus (so called "Harris mud crab"). They prefer rubble, man-made structures, and vegetation as foraging habitats, and experimental studies demonstrate a strong preference for even unfamiliar structural elements, including plastic Army men. P. macrodactylus are considered as osmoregulators and just like their related resident species. The similarities among these species often puts them in direct competition for dietary resources. What's more, when densities of these competing species reach a critical limit, cannibalism has been reported.

The mode of reproduction is called dioecism which means that an individual shrimp can have male reproductive system or a female reproductive system. In shrimps, a courting ritual is common and mating consists of indirect sperm transfer which is through olfactory and tactile cues. For the regulator shrimp, the general mating process happens when a male shrimp is at a ninety degree angle from a female shrimp. The male shrimp then transfers its spermatophore onto an area on the female's abdomen. Within 6–20 hours, the females begin to produce eggs. For P. macrodactylus, their mating season is from mid April to early October.