Processa edulis

Lore & Background

Processa edulis is a moderate-sized shrimp, with males reaching 12–34 mm and females 25–52 mm in length. Its rostrum is short, not extending beyond the eyes, and tipped with a tooth and a point. The first two pairs of legs are asymmetrical: leg 1 has a claw on the left and a clamp on the right; leg 2 has a clamp on both sides, but the left leg is longer than the right. During the day the shrimp is white, but at night it becomes pink or red, with tiny white spots marking the chromatophores; the red color comes from astaxanthin and derived esters.

It occurs in the eastern Atlantic between 55° and 33° North, the North Sea, the English Channel, the Mediterranean Sea, and the Black Sea. It inhabits shallow soft bottoms in seagrass beds and eelgrass flats, and is common in Posidonia beds in the English Channel, sometimes found among seaweeds on sandy seabeds. By day it hides among foliage or buries in sand, emerging at night to feed.

This shrimp feeds on small invertebrates such as nematodes, polychaete worms, amphipods, isopods, ostracods, decapod larvae, foraminifera, molluscs, and fish eggs, and occasionally diatoms and fragments of seagrass and seaweed. Stomach content analysis of specimens from near the Ebro Delta showed negligible plant matter, leading to the hypothesis that it is exclusively carnivorous and plants are ingested only by accident. It tears prey with its asymmetric front legs.

Reader's Guide

Processa edulis is believed to be a protandric hermaphrodite, starting life as a male and becoming female as it grows larger. This is supported by the appearance of large numbers of moderate-sized females in mid-July, coinciding with a sharp decline in males. Small females are very rare at any time of year, and the few present may be undifferentiated juveniles; large males are non-existent. Breeding occurs between March and September, with several batches of eggs laid. The female attaches green eggs to her abdominal legs; they become grey before hatching after three to four weeks. Planktonic larvae develop for another three to four weeks before settling on the seabed to metamorphose into juveniles.

Although edible, this shrimp has been little studied, perhaps because it is only caught when trawling takes place at night. Laboratory studies on its circadian rhythm show that in constant darkness it maintains its rhythm, emerging at expected night and remaining buried during expected day. When exposed to artificial light stimuli, all individuals exhibited a synchronized strong reaction, emerging from sand 15–20 minutes after the light was turned off. The species is notable for its color change, asymmetric legs, and nocturnal feeding behavior, but remains understudied in the scientific literature.

Did You Know?

• Processa edulis changes color from white during the day to pink or red at night due to astaxanthin and derived esters.
• In laboratory constant darkness, it maintained its circadian rhythm, emerging at expected night and staying buried during expected day.
• Stomach content analysis from near the Ebro Delta suggests it is exclusively carnivorous, with plant matter ingested only by accident.