Eusergestes similis (Hansen, 1903)

Common name(s): Pacific sergestid

Synonyms:  Sergestes similis
Phylum Arthropoda
 Subphylum Crustacea
  Class Malacostraca
   Subclass Eumalacostraca
    Superorder Eucarida
     Order Decapoda
       "Natantia" (shrimp--not a formal taxonomic category)
        Suborder Dendrobranchiata
         Family Sergestidae
Eusergestes similis, captured off Point Conception, CA May 1996.  Photographed live swimming in a swim tunnel.  Total length approximately 6 cm.
(Photo by: Dave Cowles, May 1996)
Description:  This mesopelagic, mostly oceanic, vertically-migrating shrimplike prawn is not a true (Caridean) shrimp because the epimera of abdominal segment 2 do not overlap those of abdominal segments 1 and 3 (photo).  The species is "half-red", like many mesopelagic vertical migrators, meaning that part of its body is clear while other parts, especially the area around the gut and internal organs, is bright red.  As with most sergestids the rostrum is short, less than 1/10 the length of the carapace.  The tiny rostrum points obliquely upward and forward and has an acute tip (photo).  The flagellum of the antennule is very long, much longer than the entire body.  The antennules are held extending forward and outward at an angle from the body for a distance of more than 1 body length, then the long flagellae trail backward from an inflection point.  Males to 18.4 mm carapace length and 6.1 mm total length, females to 17.8 mm carapace length and 57 mm total length.

How to Distinguish from Similar Species:  The only other sergestid common in this area, Sergia tenuiremis, has no supraorbital or hepatic spine, no Organ of Pesta, is not bioluminescent, and has a tubercle on the inner margin of the eyestalk.

Geographical Range:  North Pacific Ocean, Japan through Alaska and the southwest Bering Sea south along the Amereican coast to the Gulf of California; off Chile, and in the eastern South Atlantic.

Depth Range:  Surface (at night) to 1200 m; most common 150-500 m during the day (vertical migrator)

Habitat:  Mesopelagic, Oceanic (offshore).  Also found nearer shore in deeper areas of the continental shelf.

Biology/Natural History:  Since red light does not penetrate to the daytime depths of this prawn, the red parts of its body are effectively black.  This species has organs of Pesta, which are bioluminescent, ventral to the red visceral mass.  The organs of pesta have a shiny reflector to make the bioluminescence directional.  As the animal moves through the water it turns the organs of pesta so that the bioluminescence is directed downward, thus effectively erasing its shadow.  In situ studies in Monterey Bay showed that this species swims an average of 7.4 cm/s, or about 1.5 body lengths per second.  Propulsion is exclusively by the pleopods, which beat at 4 Hz.  The animal swims continuously, and remains above the oxygen minimum layer.  It ascends near the surface at night and descends to deeper water during the day.  The antennae respond to tactile stimuli and probably to vibrations in the water.  This species matures in 1 or 2 years.  Peak spawning is during the summer.  May die after spawning.  Feeds on large copepods and euphausiids.  Predators include albacore, rockfish, sei and fin whales, and snipe eels.

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Dichotomous Keys:
  Kozloff 1987, 1996
  Wicksten, 2009
  Word, Jack and Danuta Charwat, 1974.  Key to Shrimp Common in Southern California Trawl Catches.  Southern California Coaster Water Research Project.  41 pp.

General References:
  Butler, 1980

Scientific Articles:
Barham, E.G., 1956.  The ecology of sonic scattering layers in the Monterey Bay area.  Ph.D. Dissertation, Stanford University, Palo Alto, CA

Childress, J.J., 1975.  The respiratory rates of midwater crustaceans as a function of depth of occurrence and relation to the oxygen minimum layer off southern California.  Comp. Biochem. Physiol. A Comp. Physiol.  50: 787-799

Childress, J.J. and M. Nygaard, 1974.  Chemical composition and buoyancy of midwater crustaceans as a function of depth of occurrence off southern California.  Marine Biology 27: 225-238

Cowles, David L., 1994.  Swimming dynamics of the mesopelagic vertically migrating penaeid shrimp Sergestes similis:  Modes and speeds of swimming.  Journal of Crustacean Biology 14(2): 247-257

Cowles, David L., 2001.  Swimming speed and metabolic rate during routine swimming and simulated diel vertical megration of Sergestes similis in the laboratory.  Pacific Science 55(3): 215-226

Genthe, H.C., Jr., 1969.  The reproductive biology of Sergestes similis (Decapoda: Natantia).  Marine Biology 2: 203-217

Hendrickx, Michel E. and Estrada Navarrete, Flor Delia, 1996.  Los Camarones Pelagicos (Crustacea: Dendrobranchiata y Caridea) del Pacifico Mexicano.  Comision Nacional para el Conocimiento y Uso de la Biodiversidad.  157 pp.

Omori, M., 1979.  Growth, feeding, and mortality of larval and early postlarval stages of the oceanic shrimp Sergestes similis Hansen.  Limnology and Oceanography 24: 273-288

Omori, M. and D. Gluck, 1979.  Life history and vertical migration of the pelagic shrimp Sergestes similis off the southern California coast.  Fish. Bull. 77: 183-198

Pearcy, W.G. and C.A. Forss, 1966.  Depth distribution of oceanic shrimps (Decapoda: Natantia) off Oregon.  Journal of the Fisheries Research Board of Canada 23: 1135-1143

Pearcy, W.G. and C.A. Forss, 1969.  The oceanic shrimp Sergestes similis off the Oregon coast.  Limnology and Oceanography 14: 755-765

Web sites:

General Notes and Observations:  Locations, abundances, unusual behaviors:

This species is abundant off our Pacific coast, and is often the dominant mesopelagic vertical migrating shrimp in our region.

Most of this species' body is transparent, with small red chromatophore spots.  The internal thoracic organs are opaque red or black.  The organs of Pesta (not readily visible in this photo) are ventral to the dark thoracic organs.  Note the tiny rostrum (the small red dorsal bump behind the eyes).  The long second antennae trail out to the right of this photo.  Swimming is almost exclusively by the pleopods.  Photo by Dave Cowles

For an .mpg video of this species swimming in a swim tunnel, click Here (8 megabytes).  Note in the video (and in the image at the top of this page, which was taken from the video), that the animal turns downward when exposed to the light of the camera.  This is because it is a vertical migrator.

Eusergestes similis can be distinguished from Sergia tenuiremis by the small, acute, upward-pointed rostrum and by the presence of a supraorbital and a hepatic spine.
This photo is of a preserved individual.

The epimera of abdominal segment 2 does not overlap those of segments 1 and 3, showing this species is not a Caridean shrimp.
Photo from a preserved specimen

The telson is shorter than the uropods, and both telson and uropods are fringed with setae.
Photo from a preserved specimen

Authors and Editors of Page:
Dave Cowles (2006):  Created original page