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  Predators & defenses
   
 

Predators of eggs and zoea larvae of shrimps include chaetognaths, hydroid medusae, jellyfishes, filter-feeding fishes such as herring, and zoea larvae of other decapods. Predators of adult shrimps include fishes (sculpins, salmon, flatfishes), seagulls, cormorants, crabs, and even (for mud shrimps) gray whales. 

Defenses of shrimps include fast swimming, quick withdrawal into burrows or crevices, pinching with the chelipeds, protective exoskeleton, and camouflaging coloration. Many or most alpheid shrimps have an interesting behaviour that may be defensive, and that is to "snap" their large claw.

NOTE in one study in southern California a single tide-pool after poisoning with rotenone yields over 200 individual fishes respresenting 22 species.  Stomach analyses of the 4 most abundant species (representing 75% of all individuals collected) indicate a strong preference for small crustaceans, including isopods Cirolana harfordi and Idotea spp., several species of amphipods, and decapods (chiefly shrimps Spirontocaris picta and Crangon sp.), as well as some polychaetes, notably Platynereis agassizi.  Mitchell 1953 Am Midl Nat 49: 862.

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Predators

Little appears to be known about predators of eggs and zoea larvae of shrimps, so the emphasis here will be on predators of adult shrimps.

 

 

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Research study 1
 

photograph of a cormorantExamination of regurgitated “morning pellets”, chick regurgitations, and reference to literature data on stomach content of 3 species of pelagic cormorants in localities ranging from Alaska to Baja California discloses only a small dietary reliance on shrimps (e.g., 4% of 9519 prey items from cormorants in the Farallon Islands, California are crustaceans, most likely shrimps).  An apparent greater reliance on crustaceans (shrimps) in diets of cormorants in other coastal areas, such as Kodiak Island, Alaska (33% of 15 prey items) and Mandarte Island, British Columbia (20% of 103 prey items) should be viewed with caution based on the small sample sizes.  Ainley et al. 1981 The Condor 83: 120.

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Research study 2
 

graph showing effect of predation by staghorn sculpins on survival of ghost shrimps Neotrypaea californiensisIn Coos Bay, Oregon a principal predator of ghost shrimps Neotrypaea californiensis is the staghorn sculpin Leptocottus armatus.  Deployment of predator-exclusion cages over a summer period, each cage measuring 1m x 1m x 12.5cm (height), shows that survival of shrimps within the cages is significantly greater than survival outside of the cages.  In view of the high density of sculpins in the area and the convincing results from the cage experiments, the author suggests that predation on ghost shrimps is an important factor determining species composition of tide-flats in the study area.  Posey 1986 J Exper Mar Biol Ecol 103: 143.

NOTE  simultaneous deployment and monitoring of CONTROL cages consisting of ones without roofs (actually with a partial roof to mimic current disruption) and ones without sides, indicate no significant cage-artifact effects

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Research study 3
 

drawing of a staghorn sculpin as an example of a predator of mud- and ghost-shrimpstable showing % of diet of staghorn sculpins in Grays Harbor, WashingtonStudies on summer diets of staghorn sculpins Leptocottus armatus in Grays Harbor, Washington show a predominant representation of shrimps, including ghost shrimps Neotrypaea californiensis, crangonid shrimps, and mud shrimps Upogebia pugettensis.  In total, these 3 prey types comprise almost half (46%) the summer diet of the sculpins.  Armstrong et al. 1995 Fish Bull 93: 456.

 

NOTE  the index IRI (Index of Relative Importance) used in the table takes into account both number and mass of prey consumed

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Research study 4
 

On mudflats in Willapa Bay, Washington ghost shrimps Neotrypaea californiensis are eaten extensively by white and green sturgeons (Acipenser transmontanus and A. medirostris, respectively).  In one study in summer, the shrimps comprise about 50% of the gut biomass of the predators. In comparison, 2 other burrowing shrimp species N. gigas and Upogebia pugettensis, and 2 species of carid shrimps, are preyed on to insignificant extents. The authors note that when sturgeons were more abundant, they likely exerted an important “top-down” control on population numbers of N. californiensis shrimps.  Dumbauld et al. 2008 Environ Biol Fish 83: 283.

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  Defenses: fast swimming
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Research study 1
  series of drawings showing how a shrimp uses its tail flip to accelerate backwardsphotograph of shrimp Pandalus danae
Most or all shrimps have a fast escape swimming reflex involving abdominal flexion.  In the carridean shrimp Pandalus danae, studied at Friday Harbor Laboratories, Washington, the flexion lasts about 30msec and accelerates the shrimp backwards at velocities greater than 100m . sec-1.  During the tail-flip, the uropods spread out to form a fan that is twice the width of the body. The motion is not linear; rather, the body tends to rotate through an angle of about 75o.  Interestingly, drag forces produced by the flexion are at least partly compensated for by the force of seawater being forcibly expelled from the space between the folded cephalothorax and abdomen.  The escape therefore combines rapid sculling with an additional small jet-propulsive force.  The authors use complex mathematical analyses combined with fast-motion cinematography to show that as body size increases, rotational forces come more into play, and a smooth transit from one point to another during fast escape becomes increasingly disrupted. The authors calculate that a body length of 6cm for Pandalus uniquely maximises the distance tranvelled during an escape event.  Daniel & Meyhöfer 1989 J Exp Biol 143: 245.
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  Defenses: claws: covering, biting, snapping
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Research study 1
  photograph of ghost shrimp Neotrypaea californiensis with major cheliped reflected back over its body, as though in protective mode, courtesy Male ghost shrimps Neotrypaea californiensis sometime fold their single large cheliped back on itself, as though protecting the head and carapace (see photo). Photograph courtesy MacGinitie & MacGinitie 1968 Natural history of marine animals McGraw-Hill, NY.
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Research study 2
 

photo/drawing of snapping shrimp Alpheus californiensisdrawings of snapping claw of Alpheus californiensisThere are a dozen or so species of alpheid or “snapping1 ” shrimps living in southern California.  The shrimps often live in pairs within their burrows.  Both males and females have one or other claw enlarged, and it is the large claw that creates the snapping.  An early description2 for the southern California pistol-shrimps Alpheus californiensis and Synalpheus lockingtoni  is provided by researchers at Scripps Institution of Oceanography, La Jolla, California.  The former species in the La Jolla area tends to be more active nocturnally, after individuals emerge from their burrows to forage for food, and sound levels are higher.  The sound, described in more detail in the Research Study to follow, is produced by a sudden closing of the hammer portion of the snapping claw that produces a vigorous jet of water (see drawings on Right).  The sharp gush of water may serve to frighten away potential predators or a predator may sometimes even be killed by a direct blow of the descending mallet, but the authors note that the biological significance of the accompanying loud sound is unclear.  A pair of suckers3 on the malleus portion of the claw contact and hold, or help to hold, the hammer in its open position.  The cocking arrangement works well, as evidenced by the authors being able to firmly cock even the hammer of a freshly dead or preserved specimen so strongly that some force is needed to break the contact.  The authors admit to not understanding the way in which the sound is produced.  If the snapping claw is lost through injury, it is replaced at the next or following moult by a new one, but not in its former location; rather, the former location grows a new pinching claw, while the pinching claw becomes the new snapping claw.  Finally, preferred habitats for snapping shrimps around La Jolla are shale rock, cobble, or boulder fields at 20-40m depth (see graphs4 below).  Johnson et al. 1947 Biol Bull 93 (2): 122.

NOTE1  worldwide there are several hundred alpheid species, most or all living in tropical and subtropical regions.  A SCUBA-diver on a coral reef is treated both day and night to a cacophony of snapping and cracklings, similar to, as other authors have described it, the sound of burning dry twigs

NOTE2   this post-war study appears to have been initiated by the U.S. Navy through its interest in the extent and causes of undersea “crackle”, a military nuisance known to be associated with shrimps but not well understood prior to World War II

NOTE3   these have comparable function to the opposing “protuberances” described in Research Study 2 below; apparently this cocking device has different morphologies among the numerous species of snapping shrimps

NOTE4   abundances of shrimps in the graphs are expressed by the amount of noise being produced in different habitats measured in decibels above a reference sound pressure of 0.0002 dyne/cm2.  The strongest components of the noise are in the frequency range 2-15kc

  graphs showing distributions of snapping shrimps around La Jolla, California in relation to water depth and substratum
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Research study 2.2
 

diagram of cocking mechanism of a shrimp Alpheus californiensisThe snapping claw’s cocking mechanism is described for Alpheus californiensis by a researcher at the University of Virginia.  The opener muscle in the propus contracts and raises the dactyusl portion into the fully open or cocked position.  At this time opposing discs on the 2 segments meet precisely and join (see drawing).  The exoskeleton forming each disc is smooth and hard, but that of the propus is covered with a soft epicuticle that molds itself to the surface of the opposing dactylus disc when the discs come together.  So, what holds the discs together? There are no tiny latching devices, a vacuum mechanism is not involved (actual force measured is an order of magnitude greater than a theoretical vacuum-generated force), nor is there evidence of any sort of temporary adhesive. Rather, the author proposes that the discs are held together by cohesive forces of water between the 2 surfaces.  When the closer muscle in the propus contracts, the discs remain joined until the tension rises sufficiently to overcome these cohesive forces.  At release, the closer muscle acts like a stetched spring and causes the dactylus to snap closed with great speed and force.  Ritzmann 1973 Science 181 (4098): 459.

NOTE  a small scratch on either of the discs completely disrupts the ability of the shrimp to keep the dactylus open.  The epicuticle appears to be secreted by subcuticle tegumental glands via pores opening onto the disc surface

NOTE  cohesive forces are those, such as hydrogen bonding, that act between molecules causing them to resist separation. Rain falls as droplets because this strong cohesion pulls the molecules tightly together.  But, in the case of the discs being pressed together in the shrimp, there must also be adhesion generated, perhaps by electrostatic forces, between the water and the exoskeletal surfaces. This aspect is not addressed by the author

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Research study 3
 

photographs illustrating the snapping action of a snapping shrimp Alpheus heterochaelisA later description by researchers in Chicago for Alpheus heterochaelus from the Gulf of Mexico, involves a cocking of a plunger-like portion of the dactylus section of the claw by strong muscles (see illustration).  The plunger momentarily locks in place and soon comes under considerable tension from opposing contracting muscles.  When the “snapping point” is released, the plunger snaps into a pocket on the propus portion of the claw.  The cocking mechanism appears to differ in different species, but the easiest to visualise is a pair of opposing protuberances that temporarily interlock until a large closer muscle builds sufficient tension. The snapping action is exceedingly fast, requiring only 300µsec1 .  The pressure of the pounding plunger or mallet creates a high-velocity water jet that squirts from the claw.  The water-jet velocity is high enough to cause an instantaneous drop in pressure which, in turn, causes tiny micro-bubbles in the seawater to come out of solution. These instantly coalesce into a bubble of about 3-4mm in diameter (for a large shrimp such as the one illustrated) that violently implodes when it contacts higher pressure, thus creating the snapping sound.  On collapse, the bubble breaks into a cloud of small bubbles that quickly re-dissolves.  The snap, powerful enough to break a scratched aquarium jar2  , is known to be involved in stunning prey and perhaps in deterring predators but, as these activities would seem unlikely to create such a continuous underwater din, a more likely additional function is intraspecific communication, perhaps in attracting females3 or warning off competing males.  Versluis et al. 2000 Science 289: 2114; see also Ritzmann 1974 J Comp Physiol 95: 217 for a neurophysiological account of the snapping process in Alpheus californiensis. Photograph courtesy Kevin Lee, Fullerton, California diverKevin. For an effective, if somewhat popularised, account of how snapping is used to stun a prey by A. heterochaelis, see the video SNAPPING SHRIMP.

photograph of a sculpin with shrimp in mouth, courtesy Kevin Lee, Fullerton, CaliforniaNOTE1  note that this is NOT in msec: 300msec is about one-third of a second; 300µsec is 1,000 times faster.  The researchers use a high-speed video camera to record at over 40,000 frames per second

NOTE2  this is described on p. 277 of MacGinitie & MacGinitie 1968 Natural history of marine animals. McGraw-Hill Book Co. 523pp.

NOTE3 an earlier study on Alpheus heterochaelis at Duke University Marine Laboratory, Beaufort, NC shows that larger males have larger claws with a proportionately larger “snap”.  Data from field-caught mating pairs reveal that these larger males mate with larger females, presumably leading to greater production of offspring.  Since the shrimps are generally more active at night, then power of snap will reveal not only reproductive “attractiveness”, but also competitive ability.  Schein 1975 Mar Behav Physiol 3: 83. 


Sculpin with a snapping shrimp Alpheus bellimanus that didn't get away. The
species name bellimanus means "beautiful hand" in Latin, and these claws are
visible along with the shrimp's orange antennae protruding from the mouth 1X

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  Defenses: camouflage
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Research study 1
 

photographs showing pattern disruption in shrimps Crangon handiIn an early description of a new species Crangon handi in northern California the authors describe several colours produced by chromatophores, including white, black, brown, green, orange, purple, pink, and yellow.  They remark that the resulting disruptive pattern blends in “superbly with its surrounding coarse sand substrate” (see photos).  However, would simple cryptic camouflaging require such a sophisticated array of chromatophore types?  One guesses that the colours may have multiple functions, possibly along the lines proposed for other species of colorful shrimps (see Research Studies to follow). Kuris & Carlton 1977 Biol Bull 153: 540.

NOTE  lit. “colour” “carry” G., referring to special cells containing pigments.  The pigments can be contracted within the cell to make them less obvious, or dispersed to make them more obvious

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Research study 2
 

drawings of several chromatophore types in the shrimp Heptacarpus pictisdrawing of striped colour-morph of shrimp Heptacarpus sitchensisThe marvelous colour polymorphisms exhibited by some species of shrimps, such as Heptacarpus (pictis) sitchensis and H. paludicola, suggest a defensive function, perhaps crypsis camouflaging or disruptive camouflaging.  Both species have 5 basic colour morphs: 1 transparent and 4 coloured (see drawings and photographs below depicting several of these morphs in H. sitchensis.  Some colour patterns in Heptacarpus are composed of bands, stripes, and spots that appear to disrupt the body outline.  Development of the patterns is related to size (age) and sex.  For example, in populations of H. sitchensis from Cayucos, California and H. paludicola from San Juan Island, Washington, prominent coloration is a characteristic of maturing and breeding females, and of larger males. The colours are caused by movement of pigments within chromatophores located in the hypodermis beneath the cuticle, which is itself transparent.

Both species have 4 basic chromatophore types: red-white, red, yellow, and red-yellow but other colours are generated depending on the relative amounts of paired pigments (see diagram on Left).  For example, note in the diagram that in the basic red-yellow type, which is aquamarine, a more dense packing of red pigment gives a red-brown colour, while a more dense packing of yellow pigment gives a brown colour. Combined with pigments in some cells are crystals that reflect white, and variable extents of these and the red pigment create colours of white to shades of pink. At night H. pictus is transparent blue or aquamarine, and normal daytime coloration is restored within 15min of exposure to constant light.  The author remarks that each colour morph has a common environmental colour in its pattern. For example, there is the green of green algae, white and pink of dead and living coralline algae, and various shades reflecting tidepool litter. The author suggests that because the shrimps appear to be under heavy predation pressure by fishes, the daytime colour patterns may act as disruptive camouflage to hide the shrimps from these visually-hunting predators.  At night, especially if the shrimps were to swim up off the bottom, the transparency may be an adaptation for silhouette concealment.  Bauer 1981 Mar Biol 64: 141. Photographs courtesy Raymond Bauer, U Louisiana, Lafayette, Louisiana.

NOTE  the term chromatosome is used by the author to denote general colour units in shrimps and perhaps other organisms. A chromatosome may comprise one or more chromatophores, or pigment-bearing cells, sometimes bearing reflective white crystals. Thus, a red-white chromatosome in Heptacarpus is made up of a red pigment-bearing chromatophore with white crystals. The colour produced will vary from red through mauve to white depending upon the extent of dispersion of the red pigment

   
 

photograph of colour morph of shrimp Heptacarpus sitchensis. courtesy Raymond Bauer, U Louisiana, Louisiana
Bands and stripes in this morph of Heptcarpus sitchensis possibly function in disruptive camouflage

photograph of colour morph of shrimp Heptacarpus sitchensis. courtesy Raymond Bauer, U Louisiana, Louisiana
Colour units in this morph are red & white chromatophores, camouflaging the shrimp on a coralline algae background
photograph of red-white chromatosome in a shrimp Heptacarpus sitchensis courtesy Raymond Bauer, U Louisiana, Louisiana
The purple shade are produced from combinations of red astaxanthine pigment & white crystals
 
drawing of colour morph of shrimp Heptacarpus sitchensis
This morph is a variant of a speckled version. The green colour is produced from yellow chromatophores beneath the blueish-coloured exoskeleton
drawing of one of the many colour morphs of the shrimp Heptacarpus sitchensis
This morph is primarily shades of green produced by yellow chromatophores, with bands of red produced by red chromatophores as shown in the next figure on the Right
photograph of red/yellow chromatosome of the shrimp Heptacarpus sitchensis courtesy Raymond Bauer, U Louisiana, Louisiana
Red coloration comes from red or red/yellow chromatosomes. The actual colour, reddish-brown to black, depends upon degree of pigment dispersion
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Research study 3
 

With such a broad repertoire of colour variation, is there any evidence that Heptacarpus actually colours itself to match a background for cryptic camouflaging? In a later study at Friday Harbor Laboratories by the same author on collections of H. sitchensis and H. paludicola from San Luis Obispo, California reveal highly variable colorations over location and time, especially in the latter species, but a shrimp’s color and the background colour of the substratum on which it is found are not correlated. In the field the shrimps do not remain on one type of substratum for long periods; rather, they move about frequently.  Laboratory experiments additionally show that the shrimps do not change their colours to match those of test backgrounds.  The author remarks that in many respects the colours appear more to be disruptive, rather than concealing.  The study further reveals that there is no significant rapid colour change during the day, but at night the pigments concentrate in the chromatophores and produce marked colour changes.   The author suggests that the colour variations, especially in H. paludicola, may result from apostatic selection, a circumstance in which a predator preys disproportionately on a common colour morph, allowing the rarer morphs to be protected until the predator is forced to switch.  This selective process maintains the different colour morphs in the population. Bauer 1982 Mar Behav Physiol 8: 249.

NOTE  the  author makes a point that the shrimps do not seek out backgrounds that “would seem to be a matching background for concealment, thus acknowledging that what we see may not be what the shrimp sees, or what a predatory fish may see.  In this regard, recent studies on fish coloration in the tropics indicate that there are strong ultraviolet components in many of the colours, thus raising the possibility of a hidden world of colours and colour-messages not perceived by humans

NOTE  when a pigment concentrates in a chromatophore, its colour blanches. Conversely, when a pigment disperses in a chromatophore, the colour is revealed. Concentration of red pigment in a red/white chromatosome, then, leads to a general whitening of the body

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Research study 4
  photographs of anemone Cribrinopsis fernaldi and its symbiotic shrimp LebbeusThe colorful candy-stripe shrimp Lebbeus grandimanus commonly inhabits the tentacles of the sea anemone Cribrinopsis fernaldi. The anemone is crimson, pink, or white in colour, while the shrimp is quite gaudy. The shrimp crawls among the tentacles with complete impunity. This may or may not be camouflage defense, but the shrimp presumably must benefit from the protection conferred by its host's nematocysts. Perhaps it is a form of warning coloration. No research appears to have been done on the relationship. Photograph of shrimp courtesy Dave Cowles, Walla Walla University, Washington wallawalla.edu
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Research study 5
 

photograph of shrimp Lissicrangon stylirostrus camouflaged on bottom gravel, courtesy Kuris & Carlton 1977 Biol Bull 153: 540Bay shrimps Lissocrangon stylirostris in southern Oregon are preyed on by English sole and staghorn sculpins.  An ability to camouflage against bottom clutter may help defend the shrimps against these fishes and other visual predators.  Jarrin & Shanks 2008 J Crust Biol 28: 613.  Photograph courtesy Kuris & Carlton 1977 Biol Bull 153: 540.


 

Shrimp Lissocrangon stylirostrus camouflaged on a
gravel substratum in a laboratory aquarium 0.8X

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