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  Predators & defenses
  Predators of intertidal and semiterrestrial isopods include birds, fishes (sculpins, greenlings, salmon, and ling cod), crabs, and beetles. 
 
Research study 1
 

In one study in southern California a single tidepool after poisoning with rotenone yields over 200 individual fishes representing 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., amphipods several species, and decapods chiefly shrimps Spirontocaris picta and Crangon sp., as well as some polychaetes Platynereis agassiziMitchell 1953 Am Midl Nat 49: 862.photograph of oniscid isopod Armadillidium vulgare

NOTE  another California study shows surprisingly that the non-native isopod species Armadillidium vulgare provides about 12% of total food-energy consumed by salmon Oncorhynchus mykiss in 2 streams in the Big Sur region over 17mo of their life history.  The surprise comes not in the fact that the isopods are eaten by the young salmon, but that so many fall or are blown by the wind into the streams.  If not eaten, the isopods should be able to tolerate immersion for a few hours while possibly attempting to regain the stream bank.  Rundio & Lindley 2008 Trans Am Fisheries Soc 137: 467

 

Oniscid isopod Armadillidium vulgare 3X

 

 
Research study 1.1
 

Observations on fish predation on about 20 taxa of zooplankters in nearshore waters of Santa Catalina Island, California reveal that all of the 6 major fish species involved in the study (rockfishes 2 spp., salema, queenfishes, surf perches, kelp perches) subsist to at least some extent on isopod prey, including Exosphaeroma, Pentidotea, Cirolana, and others.  About 95% of the predation on isopods occurs at night versus only 5% during the day.  The authors provide day- and night-feeding data for the fishes for all taxa.  Hobson & Chess 1976 Fishery Bull 74 (3): 567.

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Protective exoskeleton & fast running Defenses include protective exoskeleton & fast running, considered in this section, and SWIMMING, HIDING/CLINGING/BURYING/NOCTURNALISM, and CAMOUFLAGE considered in other sections. Some species of terrestrial isopods have repugnatorial glands (not dealt with here).
 
 
Research study 2
 

photograph of a semiterrestrial isopod Ligia pallasii
The exoskeleton of isopods may function for protection in 2 ways. First, it provides a tough, physical barrier against potential predators and, second, its sheer bulk in relation to soft tissues must present a less-than-nutritious "package" to a predator. No research has been done on these matters.

Most intertidal isopod species are slow-moving.  However, semiterrestrial ligiids Ligia pallasii and L. occidentalis, are quick runners, attaining velocities of 2-3 body lengths . sec -1.  When running from a perceived threat and confronted by a vertical drop of any height, both species unhesitatingly fly off into space, land, right themselves, and continue running.

NOTE the funemushi of Japan, L. exotica, are much faster at  4-5 body lengths . sec -1, but are by no means the fastest of the 30 or so species of Ligia.  Taylor & Carefoot 1990 p. 121 In, The Biology of Terrestrial Isopods III (Eds. Juchault & Mocquard) U Poitiers, Poitiers, France.

 
Research study 3
 

drawings illustrating eye position on the body of an isopodWhat sort of vision do isopods have?  As in all arthropods, eyes are of the compound type, that is, are comprised of multiple units called ommatidia.  An adult Ligia has 700-1500 ommatidia in each eye, depending upon species.  The eyes are sessile, uniformly black, and occupy almost the entire lateral region on either side of the head. Drawing of L. exotica on Left from Keskinen et al. 2002 Biol Bull 202: 223.

drawing showing morphology of a single ommatidium of isopod LigiaEach ommatidium (see diagram on Right) in an adult eyes is 40-90µm in diameter (depending upon species) and the visual angle is about 30o Ligia’s vision is up to 10 times more sensitive at night than in the day.  Part of this owes to differential movement of pigment granules within each ommatidium.  During the day the granules cluster in such a way as to block light from impinging on the retinal cells.  At night the pigment granules disperse, allowing more light to strike the retinal cells.  The authors attribute this adaptation, in part, to the ability of Ligia to perform complex escape behaviours in the presence of predators in both bright and dim light.  Each ommatidium theoretically produces a single image in the brain, but current thinking on arthropod vision is that these multiple images are resolved into a single image. No estimates of visual acuity in an isopod are available. Ligia species are, however, fast-moving and, according to the authors, have “keen” eyesight. Diagram of ommatidium from Hariyama et al. 2001 J Exp Biol 204: 239 for Ligia exotica in Japan; see also Edwards 1969 Tissue & Cell 1: 217.

NOTE  lit. “eyes” G. These observations and drawings are for Ligia exotica in Japan. This species, unlike the west-coast Ligia pallasii or, to a lesser extent, L. occidentalis, is active both day and night

NOTE  this angle represents the optical “catchment” of each ommatidium


A single ommatidium of Ligia has only 7 retinal cells, each 100um in length,
shown here as if in daylight. Five of these are maximally sensitive at 520nm
(in the middle, green part, of the visible spectrum for humans), while the other 2
are maximally sensitive at shorter wavelengths (i.e., towards the red end of
the spectrum). Pigment granules visible on the inner sides of the retinal cells
migrate depending on time of day. During nighttime, the granules disperse into the
cytoplasm of the retinal cells, thus allowing more light to impinge on the cells.

 
Research study 4
 


In addition to spatial resolution, another important photoreceptor property that needs to be known to assess Ligia’s visual system is temporal resolution.  This is defined by flicker-fusion frequency (FFF), which is a measure of “quickness” of image processing.  It is the freqency at which the “flicker” of a repeating image cannot be distinguished (the value for humans in bright light is 60 . sec -1).  It is generally high in terrestrial and rapid-moving, daytime-active species, and lower in aquatic organisms.  Only one value for FFF is reported for ligiids, that of 120 . sec -1 for Ligia occidentalis in California.  The value is close to that recorded for some insect eyes, and apparently is unusually high for a crustacean.  Ligia’s eye, then, is highly sensitive to movement – a useful feature in avoiding attack by shorebirds and other fast-moving predators.  Ruck & Jahn 1954 J Gen Physiol 37: 825. Photo courtesy Jackie Soanes, Bodega Bay Marine Laboratory, California

NOTE  comparable FFF values are 250-300 for honeybees, 70 for octopuses, and 300 for a fly. This explains, in part, why it is so hard to smack a fly

Ligia occidentalis 1.8X

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