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  Predators & defenses
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  Camouflage
 

Defenses include camouflage, considered in this section, and PROTECTIVE EXOSKELETON & FAST RUNNING, SWIMMING, and HIDING/CLINGING/BURYING/NOCTURNALISM considered in other sections. Some species of terrestrial isopods have repugnatorial glands (not dealt with here).

 
Research study 0
 

graph showing diurnal rhythm in chromatophore activity in the isopod Ligia occidentalisgraph showing chromatophore responses in the isopod Ligia occidentalis to different backgroundsA study at Hopkins Marine Station, Pacific Grove, California provides information on pigment responses and functional significance of chromatophores in the semiterrestrial isopod Ligia occidentalis.  This species, unlike its more nocturnal congenor L. pallasii, tends to be active during daytime hours.   According to the author, it has both black (melanophores) and yellow (xanthophores) chromatophores, and both types exhibit a diurnal rhythm, contracting during the night and expanding during the day (see graph on Left).  This pattern is maintained for a period of about a week by individuals kept continuously in a dark box, after which it diminishes in a day or two.  By this time the yellow xanthophores are becoming occluded by deposition of melanin pigments.  If maintained for 6h in constant light on either yellow or black backgrounds, the respective chromatophores either expand or contract in accordance with the background the animals are on to produce a camouflaging effect.  Thus, on a yellow background the xanthophores expand and the melanophores contract, and on a black background, the opposite occurs (see graph on Right). When individuals after 6h are shifted to the other background, the chromatophores change accordingly (although not so responsively).  If maintained on a certain background for several days, there is a tendency for the background-matching chromatophore type to begin to overgrow the other chromatophore type by greatly extending its pigment-carrying processes.  The author considers the chromatophore changes to be of adaptive value in camouflaging.  Armitage 1960 Crustaceana 1 (3): 193.

NOTE  in the graphs a value of 1 on the ordinate axis "Chromatophore stage" signifies full contraction of the chromatophores and 5 signifies full expansion.  Chromatophore activity in other crustaceans, such as crabs and shrimps, is controlled by neurosecretions released from the eyestalk and in shrimps, which may have several different chromatophore types, it is thought that each is controlled by a different neurosecretion

 

These 3 views of Ligia occidentalis strongly suggest a camouflaging strategy.

These ligiid isopods have yellowish pigment deposits under their cuticles, a feature not mentioned in the above research paper. Based on their consistent relative sizes and locations, these deposits would seem to be a genetic feature of the species. The other west-coast ligiid species L. pallasii, also has such fixed deposits (see example photo in Research Study 2 below). There appears to be a good research project waiting to be done on this subject. Photographs courtesy Jackie Soanes, Bodega Marine Laboratory, California

photograph of sea slater Ligia occidentalis on a reddish background, courtesy Jackie Soanes Bodega Marine Laboratory, California photograph of sea slater Ligia occidentalis on a background of barnacles, courtesy Jackie Soanes Bodega Marine Laboratory, California photograph of sea slater Ligia occidentalis submerged in a tidepool, courtesy Jackie Soanes Bodega Marine Laboratory, California
  A background of red encrusting algae 1.5X A background of barnacles 1.5X Submerged in a small tidal channel 1.5X
 

This photograph shows a newly emerged Ligia exotica from Hawai'i with transparent exoskeleton,
and is included here to illustrate pigment deposit, chromatophore, and body-tissue colours. This
Ligia
species appears to have only black chromatophores, and body colours vary from grey to
blackish. The small yellowish speckles may be pigment deposits, but it is not know what happens to
these in the adult. Visible through the transparent exoskeleton is the tan-coloured gut systewm.

INSET: close view of melanophores and yellow-speckled pigment deposits

photograph of isopod Ligia sp. newly emerged from the brood chamber showing nice chromatophores
 
Research study 1
 

photograph of isopod Idotea montereyensis camouflaged on a blade of surfgrass Phyllospadix scouleriIn wave-exposed rocky coastal areas such as around Dillon Beach, California the isopod Idotea montereyensis inhabits red algae in protected pools and channels, and surfgrass Phyllospadix scouleri in the lower intertidal regions (see photo on Left).  Larger, reproductively mature individuals hang out on surfgrass, while smaller, younger individuals that cannot hold on to the Phyllospadix blades live in the red algae. 

The isopod occurs in red, green, and brownish colour varieties that usually match the colour of the plant on which it is found.  A red animal can range from dark red to pink, depending upon the colour of alga that it is associated with.  Green animals vary from dark green to yellowish, matching either the dark blades of Phyllospadix or its almost yellow roots.

Colour change in Idotea requires light and is mediated by 3 means: 1) pigmentation in the cuticle, 2) chromatophores in the dermis, and 3) colour of body tissues (see photo of a juvenile Ligia below which shows examples of chromatophores).  Chromatophores are mainly red, with a few white and black ones.  Cuticle pigments are green, red, and brown.  While chromatophores expand or contract in about 30min, change in cuticular pigments requires a much longer time. A red individual has red cuticular pigments, fully expanded red chromatophores in the dermis, and dark yellow tissues.  A green individual has green cuticular pigments, contracted chromatophores, and light yellow tissues.  Brown individuals have brown or green/red cuticular pigments, variously expanded red chromatophores, and yellow tissues.  The animals readily change from one colour to another with change in habitat.  At moult some of the colour is lost with the cast-off exocuticle, and new pigment to match the substrate colour is added during hardening of the new cuticle.  As well, pigment can be added during intermoult to a new cuticle layer secreted under the old.  Colour change takes 2-4wk depending upon duration of moult cycle that itself depends upon age of the individual.  The extent (and direction) of colour change is made more obvious by the fact that the posterior half of the cuticle is shed first and then, a day or two later, the front half.

During the year there is a massive exchange of individuals between the Phyllospadix and red-algal populations.  Young that cannot hold on to the surfgrass blades are swept inshore, while adult animals in the red algae move to the surfgrass where there is presumably less competition.  The red-algal habitat provides a protected area for development and growth of the juveniles.  The ability to change colour to match the substratum colour allows Idotea to utilise a wider variety of intertidal plants, with greater safety from predators, than would be possible without colour change.  The author concurs with other researchers that the various colours likely function as camouflage from visual predators such as fishes and birds.  Lee 1966 Ecology 47: 930; see also Lee 1966 Comp Biochem Physiol 18: 17.

NOTE  the solid green colour is produced by a mixture of a blue canthaxanthin-protein in the cuticle, and a yellow lutein in the cuticle and body tissues

 
Research study 2
  No experiments appear to have been done on the function of colour-camouflaging in intertidal/supratidal-inhabiting west-coast isopods. Here are a few examples of possible camouflaging, including 2 views of a single Idotea montereyensis on surfgrass as discussed in Research Study 1 above. Of course, in the context of protection from predators, whether an isopod is camouflaged or not depends on the species that may be hunting it for food. Birds use sight almost exclusively to locate prey, while fishes use both sight and scent. While birds are more likely to see things as humans see them, fishes may be receiving different images depending upon their extent of ultraviolet vision. If this is the case, then what a fish perceives may be quite different than what we perceive in the images below.
 
photograph of isopod Idotea montereyensis camouflaged on a blade of surfgrass Phyllospadix scouleri
Idotea montereyensis on surfgrass Phyllospadix, 2 views 0.5X
photograph of isopod Idotea wosnesenskii camouflaged on rockweed Fucus gardneri
Idotea wosnesenskii camouflaged on rockweed Fucus gardneri 1X
photograph of isopod Idotea sp. camouflaged on coralline algae Serraticardia macmillanii
Idotea sp. on coralline alga Serraticardia macmillanii 0.7X
photograph of isopod Idotea sp. crawling on brown kelp Macrocystis integrifolia
Idotea sp. crawling on brown kelp Macrocystis integrifolia

Ligia pallasii crawls in its natural intertidal habitat 0.7X
 
Research study 3
 

photograph of isopod Idotea montereyensis courtesy Lisa Needles In a follow-up investigation to Research Study 1 above the author provides more information on the colour-matching process and its possible role in camouflaging in Idotea montereyensis.  There are 3 colour varieties, namely, red, green, and brown, and individuals of each variety can be found on similarly coloured algae.  Chromatophores are of 3 types (red, black, and white), and body colours represent a combination of pigments in the cuticle and/or differential expansion and contraction of chromatophores.  Animals switched to differently coloured substrata change colour within 30min, the extent of change beng directly related to the level of light reflected from the substratum.  A green individual is green because of other types of pigments in its cuticle (see Research Study 1 above).  When a green isopod is placed on a green background it contracts its chromatophores to punctate form and the green cuticular coloration is shown to full advantage.  Later the chromatophores expand to a state little different than before the isopod was transferred, but the overall colour remains green.  In contrast, red coloration is obtained by expansion mainly of red chromatophores; again in response to the amount and wavelength of light reflected from the substratum.  Differential expansion and contraction of the red (and possibly other) chromatophores allows red individuals to adapt their coloration to a wide variety of different shades of red algae.  In the words of the author, the isopods can match their colour to that of the background “with astonishing accuracy”.  A red animal can more easily adapt to a green background than a green one can to a red colored substratum.  At night, pigments tend to be more concentrated in the chromatophores in response to the decreased amount of light reflecting from the substratum.  Lee 1972 J Exp Mar Biol Ecol 8: 201. Photograph courtesy Lisa Needles.

NOTE  known as an albedo response

Red/brown-colored Idotea montereyensis 1.5X

 
Research study 4
 

Another study at Hopkins Marine Station, Pacific Grove, California provides comparative data on colour-camouflaging in the related isopod Idotea resecata.  On the coast of California this species occurs in 2 colour varieties, matching the colours of its 2 main algal substrata, the brown kelp Macrocystis pyrifera and the green eelgrass Zostera marina.  At no time do the authors find the 2 colour-morphs intermixed among the 2 plants.  As found for Idotea montereyensis, I. resecata also uses both cuticular pigment deposits as well as chromatophore colours to modify its overall colour.  Green individuals invariably have punctate (i.e., pigment is concentrated) chromatophores, with bright green cuticles and light yellow internal tissues.  In comparison, brown individuals have moderate to fully expanded chromatophores, brown cuticles, and dark yellow internal tissues. The brown colour actually results from layers of pigments - bright red pigments in the exocuticle and green pigments in the endocuticle.  The combination to our eyes exactly matches the colour of the brown kelp Macrocystis.  Unlike in other idoteid species, colours do not change in response to change in background colours – a brown individual on Zostera and a green individual on Macrocystis maintain their original colours for up to a month, at least.  To explain this the authors suggest that the 2 differently coloured populations may be distinct physiological (or biochemical) races, a suggestion that might be worth some follow-up research.  Lee & Gilchrist 1972 J Exp Mar Biol Ecol 10: 1.

NOTE  in I. resecata the cuticular colour results from deposition of several types of carotenoids and carotenoproteins.  Pigments identified include alpha-carotene, beta-carotene, echinenone, mono-hydroxy-b-carotene, canthaxanthin, 4-hydroxy-4—keto-b-carotene, lutein, zeaxanthin, flavoxanthin, and violaxanthin.  In no case does any idoteid species studied utilise plant pigments (e.g., red phycoerythrins, green chlorophyll, or brown fucoxanthins) to produce their colours. The major pigments in all 3 species of idoteids studied by the researchers are xanthophylls and canthaxanthin.  Change from brown to red to green in all species involve similar trends in pigment changes, namely, 10% increase in xanthophylls accompanied by decreases in canthaxanthin and echinenone.  Green representatives of both species notably have green carotenoproteins in their exo- and endo-cuticles. The authors use 14C-labelling techniques to study metabolic-conversion pathways of several of these pigments

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