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  Predators & defenses
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Swimming

  Defenses of isopods involving swimming, are discussed in this section, while other defenses including PROTECTIVE EXOSKELETON & FAST RUNNING, HIDING/CLINGING/BURYING/NOCTURNALISM, and CAMOUFLAGE considered in other sections. There has been no research done on swimming as a defense in west-coast isopods.
 
Research study 1
 

photograph of a sea slater Ligia pallasiiWhile it is unusual for Ligia species to enter the ocean willingly, when they do after being chased or after being washed off a rock by waves,, they are effective swimmers, using a kind of butterfly stroke, combined with vigorous beating of the pleopods.  At 15oC Ligia pallasii can swim at 3 body lengths . sec -1 for about 20sec.  For a large male this is equivalent to about 12cm . sec -1. The swimming path is straight, but the isopod undulates in a vertical plane with each propulsive stroke of the body.  All other terrestrial isopods have lost the ability to swim. Carefoot & Taylor 1995 p. 47 In, Terrestrial Isopod Biology (ed, Alikhan)  AA Balkema, Rotterdam.

 

 

A large male Ligia pallasii crawls on a piece of kelp 1.3X

 
Research study 2
 

photograph of isopod Ligia occidentalis under water courtesy Jackie Soanes, UCDavis and Bodega Bay Marine Laboratory, California
When underwater, Ligia species appear able to use vision to find their way back to the shore.  This is unlikely to involve visual recognition of bottom features; rather, it may be simple discrimination between the darker shore image versus the lighter sea image.  If experimentally released into the sea in shallow depth Ligia turns towards the darker shore and swims towards it and downwards.  On contact with the bottom, it then crawls up the slope to the shore. If released into the sea at mid-day further offshore where the sea bottom is indiscernible (to the human eye), Ligia’s swimming orientation is random if the sky is cloudy, but away from the sun if the sky is clear.  If released into the sea at night in total darkness, Ligia sinks to the bottom and, if it contacts a sloped shoreline, crawls up the slope until it emerges. See also Taylor & Carefoot 1990 p. 121 In, The Biology of Terrestrial Isopods III (Eds. Juchault & Mocquard) U Poitiers, Poitiers, France. Photo courtesy Jackie Soanes, UCDavis and Bodega Bay Marine Laboratory.


Ligia occidentalis in a tidepool at Bodega Bay, California 0.6X.
There is an isopod-looking moult in the water at the upper left,
and it may be that this individual moulted while underwater

 
Research study 3
 

photo/schematic of branchial chamber of isopod Idotea wosnesenskiiSpecies of Idotea are capable swimmers, but appear to swim only drawing of swimmining posture of isopod Idotea wosnesenskii from belowwhen dislodged from their rock and algal substrates, and not specifically as an escape from predators (although little is known on this subject).  In a study on Idotea wosnesenskii and I. resecata at Friday Harbor Laboratories, Washington the author uses high-speed videography to determine swimming kinetics of the pleopods. The pleopods, 5 pairs in total, are protected by 2 large plates, or operculae (see schematic on Right). These are held open when swimming. Each pleopod is double. Pleopods 1-3 are used for swimming and gas exchange, while pleopods 4-5 are used strictly for gas exchange. Both species swim on their backs with the anterior 3-4 pairs of walking legs folded neatly inwards and the remaining posterior pairs held outwards, possibly for stability (see drawing on Left).

graph showing angle of power strokes of pleopods 1-3 of isopod Idotea wosnesenskii

 

 

Power strokes occur sequentially, commencing with the 3rd pleopods and ending with the 1st pleopods.  The 3rd pleopods have a short power stroke, the 2nd pleopods an intermediate power stroke, and the 1st pleopods a long power stroke (see graph on Left). Each pair of pleopods also sweeps a wider arc than the one before, perhaps adding to gas-exchange efficacy. graph of swimming speeds of isopods Idotea resecata and Idotea wosnesenskiiAt the end of the power strokes there is a pause, followed by simultaneous recovery of all 3 pairs. Each pair of biramous pleopods acts as a unit, so the stroke has the appearance of a 4-bladed fan. During recovery the 4 components compress inwards and overlap to form a more hydrodynamically streamlined shape.  This reduces drag on the recovery stroke, estimated by the author to “to negligible levels”. 

Stroke amplitudes of the 3 pairs are more or less constant, while stroke frequencies are proportional to swimming speed (see graph on Right). The author suggests that the unusual stroke pattern may be related to the gas-exchange function of the pleopods.  Alexander 1988 J Exp Biol 138: 37; see also Alexander et al. 1995 Invert Biol 114: 169.

NOTE  the number and position of legs folded inwards varies with species: in I. wosnesenskii leg-pairs 1-3 are folded inwards, while in I. resecata leg pairs 1-4 are folded in

 
Research study 4
 

graphs of body width and mass of isopods Idotea wosnesenskii and Idotea resecata in relation of body lengthFurther comparison at Friday Harbor Laboratories, Washington of swimming speeds in the isopods Idotea scatter plot of swimming speeds of isopods Idotea resecata and Idotea wosnesenskii over a range of body masseswosnesenskii and I. resecata indicates that the former is significantly slower than the latter (12 vs. 21cm . sec -1, respectively, for individuals of similar size; see graph on Left). Interestingly, in neither species does swimming velocity correlate significantly with body mass. 

Habitats and habits of life differ for the 2 species, with I. wosnesenskii usually being found clingling to or crawling amongst intertidal algae such as Fucus and rarely, if ever, swimming, and I. resecata usually being found clinging to eelgrass blades (Zostera spp.) or algae and swimming often.  Idotea wosnesenskii has a much more bulky and less streamlined shape than I. resecata (see graphs on Right. Alexander & Chen 1990 J Crust Biol 10: 406; line drawings courtesy Brusca et al. Guide to coastal marine life of California and Tree of life.

NOTE the authors present standard-error bars in their original data presentation but, as these are calculated on replicate runs on the same individuals and, in 5 instances, combine data for individuals tested on different days, they have been removed in the graph shown above

 
Research study 5
 

Any object moving through a fluid medium, like a swimming isopod, generates hydrodynamic drag,  expressed as a drag coefficient.  Such drag coefficients are usually calculated based on either total surface area or maximum cross-sectional area (i.e., frontal area), and there is apparently no uniform convention as to which is used.  An implicit assumption in studies of swimming animals apparently is that either reference area will provide a useful estimate of drag.  However, a comparison of 2 swimming isopods Idotea wosnesenskii and I. resecata in San Juan Islands, Washington shows that drag coefficients differ considerably between the 2 species depending on which reference area is used.  Thus, the drag coefficient for I. wosnesenkii based on total surface area is significantly greater than that for I. resecata, but the reverse is true for drag coefficients calculated on the basis of frontal area. The differences relate to the different body shapes of the 2 species: I. wosnesenskii is blunter and shorter, while I. resecata is more elongate (see line drawings in Research Study 4 above). The author concludes by suggesting that, given no strong basis for choosing either reference area, data be provided for both when swimming animals (i.e., complex shapes) are being compared.  Alexander 1990 Biol Bull 179: 186; for more on drag coefficients in these isopods, see Alexander & Chen 1990 J Crust Biol 10: 406.

NOTE  this is a function of Reynolds Number, a dimensionless ratio of inertial resistance to viscous resistance for a flowing fluid, named for the British scientist Osborne Reynolds (1842-1912)

NOTE  a major difference in the 2 reference areas, and one reflecting popularity of use, is that frontal area is easier (and more accurate) to measure than total surface area

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