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  Physiological ecology
   
  Aspects of the physiological ecology of shallow-water marine isopods are considered here, while the special physiological challenges that confront semiterrestrial forms are dealt with elsewhere in the ODYSSEY: EVOLUTION TO LAND.
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Gas exchange

  Gas exchange is considered here, while topics of OSMOTIC REGULATION and BEHAVIOUR are dealt with in their own sections.
 
Research study 1
 

photo/drawing composite showing location of gas-exchanging pleopods in an isopod Idotea wosnesenskiiThe 5 pairs of pleopods on the ventral surface of the abdomen in marine isopods function both for locomotion (swimming) and gas exchange.  A pair of large opercula (modified uropods) protects the pleopods and together these units form a branchial chamber.  The pleopods beat cyclically to ventilate the chamber. The question arises as to whether all pleopods function in gas exchange, or is there a division of labour between them?  Put in another way, have the swimming pleopods retained or lost their gas-exchange capability?

This is examined in Idotea wosnesenskii at the Oregon Institue of Marine Biology by removing sets of pleopods (1-3 and 4-5) in different animals, and measuring before-and-after ventilation and oxygen-consumption
(VO2) rates. To control for effects of removing appendages the authors create a control group with a pair of walking legs removed in the same way that the pleopods are removed.histogram showing effect of pleopod removal on oxygen uptake in isopods Idotea wosnesenskii

Results show that removal of the 3 pairs of swimming pleopods (1-3) does not significantly affect oxygen consumption (only 2% reduction), while removal of the 2 pairs of gas-exhanging pleopods (pairs 4-5) significantly reduces oxygen consumption by 24% (see graph on Left).  Removal of a pair of walking legs in control animals does not significantly affect oxygen consumption (4% reduction). Note that ventilation rate of Pleopods 4-5 increases significantly after removal of the swimming pleopods (1-3), suggesting that these (Pleopods 1-3) also play an important role in ventilating the branchial chamber.  The answer to the authors’ research question, then, is that a division of labour with respect to gas exchange does exist in Idotea. Alexander & Chen 1989 Comp Biochem Phys 94A: 689.

NOTE  each pleopod is actually double, being made up of an exopodite and endopodite, so there are actually 10 flapping plates plus a single operculum on each side of the abdomen

NOTE  appendages are removed by first crushing at the base, then snipping off with scissors – a method that seems to reduce hemolymph loss

 
Research study 2
 

photograph of isopod Idotea wosnesenskii crawling on kelpFurther studies on Idotea wosnesenskii collected around Charleston, Oregon show that the ventilation is a 2-cycle operation. At the start of a beat cycle, the pleopods move medially and ventrally, and water is sucked in between them.  When they close, they move laterally and dorsally, and water is forced out.  drawing of movement of water through the branchial chamber of an isopod Idotea wosnesenskiiExperiments employing dye and small plastic particles indicate that water typically takes about 3 complete cycles to pass through the branchial chamber (see drawing at Right).  At rest, the pleopods are held in a flat stack on the ventral surface of the abdomen.  Alexander 1991 J Zool Lond 224: 607.

Paths taken by particles released
at different locations (X) in the
branchial chamber ofI. wosnesenskii

 
Research study 3
 

graph showing difference in flexural stiffness in the 5 pleopods of isopod Idotea wosnesenskii
The division of labour in isopod pleopods for locomotion (Pleopods 1-3) and gas exchange (Pleopods 4-5) prompts the question as to whgraph showing flexural stiffness in 5 pleopods of isopod Idotea resecataether they differ mechanically and/or morphometrically commensurate with their different functions.  A study on Idotea wosnesenskii and P. resecata collected in San Juan Islands, Washington and Charleston, Oregon show, in fact, that flexural stiffness (bendability) is significantly greater in the first 2 pairs of pleopods than in the last 2 pairs, while that in the 3rd pair is intermediate (see above graphs). This greater stiffness is partly explained by greater overall thickness and thicker cuticles in the first 2 pairs than in the last 2 pairs (Pleopod 5 is only about 30-60% of the thickness of Pleopod 1 in the 2 species). While the swimming pleopods do not make a significant contribution to gas exchange, all pleopods work together as a ventilatory pump, beating to drive water through the branchial chamber.  Alexander et al. 1995 Invert Biol 114: 169. Photo of Idotea resecata courtesy Heidee Leno & Dave Cowles, Walla Walla University, Washington www.wallawalla.edu.

NOTE  the schematic on the Left shows how flexural stiffness is measured in one of the podites. Force is applied at the location shown and the amount of force required to deflect the podite to different distances is recorded. To obtain a value for flexural stiffness for a pleopod (as shown in the above graphs), the 2 values for exopodite and endopodite are averaged for each deflection distance

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