24 



Comparative Animal Physiology 



Limited Osmoregulation : Invaders of Brackish Water. Eriphia spiniformis 

 and Pachygrapsus mannorahis, --- Cancer pagitriis (Platycarcinus, Fig. 13 

 curve E), -^•^' and Cancer magister, Hemigrapsus nudus (Fig. 13, curve D), 

 and Hemigrapsus oregonensis (Fig. 13, curve B) '-- show a slight ability to 

 remain hypertonic in dilute sea water (Fig. 13, curves B and D; Fig. 14, curve 

 E). These crabs are isotonic or nearly so in sea water and in higher concen- 

 trations, and hypertonic but not constant in blood concentration in increasing- 

 ly dilute media. They are rock crabs occurring along the shore, sometimes in 

 tidal pools. One important factor in their regulation is low permeability to 

 water and salts. Figure 9 shows that in 66 per cent sea water Cancer swells 



2.5 — 



2.0 



Fig. 13. Blood concentrations (Ai) as a function of external concentrations (Ao) in 

 several Crustacea which are labile in hypertonic media and show some regulation in 

 hypotonic media. A Pachygrapsus crassipes; • Heniigrapsiis oregonensis; ■ Rhithro- 

 panopeus harrisii; Q Hemigrapstis nudus. Data from Jones.^"" 



less than does Maja in 75 per cent sea water, but that Cancer recovers more 

 slowly in volume. Hence Cancer is less permeable to both water and salt. In 

 a hypertonic medium (5.1 per cent salinity) the blood of Eriphia reached a 

 concentration equal to that of the medium only after 72 hours, whereas Maja 

 reached the same concentration as the medium in 12 hours (Fig. 11). --- 



A useful method for investigating salt permeability is to place crabs in sea 

 water containing small amounts of iodine and to measure the amount taken 

 up. When Cancer was bathed in sea water containing 0.5 per cent iodine, the 

 crab took up 0.35 to 0.48 mg. iodine / cc. of blood after 7 to 8 hours. '^^ When 

 the intestine was plugged the uptake was the same. When a tube containing 

 0.75 per cent sodium iodide was placed on the surface of the carapace. Cancer 

 took up in 50 hours an amount corresponding to 0.048 mg./cc. of blood. 

 Hence there may be some ionic uptake directly through the shell but most 

 of the exchange is across the gills. The poikilosmotic crabs Portiinas and Hyas 



