278 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 8 



Strongylocentrotus franciscanus (A. Agassiz) 

 Plate 45, Fig. 23 



Toxocidaris franciscana A. Agassiz, 1863, p. 22. 

 Strongylocentrotus franciscanus A. Agassiz, 1872a, p. 163. 



Mortensen, 1943a, p. 242, pi. 28, figs. 

 1-7; pi. 29, figs. 1-4. 



There are in the Velero collection 343 specimens of Strongylocentrotus 

 from 24 stations which are here referred to this conspicuous sea-urchin. 

 In point of size it rivals Tripneustes. The latter reaches perhaps a greater 

 diam.eter of the test, 150 mm as against 146 for the present species but the 

 latter has such long, heavy spines, 50 mm or even more in some individuals, 

 it is evidently the largest of the west coast Echini. Young individuals with 

 a test diameter of 10-15 mm have the relatively slender spines about two- 

 thirds as much, while adults with a diameter of 100 mm or more are satis- 

 fied with spines only a third of that. In two specimens at hand from Point 

 Vicente, California, w^hich are rather more than 100 mm in diameter the 

 number of coronal plates (and hence the number of primary spines) is 

 considerably more than normal and the length of the primaries is markedly 

 less, only 20-30 mm. The appearance of these specimens is thus strikingly 

 different from normal specimens but the differences are probably associated 

 with some unusual feature of their habitat, such as excessively strong surf 

 or tidal currents. Further collecting at Point Vicente and study of local 

 conditions might explain this curious anomaly. 



The color of normal franciscanus when young is a light fawn brown 

 but with growth the color darkens and in most individuals it becomes a 

 very deep purple or even almost black. Curiously enough many individuals 

 instead of becoming very dark remain light colored until half grown and 

 are then a dull and pale "old rose" or pale violet. With increased age and 

 size the dark color becomes predominant and the big adults are nearly if 

 not quite black. Other individuals never become black at all but even 

 when full-grown are light-colored, a peculiar shade of brown, lightest at 

 the base of the spines, darkest near the tips. Curiously enough such speci- 

 mens if placed in a dilute solution of formalin (4 per cent) to which a 

 pinch of corrosive sublimate has been added turn bright green, strikingly 

 different from any normal shade of the living urchins. On being dried, the 

 green disappears more or less and the urchin becomes brown once more but 

 darker, especially on the distal portion of the spines. It is conceivable that 

 the colors are an indication or a sex dimorphism and an investigation into 

 the matter of a possible correlation of color and sex might prove rewarding. 



