like Ostrea virglnica , 0. chilensis , 0. glgas and others. Pearl oysters, lirnl;!^ attached 

 to the su'bstrat'uin ty a oyssus, maj' tie found in large quantities f,Tov;ing i.n close proxirity 

 to each other, tut \;ithout forming clusters. On the hottom of the sea, the pearl oyster 

 grows in a vertical position, or slightly inclined, at ahout 35 degrees to the horizontal 

 surface. Objects to which the oyster attaches itself vary greatly, hut small rocl's and 

 corals are nost freouently used. Occasionally pearl oysters are found on semisoft botton 

 nade of a mixture of mud and sand, hut this type of hottom is not tjrpical for the species. 



Along the Pacific coas-t of Panama, Pinctada mazatlanica is found in shallow water 

 along the mainland and aro-und the islands in depths prohahly not exceeding lS-20 fathoms. 

 It is quite possible that the oysters live in deeper water, hut since commercial divers do 

 not descend helow 12 fathoms, the maximum depth at which the pearl oysters nay he found is 

 not definitely known. 



Around the Pearl Islands in the Gulf of Panama, and near Coiha Island in the 

 Gulf of Chiriqul, the pearl grounds are likely to he found in relatively narrow chatmels 

 between the small islands, vrhere rapid tidal currents provide a good exchange of water. 



Pearl oysters do not occur in the tidal zone ahove lov; water level. The reason 

 for this is found in the peculiarities of the closing mechanism of the addvictor muscle. ITo 

 physiological studies have ever heen made \;ith Pinctada mazatlanic a, hut my experiments 

 v;ith the related species of nearl oyster of the Hawaiian Islands, P. galtsoff i Bartsch, 

 shov; that when the oyster of this species is tal<:en out of water it is not ahle to keep the 

 valves of its shell closed. After a brief exposure to air, the adductor muscle of the 

 oyster relaxes, the valves gape, the sea water retained between the shells is spilled and 

 the nollusk dies. The gaping of the valves may be prevented if the oysterp, are tightly 

 packed in boxes between layers of seaweed. If the behavior of P. mazatlanica is similar to 

 that of P. galtsoffl , it would explain its absence from the tidal zone. It is generally 

 knovm that in edible oysters of the genus Ostrea the adductor rauscle has the ability to re- 

 main contracted for days or even v/eeks, keeping the valves tightly closed. Because of this, 

 edible oysters remain alive for a long time after being taken out of water. 



In the Gulf of Panama and in the Gulf of Chiriqui, the distribution of the pearl 

 oyster is restricted to a rather narrow shelf of hard bottom. Around the islands, this 

 zone of suitable bottom is limited by a 20-fathom contour. Along the mainland shores, it 

 is confined to a S-fs-thom depth, below which the botton, as sho\m by the Coast and Geodetic 

 Survey charts, becomes soft and muddy. 



Jrom the information I received from various persons familiar with the pearl 

 industry in Panama, I conclude tliat no extensive pearl-oyster beds have ever existed along 

 the continental shores of Panama, although a few specimens of pearl oj'ster may be found 

 here and there along the coast. In selecting my sampling stations, I was influenced by the 

 past experience of pearl divers, and made observations in the locations which in the past 

 30 years were knoi^m to be productive in pearl oysters. These grounds were primarily con- 

 fined to the sheltered and relatively narrov/ channels between the islands of the 

 Archipielago de las Perlas and around Coiba Island in the Gulf of Chiriqui (Pig. 6). 



sdb;>iaeike topogeapbt op the gulf op Panama 



The botton of the Gulf of Panama gently slopes seav/ard for a distance of about 

 one hundred miles v/here at the edge of the continental shelf the depth over a short distance 

 of 10 to 15 miles rapidly increases to 100 fathoms. Within the next I5 miles in the 

 southern and southeastern direction the depth reaches 1,000 fathoms. Considerable portion 

 of the Gulf is occupied by many small and large volcanic islands which form the Archipielago 

 de las Perlas. At the northern end of the Pearl Islands, a submarine valley beginning at 

 the 20-fathom contour at the mouth Of Eio Chepo (Bayano River) can be traced for a distance 

 of about SO miles along the western side of the Islands. According to Terry (19^1) the 

 valley is perceptible to the UoO-fathom line, where radiating channels suggest a delta. 

 Terry thinks that the relation of this submarine valley to the ancient Bayano (Chepo) River 

 is unmistakable. 



lU 



