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Size of Eggs and the Number Incubated

mm, fish, average, diameter and measured


The eggs, while enormously large, show considerable variation in size, running from 15 to 25 mm. in diameter. Heretofore, however, sufficient measurements have not been made to enable one to say with any definiteness what is the size of the average egg of any oral gestator. For the gaff-topsail hundreds of measurements have been made and from them the following diameters are given as covering both normal and abnormal sizes of eggs. It has been constantly observed that the non-fertile eggs of this fish are much larger than the fertile ones. In 1908, at one catch 11 such eggs were taken and measured in 3 diameters. They ran in millimeters, 17 by 18 by 20, 18 by 19 by 20, 18 by 19 by 21, 18 by 20 by 21, 19 by 20 by 21, 21 by 21 by 22, 22 by 22 by 22, 22 by 22 by 23 (two), 22 by 22 by 25, 23 by 24 by 27. The largest fertile eggs measured 18 by 18 by 19, 19 by 19 by 20, 19 by 19 by 22, 20 by 20 by 21. Incidentally these measure ments show that the eggs are rarely round, some are markedly ellip tical—one 22 by 22 by 26 mm.

To ascertain the average size, the following data are selected out of the large amount available. Of live eggs 138 were measured as follows: Longest diameter only: 17.5 mm., two eggs; 18, nine; 18.5, eleven; 19, thirty-nine; 19.5, ten; 20, thirty-eight; 20.5, eight; 21, nineteen; 21.5, twenty-two. Average of 138 live eggs 18.8 mm. Of preserved eggs, 189 were measured as above: 16.5 mm., three eggs; 17, one; 17.5, one; 18, four; 18.5, two; 19, thirty-four; 19.5, eleven; 20, sixty-four; 20.5, ten; 21, forty-nine; 21.5, five; 22, five. Average of 189 dead eggs, 20 mm. The average of the 327 eggs is 19.5 mm. From the above it will be seen that the largest number of eggs meas ure 19, 20, and 21 mm. in longest diameter, the 20-mm. eggs being the most abundant-102 out of 327. Normal eggs of this size are shown in figure 8, plate in, while the egg of figure 9, plate iv, is about the same size but has an older embryo.

In connection with the foregoing data, the following measurements should be given, since they are of the smallest eggs, taken as a lot, which have been found in the six years' search. They measured in longest diameter: 15 mm., one; 16, three; 16.5, one; 17, four; 17.5, one; 18, ten; 18.5, one; 19, seven. The average for the 28 eggs is 17.7 mm. This average, however, would be much lower if the last three lots of eggs of normal size were omitted, or even the one last lot only. These eggs were taken on July 21, 1910, and contained embryos having black stippling on the dorsal surface. At the same

time there were obtained, from another fish, 5 lame which seemed to be from 2 to 3 weeks older than these embryos. All these things lead to the conjecture that possibly these smaller eggs are a lot coming to maturity later and are of a second laying. On these points see page 31.

Not only is there great variation in the size of the eggs, but also in the number carried. Twice I have taken fish with only 2 eggs in the mouth. The smallest incubators caught during this research were 13 and 15.5 inches over all. Each carried 4 eggs. Other small numbers of eggs incubated were 2, 5, 8, 11 (in 2 fish), 13, 14, 15 (in 2 fish), 16, etc. The largest number found was 55 in a 22-inch male. Notable also are 50 (22-inch fish), 45 (taken twice), 38 (22- and 20-inch fish), 36 (from a notably small fish 17.25 inches long), 35 (22-inch), 32 (fish, 20.25 inches long), and numerous catches in the twenties. Once a large male was found with only one egg in his mouth (the only instance anong hundreds captured), but from the great size of the buccal cavity it is probable that other eggs had been thrown out.

That these large numbers of colossal eggs do not, however, entirely fill the capacious buccal cavities of these fish may be readily perceived by referring to the calculations given on pages 34 and 35.

Before leaving this subject brief reference may be made to the large size and to the number of eggs which are carried by other silu roids practicing oral gestation. Evermann and Goldsborough (1902) noted that 39 eggs were taken from the mouth of a 13.5-inch male Conorhynchus nelsoni. The average size of the eggs measured was ten sixteenths by eleven-sixteenths of an inch. Von Ihering (1888) found eggs of 18 mm. diameter to the number of 3 or 4 dozen in the mouths of "large" males of Arius commersonii (Netuma or Taehysurus barbus), a salt-water catfish of southern Brazil. Day (1873) obtained 15 to 20 eggs, 0.5 to 0.6 inch in diameter from the mouths of Osteogeniosus mili taris and various species of the genus Arius in India. Boake (1866) reported the eggs of his Cingalese Arius to be about the size of small grapes. Gunther (1864) says that the eggs of Arius fissus of Cayenne are rather larger than peas. Last of all, but not least, are the eggs of that catfish of Guiana whose native name is jarra-bakka, which are declared by Wyman to measure 0.75 inch in diameter. However, it may be remarked that, although much work has been done in recent years on oral gestation in the catfishes of Guiana, none with eggs so large has ever been taken.