96 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



DESCRIPTION OF FERTILIZATION 

 TRIALS 



Sixteen manual, or artificial, fertilizations were 

 attempted to obtain developing embryos from 

 positively identified yellowfin menhaden. A single 

 female and several males were used in the first 

 trial. The ova were removed by dissection, di- 

 vided into three lots, and those in each lot mixed 

 dry with milt from a separate male. Fifteen 

 minutes later, filtered dockside water (salinity 

 20.5%o, and temperature 20.1° C.) was added 

 to each container. An hour later, approximately 

 90 percent of the eggs in one lot were fertilized. 

 The other two lots contained so few fertilized 

 eggs, perhaps because of less viable sperm, that 

 they were tUscarded. Development was arrested 

 after several hours during early cleavage. Whether 

 the failure to develop was due to decomposition, 

 stagnation, or immaturity of ova or sperm could 

 not be determined. 



Four females were used in the second trial. 

 Ova were removed by chssection, mixed "dry" 

 with milt, and 15 minutes later, dockside water 

 was added (27.27oo, 19.0° C). The apparent 

 success of fertihzation varied from 40 to less 

 than 10 percent. Two lots of eggs were placed 

 in the floating pen anchored ofT the end of the 

 dock where salinity was 26.7°/oo and temper- 

 ature 18.5° C. The remaining two lots were 

 placed in containers in the laboratory. Twelve 

 hours later, eggs in the laboratory containers 

 had failed to develop beyond early cleavage and 

 showed signs of decomposition. Samples of eggs 

 from the pens appeared normal, although in one 

 compartment, few ova were fertilized. This trial 

 fm-nished most of the developing enxbryos and 

 yolk-sac larvae used for the descriptions. 



DESCRIPTION OF EGG 



Living eggs showed an iridescent, glasslike 

 transparency, with little or no color in the yolk. 

 Iridescence disappeared when the material was 

 placed in formalin, but the chromatophores were 

 retained and accentuated as the developing em- 

 bryo and yolk became clouded. The following 

 description is based on preserved material. 



The egg is spherical and has a resilient, trans- 

 parent membrane. Under magnification of 100 

 diameters or more, the membrane surface is 



marked with fine, short lines that form no chs- 

 cernible pattern. The yolk is segmented, con- 

 tains a single oil globule, and is pale yellow. 

 The oil globule is near the vegetative pole and 

 floats uppermost throughout development. Coarse 

 granulation of the yolk appeared to be character- 

 istic of eggs not fully matured. 



Comparative measurements showed the plank- 

 tonic eggs to be slightly larger than those obtained 

 artificially (table 1). Fertilized eggs in the plank- 

 ton, siniilar in appearance and structure to those 

 artificially fertilized, were assumed to be from 

 yeUowfin menhaden. Whether the artificially 

 fertilized eggs had not reached maximum size 

 because of inunaturity, or whether naturally 

 spawned eggs swell to a greater size could not 

 be determined. Eggs, ranging from approxi- 

 mately 1.0 to 1.1 mm. in diameter, developed 

 a fertihzation membrane and perivitelline space; 

 however, the very low fertility and tlie failure 

 of naost eggs to develop beyond the earliest stages 

 of cleavage indicated that these ova had not 

 reached maturity. 



T.\BLE 1. 



-Measurements of yellowfin menhaden eggs, in 

 millimeters 



Developing yellowfin menhaden eggs from the 

 plankton were buoyant, floating just beneath the 

 surface film. Unfertilized eggs rested on the bot- 

 tom in still water. Artificially fertilized eggs 

 formed a layer above the unfertilized eggs, floating 

 off the bottom with the slightest disturbance. 



DEVELOPMENT OF THE EMBRYO 



In discussing tlie development of yellowfin 

 menhaden eggs the following tliree stages are used 

 (Ahlstrom and Counts, 1955): 



Early—from fertilizntion to closure of the blastopore. 



Middle— from closure of the blastopore to the time that 

 the separating tail begins to curve laterally away from the 

 embryonic axis. 



Late— from the timis the tail curves away from the 

 embryonic axis to the time of hatching. 



