MATARESE AND SANDKNOP: EGG IDENTIFICATION 



29 



egg development as follows: Early— from fertilization to closure 

 of blastopore. Middle— from closure of blastopore to tail bud 

 lifting off yolk, and Late — from tail bud lifting off yolk to time 

 of hatching. 



Identification Characters 

 Shape.— The vast majority of all egg types are spherical. Ex- 

 ceptions include ellipsoidal eggs as found in anchovies, En- 

 graulis and Anchoa. and slightly flattened or ovoid eggs as seen 

 in members of the families Gobiidae, Scaridae, and Ophidiidae 

 (Fig. 13A. B). A number of demersal eggs have somewhat ir- 

 regular shapes, especially those associated with large egg masses. 

 The perciform family Congrogadidae has cruciform shaped eggs 

 (Herwig and Dewey, 1982). An unidentified, star-shaped egg is 

 encountered infrequently in the Alaska region (Fig. 13C). 



Size.—T\\t average marine and freshwater fish egg size is about 

 1.0 mm. According to Ahlstrom and Moser (1980), pelagic fish 

 eggs range from 0.5 mm [Mncigiicnia (Fig. 13D)] to about 5.5 

 mm (Muraenidae). Demersal eggs may range higher in size (up 

 to 7.0-8.0 mm), e.g., members of the families Salmonidae, An- 

 arhichadidae, and Zoarcidae. Mouth brooders, e.g., in the catfish 

 family Ariidae, have among the largest eggs with sizes from 1 4 

 mm to 26 mm. 



Oil globules.— The oil globule provides useful characters in fish 

 egg identification; these include presence or absence, number, 

 size, position, color, and pigmentation. Among both pelagic and 

 demersal eggs, the most common form contains a single oil 

 globule. Eggs may lack an oil globule as in most gadines and 

 pleuronectids (Glyplocephaliis). contain only one (Icosteiis), or 

 have multiple oil globules as in the cynoglossids and triglids 

 (Symphums and Prionotus) (Fig. 13E, F, G, and H). In pelagic 

 eggs with a single oil globule, the size ranges from <0.10 mm 

 to > 1.0 mm (Ahlstrom and Moser, 1980). The position of the 

 oil globule within the yolk sac is usually posterior, but several 

 groups contain species that have an anterior placement (e.g., 

 labrids and carangids) and others have an intermediate place- 

 ment (argentinids). In some fishes, oil globules migrate during 

 embryonic development. Some members of the family Bathy- 

 lagidae initially possess multiple oil globules that eventually 

 coalesce into a single globule (Ahlstrom, 1969). Although not a 

 totally reliable character, the oil globule color can be useful, 

 especially in the identification of freshly taken demersal eggs. 

 Lastly, many species have oil globules with melanistic pigment, 

 Icosteus (Fig. 13H) and Icichthys. 



Yolk.— The degree of yolk segmentation is an important iden- 

 tification character. Yolk is usually segmented in primitive forms, 

 e.g., Etruineus (Fig. 131), and homogeneous in higher forms 

 (Rass, 1973; Ahlstrom and Moser, 1980). The opaqueness of 

 yolk found in catfishes, salmonids, and gars can be diagnostic' 

 Pigment, which may also be diagnostic, can be present dunng 

 various developmental stages from middle to late. Yolk color 

 is often important especially in demersal eggs. Among demersal 

 eggs vitelline circulation patterns within the yolk sac are useful 

 in identification.' 



' P. Douglas Martin, Chesapeake Biological Laboratory, P.O. Box 38, 

 Solomons, Maryland 20688. Personal communication, October 1982. 



Chorion. — A. number of characteristics associated with the cho- 

 rion or egg envelope can be useful in identifying fish eggs and 

 have been shown to be highly adapted to the environmental 

 conditions under which an embryo develops (Ivankov and Kur- 

 dyayeva, 1973; Stehr and Hawkes, 1979; Laale, 1980; Stehr, 

 1982). The most important character of the chorion is whether 

 it is smooth, as is in most fishes, or sculptured. Among fish eggs 

 with patterns, the size and texture (e.g., raised hexagons, pus- 

 tules) of the design are diagnostic. Raised polygonal surfaces are 

 found in several unrelated species (Stehr, 1982), e.g., Synodus 

 and Pleuronichthys (Sumida et al., 1979), and pustules occur 

 among some bathylagids and argentinids. Mugil cephalus eggs 

 (Fig. 14A), previously considered to have a smooth chorion, 

 have a raised patterned surface visible by scanning electron 

 microscope (Boehlert, this volume). In many groups of fishes, 

 the chorion has various degrees of ornamentation consisting of 

 projections, threads, filaments, or stalks which may aid in flo- 

 tation (pelagic) or attachment (demersal). In some scombere- 

 socids, e.g., Cololahis (Fig. 14B). some exocoetids and ather- 

 inids, pelagic eggs are attached to each other or to a substrate 

 by filaments. Spines are found in some myctophiforms and 

 exocoetids, and stalks occur in some demersal egg groups, e.g., 

 blenniids and Osmerus mordax. In ostraciid eggs, a patch of 

 pustules is present near the micropyle (Fig. 14C). 



Recently, thickness of the chorion has been of diagnostic value 

 (Ivankov and Kurdyayeva, 1973; Boehlert, this volume). Stehr 

 and Hawkes (1979), using scanning electron microscopy, found 

 that most marine teleosts with pelagic eggs have thin chorions 

 in relation to egg diameter whereas demersal eggs tend to de- 

 velop much thicker chorions. Color of the chorion is an im- 

 portant diagnostic character, especially for freshly taken de- 

 mersal eggs in the marine intertidal environment (Matarese and 

 Marliave, 1982). A number of freshwater demersal fishes have 

 eggs that possess a special coating associated with the chorion 

 which can be either gelatinous or adhesive, e.g., Perca. Icialurus, 

 and Notropis (Snyder, 1981). 



Penvilelline space. — Most fish eggs have a narrow- to medium- 

 width perivitelline space, but wide spaces are common in some 

 groups, especially among the more primitive fishes that have a 

 segmented yolk, e.g., Clupeiformes (Sardinops. Fig. 14D), An- 

 guilliformes, and Salmoniformes (Chauliodus. Fig. 14E) (Ahl- 

 strom and Moser, 1980). Large perivitelline spaces are also found 

 among some unrelated higher forms, such as cypnnids (Nolro- 

 pi.s). percichthyids (Morone saxatill.s). or pleuronectids (Hip- 

 poglossoides). 



Embryonic characters.— CharacXers associated with the devel- 

 oping embryo are extremely useful in egg identification, partic- 

 ularly in the middle and late stages of development. Many eggs 

 not identifiable in the early stages are easily recognizable using 

 embryonic characters such as pigment on embryo or finfold and 

 morphology. In some fishes, embryonic pigment in the late stages 

 has already undergone sufficient migration and rearrangement 

 to the point where it resembles the yolk-sac larva; this is com- 

 mon in several groups including gadiformes, e.g., Merluccius 

 (Fig. 14F), Gadus. and Theragra. and heavily pigmented flat- 

 fishes like Pleuronichthys and Hypsopsetta. Characteristic late- 

 stage pigment bands appear in Glyptocephalus (Fig. 13E). In 

 most freshwater species, pigment is not present prior to pigment 

 cell migration but appears sometime after the cells have mi- 



