HUNTER: CULTURE METHODS 



27 



sac fish larvae as food. Zooplankton is the initial food until 

 piscivorous feeding habits develop (Houde, 1972b; Mayo, 1973; 

 Hunter and Kimbrell, 1980). Piscivorous larvae manipulate their 

 larval prey and consequently are less dependent on mouth size 

 when consuming larval fish. Sibling cannibalism is common 

 under reanng conditions in such fishes. Increasing the food den- 

 sity may increase survival as may elevating the temperature, 

 thereby accelerating growth through the most cannibalistic sizes; 

 at least in scombroids sibling cannibalism declines at meta- 

 morphosis (Mayo. 1973; Hunter and Kimbrell, 1980). Sorting 

 by size and isolating the larger larvae is probably the only certain 

 method for controlling losses due to cannibalism, however. 



Phytoplankton 



Phytoplankton blooms are often maintained in larval culture 

 tanks to reduce the detrimental effects of metabolic by-products 

 which accumulate in static rearing tanks (Houde, 1974) and to 

 provide food for larval food organisms. In many cases dense 

 blooms of phytoplankton enhance larval growth and survival 

 and I recommend the practice but the mechanism is obscure. 

 The phytoplankters used are various, easily grown, small species 

 such as Chlorella. Anacystis, Nannochloris, Tetraselmis. Dun- 

 aliella. Isochrysis. Phaeodactylum and others.' They are main- 

 tained at high densities (10,000 or more cells/ml) in the rearing 

 tanks. At high cell densities larvae ingest these small phyto- 

 plankters, perhaps inadvertently (Moffatt, 1981) but they appear 

 not to be able to exist on them as a sole food source (Houde, 

 1974; Scura and Jerde, 1977). They may supplement the food 



' For a nominal fee starter cultures of manne phytoplankton can be 

 obtained from R. R. L. Guiliard. Bigelow Laboratory for Ocean Sciences. 

 McKown Point, West Boothbay Harbor, Maine 04575 USA; culture 

 methods are discussed by Guiliard (1975). 



ration either directly or indirectly through the ingestion of prey 

 having guts full of algal cells (Moffatt, 1981). Evidence now 

 exists that enhancement of growth and survival of larval Scoph- 

 ihalmus maximiis by blooms of Isochrysis and Phaeodactylum 

 is due to the inclusion in the diet of certain polyunsaturated 

 fatty acids not occurring in the normal laboratory rotifer diet 

 (Scott and Middleton, 1979). It is interesting in this regard that 

 Dunaliella which lacks the fatty acids did not enhance S. max- 

 imiis larval growth or survival. 



Effects of Culture 



Extrapolation from cultured larvae to natural populations must 

 be done with caution because culture may affect the morphology, 

 behavior and biochemistry of larvae (Blaxter, 1976). The mor- 

 phological characteristics most susceptible to modification in 

 tanks are those partially controlled by environmental conditions 

 such as vertebrae and fin ray counts. Reared larvae also may 

 be more heavily pigmented than sea caught specimens (Watson, 

 1982). This appears to be related to the expanded nature of the 

 melanophores, not to added numbers of pigment cells. In ad- 

 dition, pigmentation events may occur at smaller sizes in reared 

 material (S. Richardson, Gulf Coast Research Laboratory, Ocean 

 Springs, Mississippi, pers. comm.). Laboratory reared larvae are 

 often heavier and have deeper bodies than their wild counter- 

 parts, making some morphometric measurements on laboratory 

 specimens useless (Blaxter, 1975). The differences in preserva- 

 tion and handling between laboratory and sea-caught larvae also 

 make direct size-specific comparisons difficult. Shrinkage in 

 length may vary greatly depending on the duration larvae re- 

 main in plankton nets and shrinkage differences between reared 

 and wild specimens can be misinterpreted as morphological 

 differences (Theilacker, 1980a). 



National Marine Fisheries Service, Southwest Fisheries 

 Center, P.O. Box 271, La Jolla, California 92038. 



Identification of Fish Eggs 

 A. C. Matarese and E. M. Sandknop 



A wide variety of egg types exists among teleost fishes in both 

 freshwater and marine environments. Eggs may be pelagic 

 and nonadhesive or demersal and either adhesive or not. They 

 may possess a variety of specialized structures aiding in flotation 

 or attachment. Depending on egg type and associated repro- 

 ductive ecology, many characters are useful in identification. 

 These characters have been reviewed for pelagic marine eggs by 

 Rass(1973), Robertson (1975a), Russell (1976), and Ahlstrom 

 and Moser ( 1 980); we have liberally and extensively drawn from 

 the latter. Important characters for other egg types have been 

 discussed in part by Balon (1975a, 1981a), Hardy (1978a, b), 

 Jones et al. (1978), and Snyder (1981). Characters such as size 

 and possession of oil globules are important for all types; how- 

 ever, perivitelline space and chorion sculpturing are more im- 

 portant in pelagic eggs, while in demersal eggs special coatings. 



chorion thickness, or nature of egg deposition may be more 

 useful. 



A wealth of potential characters useful in egg identification 

 exists; however, it is still difficult to identify eggs of most species 

 with certainty. Except for late stages, few may be recognized at 

 the species level. Some characters are useful at a family level, 

 but presently it is not productive to speculate on the systematic 

 significance of any characters (see Kendall et al., this volume). 

 Presently, the main goal of taxonomy with respect to fish eggs 

 is identification. 



Regardless of egg type or reproductive ecology, a summary 

 of identification characters useful to an egg taxonomist is pre- 

 sented. Additionally, we recommend using available literature 

 for reference and encourage the building of local fish egg col- 

 lections. We follow Ahlstrom and Ball (1954) in subdividing 



