Sheepshead minnow, continued 



tions in all life stages. Observations suggest a prefer- 

 ence for salinities of 1 0-25.0%° and 21 .0-30.0%o, being 

 less common above this range than below (Gunter 

 1 945, Gunter 1 950, Reid 1 954, Kilby 1 955, Odum and 

 Caldwell 1955, Phillips and Springer 1960, Tabb and 

 Manning 1961, Franks 1970, Hudson et al. 1970, 

 Swingle 1971, Wang and Raney 1971, Martin 1972, 

 Christmas and Waller 1973, Pineda 1975, 

 Subrahmanyam and Drake 1975, Swift et al. 1977, 

 Cornelius 1984, Nordlie 1985). It has been collected 

 from an overall salinity range of 0-142.4% o . The high 

 extreme of this range is probably very close to the 

 upper tolerance limit for this species (Gunter 1945, 

 Simpson and Gunter 1956, Simmons 1957, Renfro 

 1960, Hoese 1960, Gunter 1967, Martin 1972, Ward 

 and Armstrong 1 980, Nordlie 1 985). However, it rarely 

 invades salinities higher than 80%o, possibly due to the 

 lack of food at such high salinities (Hildebrand 1957). 

 Environmental factors experienced during growth and 

 development may affect the ability of different popula- 

 tions to withstand salinity variations (Martin 1968). 



Dissolved Oxygen: The sheepshead minnow appears 

 to have a strong tolerance of hypoxia (Peterson 1 990). 

 It has been found in Chesapeake Bay in waters with a 

 dissolved oxygen (DO) content ranging from 1 to 6 

 ppm, and 20 to 90% saturation (De Silva et al. 1962). 

 It has also been taken from anoxic waters where the 

 DO content ranged from to 0.81 ppm (Odum and 

 Caldwell 1 955). "Obligate gulping" of air is believed to 

 be used in order to relieve oxygen stress. 



Movements and Migrations : This species remains in 

 estuaries throughout the year (Rogers and Herke 

 1 985). Observed movements appear to be influenced 

 by seasonal fluctuations in temperature. As tempera- 

 tures begin to drop in the fall there is a general 

 movement to warmer, slightly deeper waters. It has 

 been noted that at this time individuals can be taken by 

 trawls in these deeper waters where none were present 

 during warmer months (Gunter 1945, Simpson and 

 Gunter 1956, Breuer 1957, Springer and Woodburn 

 1960). 



Reproduction 



Mode : This species has separate male and female 

 sexes (gonochoristic), with equal (or nearly so) sex 

 ratios (Hildebrand 1919, Raney et al. 1953, Warlen 

 1964). Fertilization is external. 



Spawning : This species has an extended spawning 

 season lasting from February to October and probably 

 throughout the year in warmer waters (Kuntz 1914, 

 Hildebrand 1919, Gunter 1950, Kilby 1955, Raney et 

 al. 1953, Martin 1972, Ruebsamen 1972, DeVlaming 

 et al. 1 978). Ripe females have been collected in water 

 temperatures ranging from 1 5 to 28.5°C (Ruebsamen 



1972). Drops in salinity may initiate spawning activity 

 (Martin 1 972). Spawning can occur at depths of 2.5-61 

 cm in shallow arms of small bays, large tide pools, 

 mangrove lagoons, roadside ditches, and pools in 

 shallow, gently flowing streams over bottoms of sand, 

 black silt, or mud. Males occupy territories up to 0.3-0.6 

 m in diameter and may or may not construct nest pits. 

 Pits, when constructed, are over sand, gravel, or soft 

 mud bottoms with a detritus overlay, and are 1 0-1 5 cm 

 in diameter, 2.5-3.8 cm deep, and are centrally located 

 in well groomed, oval shaped territories. This territory 

 is defended by the male against all but ripe females. 

 Spawning may take place within or outside of the 

 territories, but not usually within the nest pit. Spawning 

 territories are typically situated adjacent to banks or up 

 to 3 m from shore and are usually associated with 

 submerged logs or rocks. The density of territories 

 may approach 1 00 per 0.9 m 2 area (Raney et al. 1 953, 

 Simpson and Gunter 1956, Hardy 1978, Ward and 

 Armstrong 1980). 



Fecundity : Sheepshead minnows are fractional spawn- 

 ers. Fecundity varies with each spawn and each 

 female. Single females spawn a number of times 

 during a single season at intervals of 1 -7 days with an 

 average of 4 spawnings per nest entry, and deposit 1- 

 3 eggs per spawning (Kuntz 1914, Hildebrand 1919, 

 Hardy 1978). Spawning throughout the year is pos- 

 sible in southern parts of the range (DeVlaming et al. 

 1978). In one laboratory study, the number of eggs 

 produced over a 28 day period per female in vitro 

 ranged from 2 to 1,028 and averaged 186 (Schimmel 

 and Hansen 1974). Another study reported from 2 to 

 24 eggs spawned by a single female on thirty occa- 

 sions from April 9 through August 1 6 with the possibility 

 that the actual number may have exceeded observa- 

 tions (Hildebrand 1919). The ovary from a single 

 female in this study contained 1 40 oocytes with at least 

 50% mature. 



Growth and Development 



Egg Size and Embryonic Development : Eggs are de- 

 mersal, develop oviparously, and are adhesive or 

 semi-adhesive by means of minute threads which stick 

 to plants, the sides of aquaria, each other, and the 

 bottom substrate. Eggs are spherical in form (1 .0-1 .73 

 mm in diameter), yellowish in color, and highly translu- 

 cent. The egg membrane is thick and heavy with a 

 visible perivitelline space between it and the vitelline 

 membrane. Small groups of minute oil globules are 

 scattered over the surface of the yolk sphere that 

 normally rests at the upper pole. Incubation time can 

 vary from 4-1 2 days: 1 2 days at 1 7.4-25.5°C and 1 1 0%o 

 salinity; 5-6 days at laboratory temperature; 5 days at 

 30°C; 4-5 days at 28°C and 30% o salinity. Hatching 

 typically occurs in spring and summer (Kuntz 1914, 

 Hildebrand 1919, Hubbs and Drewry 1959, Renfro 



171 



