Silversides, continued 



collections made in hypersaline conditions at 120% o 

 (Copeland 1 967). They have been collected in waters 

 with to 120%, salinity (Gunter 1945, Gunter 1950, 

 Simmons 1957, Renfro 1960, Copeland 1967, Franks 

 1 970, Perret et al. 1 971 , Swingle 1 971 , Christmas and 

 Waller 1973, Perret and Caillouet 1974, Swingle and 

 Bland 1974, Pineda 1975, Tarver and Savoie 1976, 

 Barrett et al. 1 978, Cornelius 1 984). Reported salinity 

 ranges of occurrence include 5.0 to 9.9%o (Tarver and 

 Savoie 1 976); 0.0 to 4.9%o and 1 5.0 to 1 9.9%o (Swingle 

 1 971); 10.0 to 24.9%o(Perretetal. 1971); 21.0 to 30.0%o 

 (Cornelius 1 984); and 22.5%o or higher (Franks 1 970). 

 However, these historical reports of disparate salinity 

 ranges are probably due to different habitat affinities 

 among the now-recognized Menidia species. M. 

 beryllina is considered to be the more euryhaline 

 species, occurring from fresh to marine salinities, 

 whereas M. peninsulae is found primarily from estua- 

 rine to marine salinties (Echelle and Mosier 1982). In 

 a study of Copano Bay, Texas, M. peninsulae was 

 predominant in seaward bays and connected tidal 

 pools (salinity range 13.5-32.5%o, mean 18.9%o). M. 

 beryllinawere predominant in freshwater streams and 

 bays (salinity range 0.1-2.3% o , mean 0.8% o ), isolated 

 pools (salinity range 2.3-20%o, mean 7.5%o), and tidal 

 creeks (salinity range 3.5-7. 8%o, mean 5.1%o). Both 

 species, their hybrids, and M. clarkhubbsi co-occured 

 in shallow bays and tidal pools (salinity range 6.0- 

 18.5%o, mean 11.4%„) (Echelle and Echelle 1997). 



Dissolved Oxygen and pH : M. beryllina can tolerate 

 dissolved oxygen (DO) concentrations as low as 1.7 

 parts per million (ppm) (Middaugh et al. 1 985), but have 

 also been collected at 9.5 and 1 1 .0 ppm DO (Barrett et 

 . al. 1 978). Collections have been made in a pH range 

 of 7.2 to 9.4 (Middaugh et al. 1985). 



Movements and Migrations : Silversides are non-mi- 

 gratory estuarine residents (Benson 1982, Middaugh 

 et al. 1 985). Diel inshore and offshore movements are 

 probably related to predator avoidance and feeding 

 (Darnell 1958, Krull 1976, Wurtsbaugh and Li 1985). 

 As juveniles grow, they are reported to move into 

 shallower waters (Darnell 1958). 



Reproduction 



Mode : Spawning of Menidia species is by external 

 fertilization of broadcast milt and roe, and egg develop- 

 ment is oviparous (Fisher 1 973). Sexes of M. beryllina 

 and M. peninsulae are separate (gonochoristic), but 

 sex ratios in these species may be skewed in response 

 to environmental conditions. In a study near Santa 

 Rosa Isiand, Florida, M. peninsulae spawned during 

 cool conditions (14.1-24.2°C) February through April 

 were 70-94% female, whereas those spawned during 

 warm conditions later in the year were 35-60% female 

 (Middaugh and Hemmer 1987b, Echelle and Echelle 



1 997). This temperature-dependent expression of sex 

 may be a reproductive adaptation to favor growth of 

 females during optimum conditions, and allow matura- 

 tion within a year (Middaugh and Hemmer 1987b). 

 Small populations of a unisexual all-female gynoge- 

 netic species complex (M. clarkhubbsi) have been 

 described from Texas (Echelle and Mosier 1982). 

 These fish produce diploid eggs without genetic re- 

 combination, and embryonic development is initiated 

 by spawning with one of the bisexual Menidia species, 

 without genetic contribution from the sperm. The 

 resulting progeny are clones of the parental M. 

 clarkhubbsi individual. This "species" may have origi- 

 nated from hybrids between M. beryllina and a now- 

 extinct progenitor species similar to M. peninsulae 

 (Echelle and Echelle 1 997). M. beryllinaxM. peninsulae 

 hybrids are known to occur in low frequency in waters 

 where the two species are sympatric, with habitat 

 affinities intermediate to the two parental species. 

 Hermaphroditic individuals have also been reported 

 (Yan1984). 



Spawning : Spawning of Menidia beryllina (reported as 

 M. audens) occurs during the day in the late morning 

 (Hubbs et al. 1 971 ), and takes place in Gulf of Mexico 

 estuaries in spring and fall as a bimodal peak. Occa- 

 sional spawning throughout the year has also been 

 reported. Ripe adults usually appear by March, but 

 sometimes as early as February, and are collected 

 throughout the year in some areas. Seasonal peaks 

 usually occur in May to June and September to Janu- 

 ary (Hildebrand 1922, Gunter 1945, Gunter 1950, 

 Simmons 1957, Hellier 1962, Hoese 1965, Garwood 

 1968, Swingle 1971, Ruebsamen 1972, Christmas 

 and Waller 1 973, Wagner 1 973, Gallaway and Strawn 

 1974, Swingle and Bland 1974, Pineda 1975, Hubbs 

 1982). Salinity has little effect on spawning condition 

 of M. beryllina, which is probably triggered instead by 

 rising temperatures or possibly changes in water levels 

 (Hoese 1965, Garwood 1968, Hubbs 1982, Middaugh 

 et al. 1985). Evidence of spawning was found over a 

 salinity range of 3.6 to 31 .5%o and a temperature range 

 of 15.0° to 32.7°C, but slowed or ceased at 30.0°C 

 (Garwood 1 968, Hubbs 1 982, Middaugh 1 985). Spawn- 

 ing of M. beryllina is probably most prevalent in tidal 

 freshwater or brackish water in the upper parts of 

 estuaries (Martin and Drewry 1978), and occurs in 

 shallow waters with gently sloping bottoms having an 

 abundance of rooted aquatic and/or inundated terres- 

 trial plants, tree roots, and dead leaves (Hildebrand 

 1922, Wilson and Hubbs 1972, Fisher 1973). It has 

 also been reported in a low to medium salinity tidal pass 

 in Louisiana (Sabins and Truesdale 1974). M. 

 peninsulaeis primarily a nocturnal spawner, and peak 

 spawning activity coincides with interruptions in cur- 

 rent velocity (Middaugh and Hemmer 1 984). In a study 

 near Santa Rosa Island, Florida, spawning activity of 



186 



