10 



CoUette (1962:139) examined specimens between 

 12.5 and 14.9 mm (presumably total length) that had 

 not yet developed pored lateral-line scales and specimens 

 between 15.9 and 17.1 mm that had one or no pored 

 scales. Our smallest specimen (14 mm in standard 

 length) had two or three pored scales anteriorly, but 

 another specimen ( 1 7 mm in standard length ) lacked 

 them. Evidently pored scales first appear at an average 

 total length of about 15 mm and at an age of slightly 

 less than 1 week. 



After about 2 weeks the lateral-line pores ranged from 

 15 to 18, nearly the full complement; an adult may 

 have as few as 13 but usually has 20—23. Squamation 

 was nearly complete; scales were lacking only on the 

 nape and, in some juveniles, on the opercles. The head 

 canals in larger juveniles were complete except for the 

 supratemporal canal, which may require many months 

 to form and sometimes never completely closes. In the 

 14-day-old darters the subocular canal was complete 

 except for one small inteiTuption. 



At the end of approximately 3 weeks squamation 

 was complete, and the opercular spine had formed. 



After 1 J/a months the green pigment had formed 

 on the sides, and the subdistal row of red dots had 

 appeared in the spinous dorsal fin; the females could 

 be sexed by their characteristic genital papillae. The 

 fish at this age are miniature adults. 



The development of scjuamation, the lateral-line 

 system, head pores, and adult bodily proportions are 

 illustrated in Fig. 7. 



The growth rate of the slough darter follows the 

 typical sigmoid pattern with the most rapid growth 

 shortly after hatching (Fig. 8) . Growth presumably 

 continues throughout life but at a slower and slower 

 rate. There is little indication of sex influence on size. 

 Although the largest specimen collected was a male, 

 the means for the sexes at various ages were usually 

 similar. The young are sexually mature at the end of 

 the 1st year. 



PHYLOGENETIC RELATIONSHIPS 



The habits and reproductive cycle of E. gracilc 

 clearly represent an evolutionarily advanced stage. Its 

 habitat and place of egg deposition are similar to those 

 reported for R. c.vf// (Winn 1958:160-161). How- 

 ever, its sexual dimorphism, spawning position, place 

 and manner of oviposition, number of eggs, late spawn- 

 ing time, short breeding period, rapid de\('lopment. and 

 lack of ])arental care all indicate a greater degree of 

 specialization than in exile. 



These characteristics also indicate more similarities 

 with the reproductive pattern of E. mictopcrca, a darter 

 that occupies a similar habitat and is in the subgenus 

 Microprrca, than with those of other species discussed 

 by Winn (1958). Differences in size of adults, type of 

 sexual dimorphism, egg size, place of oviposition. and 

 hatching time are relati\cl\ minor between llie subi;enera 



Hololepis and Microperca. Collette (1965:605) postu- 

 lated a clo.se relationship between the two groups because 

 of similar patterns of breeding tubercles. 



POPULAflON COMPOSITION 



During late winter and early spring adults were easy 

 to collect, as they tended to be concentrated in winter 

 aggregations in deep sand- or mud-bottomed pools. In 

 late spring they became scarce and apparently scattered. 

 On a few occasions no adults could be found in the 

 study area, and they remained scarce after the spawn- 

 ing period. In fact, only two adults were taken after 

 July 1 although no great time or effort was e.xpended 

 trying for adults in the postspawning season. The post- 

 spawning population in the study area consisted predom- 

 inantly of young-of-the-year. By early July when the 

 young had become large enough to be seined, the species 

 was again abundant. A series of 30—35 young could 

 be secured with a few short seine hauls. A se\ere fish 

 kill in early December terminated the study when a 

 sudden cold spell following drought froze the reduced 

 and stagnating pools. 



The prespawning population ranged in age from 

 those approaching 1 year to 4 year olds. In the sample 

 taken from the study area from February to April, 1964. 

 2-year-old fish (year-class of 1962 t made up 57 percent 

 of the population, and 2- and 3-year-old fish together 

 constituted 94 percent of the darters taken (Table 3). 

 Howe\er. the age composition may \an- from year to 

 year. 



Table 3. — Distribution of sexes and age classes in the sam- 

 ples of the prespawning population collected from the study area. 



The distribution of age classes in the prespawning 

 population sample points out the vulnerability of short- 

 season spawners to such conditions as unusually high 

 or low water le\els dining the spawning ])eriod. Of 

 the 109 adults preserved (some adults were kept alive^ 

 in the 3-monlh period, only four fish were 1 year old, 

 suggesting that the 1963 spawning was only partially 

 successful. Only three fish were 4 years old (year-class 

 of 1960^, suggesting that 1960 may also have been a 

 poor year for spawning success. \Vhether E. gracile 

 attains an age of more than 4 years is not known, but 

 it is likely that individuals 4 or more years of age nor- 

 mally make up a \ery small percentage of the population. 



It appeare that the physical habitat is the principal 

 enemy of the species, even with its wide range of toler- 

 ance, and that changes in the habitat probably control 

 ihe popul.ttion le\el. 



