MATERIALS AND METHODS 



The clams were collected from shallow 

 water in the Tred Avon River opposite the 

 Bureau of Commercial Fisheries Biological 

 Laboratory in Oxford, Md. These samples 

 were taken weekly during periods of active 

 spermatogenesis and nnonthly during periods 

 of maturation inactivity. In spring 1963, addi- 

 tional samples were collected from the 

 Potomac, Patuxent, and Chester Rivers. 



Each sannple, consisting of 10 or more 

 clams 2 or more inches long, was fixed in 

 Kahle's fluid (Guyer, 1953), dehydrated in 

 alcohol, cleared in xylene, and mounted in 

 paraffin. The gonad of each clam was then 

 sectioned at about 7 microns with a standard 

 rotary microtome. The sections were stained 

 in either Harris' or Delafield's hematoxylin 

 and counterstained with eosin. Of the 1,063 

 clams examined about 50 percent were males. 



RESULTS 



Spermatogenesis was inactive inmaleclanns 

 collected from May through July 1 961 and 1962. 

 Each alveolus consisted of follicular cells with 

 nnany atypical cell inclusions with 1 to 1 6 

 nuclei (fig. lA). In addition to these multi- 

 nucleated inclusions, groups of spernn, or 

 sperm-balls, were found in about 50 percent 

 of the males examined (fig. 1 B). Probably these 

 sperm-balls were derived from sperm that 

 were not released from the previous spawning 

 period. Coe and Turner (1938) noted bodies 

 similar in size but described them as nuclei 

 that were pycnotic. Loosanoff (1937) reported 

 that spernnatozoa remaining in Mercenaria 

 ( Venus ) mercenaria underwent cytolysis, but 

 in Mya this was not observed. 



In both 1961 and 1962, active spermato- 

 genesis began in August. Primary and second- 

 ary spermatogonia were observed along the 

 base of the alveolar walls, with secondary 

 spermatogonia protruding toward the center 

 of the alveoli. By early September spermato- 

 genesis was progressing at a rapid rate. The 

 more advanced sex cells, primary and second- 

 ary spernnatocytes, and spermatids were 

 closer to the center of the alveolus (fig. 2). 

 During this period all stages of spermato- 

 genesis could be found. 



In 1961, ripe clams were found in the latter 

 part of September, while in 1962 this stage was 

 observed several weeks earlier. Just prior to 

 spawning, each alveolus consisted of mature 

 spermatozoa, with their tails filling the center 

 of the lumen (fig. 3). Only a few of the inclu- 

 sions so prevalent during the summer were 

 present. The sperm-balls, numerous earlier, 

 had disappeared, indicating that the spernn 

 had been released into the center of the lumen. 



Spawning was first observed on October 12, 

 1961, and September 17, 1962. In partially 



spawned-out clams (fig. 4) the rows of sperm 

 were more separated, and the lumen was not 

 as fully packed with sperm tails. A few inclu- 

 sions could still be found near the alveolar 

 walls. In 1961 and 1962, spawning was com- 

 pleted by early November. Each alveolus con- 

 tained follicular cells with a few inclusions, 

 far less than found during the summer (fig. 5A). 

 In addition, many alveoli also contained un- 

 spawned sperm (fig. 5B). Instead of the spernn 

 undergoing cytolysis, they were grouped into 

 sperm-balls and retained throughout the winter 

 (fig. 6). 



A second cycle of spermatogenesis began 

 shortly after fall spawning (fig. 7). Primary 

 and secondary spermatogonia were develop- 

 ing along the base of the alveolar walls. Fronn 

 January through March little further develop- 

 ment was observed. The follicular cells had 

 only a few multinucleated inclusions. Also, 

 the gonads appeared watery. 



During April, when active spermatogenesis 

 was expected with a rise in temperature, the 

 spermatogonia apparently underwent cytolysis, 

 and the clam entered the summer or inactive 

 stage (fig. lA). In many clanns it appeared 

 that sperm were liberated fronn the sperm- 

 balls into the center of the lumen. Coe and 

 Turner (1938) reported that an aberrant mode 

 of nneiosis occurred in Mya and the multi- 

 nucleated cells (inclusions) form spernnatids 

 that later transform into spermatozoa. Atypical 

 spermatogenesis was not observed in Maryland 

 clams. 



In spring 1962, the clanns had no prinnary and 

 secondary spernnatocytes or spernnatids; dur- 

 ing spring 1963, only 1 percent of the clams 

 contained these stages of spermatogenesis. 

 In no microscopic sections were the clams as 

 ripe as during the previous fall. Instead, the 

 spernn present were surrounded by follicular 

 cells with inclusions (fig. 8). Spring spawning 

 was not observed either in 1962 or 1963. As 

 in the fall, the unspawned sperm were grouped 

 into balls and carried in the follicular cells 

 throughout the summer (fig. IB). 



In addition to the histological examination 

 of the seasonal changes in the gonad of the 

 soft- shell clam, plankton samples were col- 

 lected and studied for the occurrence of Mya 

 larvae, and the contents of bottle collectors 

 (after Thorson, 1946) were exannined for newly 

 settled Mya . In fall 1961 and 1962 Mya larvae 

 and set were found in the Tred Avon River. 

 No larvae or set were observed in spring 



1962, and no larvae were found in spring 1963. 

 Five newly set Mya were caught in two bottles 

 exposed from May 6 to June 17, 1963. The 

 presence of these juvenile clanns indicated 

 that some spawning had occurred during spring 



1963, although my exannination of the testis 

 and the plankton indicated no apparent spawning 

 had taken place. It is possible that these clams 

 developed from eggs spawned outside the Tred 

 Avon River. Pfitzenmeyer (1962) observed 



