Sperm activation in the polychaete annelid Arenicola marina was investigated using video microscopy following the in vitro and in vivo manipulation of gametes. Careful observation of spermatozoa obtained from spawning animals indicated that they were immotile immediately after their release from the gonopores, but that they subsequently became motile following dilution in sea water. It was determined that under the pH conditions of the coelomic cavity (pH 7.3) sperm motility was suppressed, whereas upon dilution in sea water buffered at pH 8.2, motility was triggered. It is hypothesised that sperm activation, under normal conditions, occurs in two stages. The first results in the liberation of free spermatozoa from sperm morulae, allowing them to be spawned; this process is stimulated by an endocrine factor and occurs in vivo during normal spawning. The second involves the switching on of the sperm flagellar apparatus, which occurs when spermatozoa are subjected to change in extracellular pH associated with their dilution in sea water. Pharmacological agents such as ammonium chloride and quinacrine are shown to stimulate the breakdown of sperm morulae and the acquisition of sperm motility. Motile spermatozoa of A. marina, in artificial sea water buffered at pH 8.2, can remain motile for over 5 h, have a beat frequency of approximately 50 Hz and have average path velocities of between 100 and 120 µm s-1. Motile spermatozoa under these conditions can also display the phenomenon of intermittent swimming.

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