Role of gamete limitation in the occurrence of ‘abnormal early clutches’ on female American lobster, Homarus americanus, in eastern Canada

Abstract

A large-scale survey in 2011–2014 found that ovigerous female American lobsters, Homarus americanus, carrying “abnormal” early clutches (< 50% of abdomen covered by eggs) were ubiquitous in eastern Canada. This could be caused by reduced production of oocytes, or insufficient sperm to fertilize all oocytes produced. To address the oocyte limitation hypothesis, ovarian fecundity was assessed in 343 pre-spawn females and clutch fecundity in 169 post-spawn females sampled from ten sites across eastern Canada in May and June 2014. All females produced and spawned enough oocytes to form a full clutch, thus invalidating the oocyte limitation hypothesis. To address the sperm limitation hypothesis, the prevalence of different “types” of sperm plug in the seminal receptacle of 1735 wild-mated, pre-spawn or ovigerous females was surveyed in July–August 2014 and 2015, and the effect of sperm plug type on fecundity/fertility was examined in 60 wild-mated, laboratory-spawning females beginning in July 2014. Among the pre-spawn wild-mated females, 88.3% had a hard plug, 5.9% a soft plug and 5.8% no plug. These sperm plug types corresponded to sharply decreasing amounts of stored semen/sperm in post-spawn females. In the laboratory, 11 of 20 females with no plug produced a full clutch that became “abnormal” through egg loss or was completely dropped within 2 weeks of spawning. Thus, sperm limitation is one likely cause of abnormal early clutches on female American lobster, which may be occasioned by fishery-induced reductions in male reproductive potential and/or density-dependent processes occurring at currently high population levels.