Growth and age differences between two male alternative reproductive tactics in the plainfin midshipman

he plainfin midshipman (Porichthys notatus Girard, 1854) is a toadfish with two dis-

tinct reproductive male tactics:‘guarder males’ and‘sneaker males’. These two tac-

tics are anatomically, physiologically and behaviourally distinct from one another at

sexual maturity, but it remained unclear whether these two male types remain fixed

or plastically transition from one to another across a lifetime. To examine this ques-

tion, we aged the sagittal otoliths (inner ear bones) of 371 adult plainfin midshipman

via the break-and-burn method. Our study showed that guarder males were on aver-

age larger and older (mean age: 7.0 years, range: 3–17) than sneaker males (mean

age: 3.4 years, range: 2–8) and females (mean age: 5.1 years, range: 2–10). There was

considerable overlap in age between guarder and sneaker males; the two tactics were

of similar body sizes at age 3, but after that guarder males were always bigger. We

used a two-factor von Bertalanffy growth functions (VBGFs) to show that males ver-

sus females and guarder versus sneaker males had divergent growth trajectories. The

VBGF estimates and distance measures from the otolith centre to the first annual

otolith mark suggest that sneakers initially grew more quickly. Our results support

the hypothesis that the two male reproductive tactics in plainfin midshipman likely

represent distinct fixed life-history pathways that differ early in their development.