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

The plainfin midshipman (Porichthys notatus Girard, 1854) is a toadfish with two distinct reproductive male tactics: 'guarder males' and 'sneaker males'. These two tactics 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 question, 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 average 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 versus 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.