Evidence for ontogenetically and morphologically distinct alternative reproductive tactics in the invasive Round Goby Neogobius melanostomus

Abstract

Alternative reproductive tactics are characterized by the occurrence of discrete alternative morphs that differ in behavioural, morphological and physiological traits within the same sex. Although much effort has been made to describe the behaviour, morphology and physiology of such alternative morphs, less effort has been invested investigating how much overlap there is in the characteristics of such morphs in natural populations. We studied random population samples of the invasive Round Goby Neogobius melanostomus from five different localities in the river Rhine system in the Netherlands. We found two morphologically and physiologically distinct male morphs which likely represent alternative reproductive tactics. Almost all mature males under 9.35 cm total length had a gonadosomatic index > 3%, suggestive of a sneaker tactic, while nearly all males above 9.35 cm has a gonadosomatic index of < 3%, suggestive of a parental tactic. Cheek size and eye diameter alone were sufficient to distinguish the two morphs. Gonads had a different relationship with size in the two morphs, indicating separate growth trajectories. The gonad mass of sneaker morphs would be ca. 7.5 times as high as the gonad mass of parental morphs of the same total length after extrapolation. Few (9%) intermediates were found, suggesting that the expression of alternative reproductive tactics is determined before the first breeding season. This contrasts with studies on other goby species, which show evidence of plastic tactics that can be affected by social circumstances. We conclude that it is possible to distinguish two alternative male morphs in the Dutch Round Goby population using morphological measurements alone. Although behavioural observations are needed to provide conclusive evidence, the difference in GSI between these morphs indicates that these morphs reflect alternative reproductive tactics.

Growth and otolith morphology vary with alternative reproductive tactics and contaminant exposure in the round goby Neogobius melanostomus

“…Both male tactics also overlapped considerably in body size, which is in contrast to the findings of Bleeker et al . () who found little to no overlap in the body sizes of the two male tactics in several Dutch N. melanostomus populations. The present data (overlapping ages and body sizes but different somatic growth rates) lend support to the idea that male ARTs in N. melanostomus represent two separate developmental pathways, becoming fixed within the first year of life.…”

“…Neogobius melanostomus is a convenient study species for four reasons. First, N. melanostomus male ARTs have been described morphologically and physiologically and are easily categorized into two distinct morphs, a conventional guarder tactic and a parasitic sneaker tactic (Bleeker et al, ; Marentette et al, ). Although males may not be capable of rapidly switching between tactics, it is currently not known if the two tactics represent sequential or fixed strategies.…”

DNA methylation patterns in the round goby hypothalamus support an on-the-spot decision scenario for territorial behaviour

“…The round goby Neogobius melanostomus displays no major inter-or intra-sexual dimorphism outside the mating season. During the mating season, however, some males display a pale, inconspicuous colouring and do not guard nests (Figure 1a), while others undergo morphological changes and develop an intense black body pigmentation and puffy cheeks ( Figure 1a) [40,41]. The black-colored males occupy a nest, court females, and guard the eggs after oviposition [41].…”

“…The black-colored males occupy a nest, court females, and guard the eggs after oviposition [41]. Bleeker et al (2017) [40] described morphological differences in size, gonado-somatic index and other morphological features between putative sneaker males and territorial males, with few intermediates present in the population. This may indicate that the round goby mating phenotype is determined before the first breeding season, and does not depend on current conditions.…”

“…(2017) [40] suggest that all males above a size threshold of 9.35 cm have the potential to eventually become territorial at some point during mating season, provided nesting sites are available.…”