Reproductive characteristics of Ratabulus diversidens and Ambiserrula jugosa (Pisces: Platycephalidae) from continental shelf waters of south-eastern Australia

Abstract

The reproductive characteristics of co-occurring freespine flathead, Ratabulus diversidens, and mud flathead, Ambiserrula jugosa, that interact with fisheries across continental shelf waters of eastern Australia were examined. Samples were collected across three depth strata and two locations on a monthly basis over two years. Males of both species matured younger and at smaller total lengths (TL) than females. Estimated TL and age (years) at maturity (L50 and A50, respectively) of R. diversidens also varied between locations, but differences were not related to differential growth. Although some mature individuals of both species occurred year-round, they were most prevalent and gonadosomatic indices greatest, between the austral spring and autumn. Mature R. diversidens almost exclusively occurred in deeper offshore waters, whereas the opposite was evident for A. jugosa. Both species displayed asynchronous oocyte development, and were thus considered capable of spawning more than once throughout each spawning season. Potential batch fecundity was positively related to TL for R. diversidens, but not A. jugosa, possibly due to the small size of the latter species. The sex ratios for R. diversidens varied between locations and length categories, and like A. jugosa the larger categories were skewed towards females, a result of divergent growth between sexes. Macroscopic and microscopic evidence indicated both species were gonochoristic. The data provide new information for fisheries management consideration and contribute to the data-poor international knowledge base of platycephalid biology.

Spawning and maturity traits of coexisting Platycephalidae (Platycephalus caeruleopunctatus, Platycephalus grandispinis, Platycephalus richardsoni) from southeast Australia

“…All three species, along with other coexisting platycephalids, Ratabulus diversidens and Ambiserrula jugosa (Barnes et al ., 2021), displayed a similar reproductive characteristic of (a) asynchronous oocyte development, which potentially allows each individual to spawn multiple times within each spawning period (Lowerre‐Barbieri et al ., 2011) and (b) an extended spawning period spanning several months (potentially year‐round for P. caeruleopunctatus and P. grandispinis ). These risk‐adverse traits allow individuals to extend and maximise egg production across a spawning period (Lowerre‐Barbieri et al ., 2011; McBride et al ., 2013), enhancing the probability of successful reproduction during times of favourable oceanographic ( e.g ., temperature, current direction, strength) and ecological ( e.g ., types and levels of prey and predators) conditions that facilitate optimal dispersal and survival of developing eggs and larvae (Lambert & Ware, 1984; Lobel, 1989; Lowerre‐Barbieri et al ., 2011; McBride et al ., 2015).…”

“…The potential depth of spawning habitat of each platycephalid may reflect a further ecological adaptation to regional oceanographic conditions (Ciannelli et al ., 2015). Mature (including those in spawning condition and recently spawned) individuals of all three platycephalids, along with coexisting R. diversidens (Barnes et al ., 2021) and sillaginid species (Gray et al ., 2014), predominantly occurred in waters >30 m depth. Oceanographic model simulations have identified that broadcast spawning in waters 30–100 m deep is optimal for transporting developing eggs and larvae to inshore nursery habitats along the east coast of Australia, potentially benefitting population renewal (Cetina‐Heredia et al ., 2019; Everett et al ., 2017; Schilling et al ., 2020).…”

“…Reproductive isolation among these sympatric platycephalids may be achieved by fine‐scale spatial and temporal partitioning of spawning activity, together with species recognition (Colin & Bell, 1991; Mercier & Hamel, 2010; Sancho et al ., 2000; Wellenreuther & Clements, 2007), even though there was a broad overlap in potential spawning times and areas inhabited among species. For example, mature (including ripe and recently spent) individuals of P. richardsoni (and R. diversidens ) predominantly occurred in the deepest depth stratum (and potentially deeper waters, Fairbridge, 1951), P. caeruleopunctatus and P. grandispinis in the mid stratum and A. jugosa the shallow stratum (Barnes et al ., 2021). Based on levels of reproductive investment and the above, all three species (and R. diversidens ) likely spawn in small discrete groups (Sadovy, 1996) and potentially limited diel periods (Bani et al ., 2009).…”

“…Divergent maturation strategies have been observed among platycephalids, with most species being gonochoristic as evidenced here, but other species protandrous hermaphrodites (Coulson et al ., 2017; Fujii, 1971; Sunobe et al ., 2016;). Skewed sex ratios favouring females in larger length groups (compared to small length groups), as observed here for P. richardsoni , have been used as evidence of sex change in some species, but length‐differentiation in sex ratios of platycephalids has mostly been shown to be the result of sexually dimorphic growth characteristics with females growing larger, but not necessarily older, than males (Akita & Tachihara, 2019; Barnes et al ., 2021; Coulson et al ., 2017). This latter scenario is the case with the platycephalids examined here (Barnes, Gray, & Williamson, 2011).…”

“…In the absence of strong competitive needs, males in contrast do not need to attain large body sizes for maximising successful fertilisation (Parker, 1992). Barnes (2013) determined that batch fecundity was positively related to total length for the three species examined here, as for other platycephalids (Barnes et al ., 2021; Gray & Barnes, 2015). This trait is widely observed among marine teleosts that display either asynchronous and synchronous oocyte development (Gwinn et al ., 2015; McBride et al ., 2015).…”