Abstract
The maternal-embryonic nutritional relationship in chondrichthyans has been poorly explored. Consequently, accurately discerning between their different reproductive modes is difficult; especially lecithotrophy and incipient histotrophy. This present study is the first to assess changes in mass throughout embryonic development of an oviparous chondrichthyan other than Scyliorhinus canicula. Heterodontus portusjacksoni egg cases were collected and used to quantify the gain or loss of wet mass, dry mass, water content, inorganic and organic matter from freshly deposited eggs (without macroscopically visible embryos) to near full-term embryos. A loss in organic mass of ∼40% found from this study is approximately double the values previously obtained for S. canicula. This raises concerns for the validity of the current threshold value used to discern between lecithotrophic and matrotrophic species. Accordingly, 26 studies published in the primary literature between 1932 and 2012 addressing the maternal-embryonic nutritional relationship in sharks were reviewed. Values for changes in mass reported for over 20 different shark species were synthesised and recalculated, revealing multiple typographical, transcribing, calculation and rounding errors across many papers. These results suggest that the current threshold value of −20% established by previous studies is invalid and should be avoided to ascertain the reproductive mode of aplacental viviparous species.
Highlights
Chondrichthyans demonstrate a diverse range of reproductive strategies which are categorised into two distinct modes: oviparity and viviparity [1], [2]
Matrotrophy refers to the developmental pattern where the external yolk sac is supplemented by maternal sources once the initial yolk stores are exhausted; this pattern only occurs in viviparous species
The embryonic development of Heterodontus portusjacksoni was studied with the aim to determine the validity of using the current threshold value to differentiate aplacental reproductive modes
Summary
Chondrichthyans (sharks, rays and skates) demonstrate a diverse range of reproductive strategies which are categorised into two distinct modes: oviparity (egg laying) and viviparity (live bearing) [1], [2]. Lecithotrophy refers to the developmental pattern where the yolk, produced by the maternal liver and sequestered in the external yolk sac (EYS), persists for the entire gestation period; providing the only source of nutrition to the embryo throughout its development [4]. While this is the case for all oviparous species, lecithotrophic viviparity exists, and is the predominant reproductive mode exhibited by chondrichthyans [1]. Yolk supplementation can occur through a variety of maternal processes such as uterine secretions (histotrophy), placental transfer (placentatrophy), or through the consumption of unfertilised eggs (ovatrophy) or sibling embryos (adelphotrophy) [1,5]
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