Mode of maternal provisioning in the fish genus Phalloceros: a variation on the theme of matrotrophy

Abstract

The placenta is a complex organ that shows high morphological diversity. Among fish, the first vertebrates that have evolved a placenta, the family Poeciliidae exhibits very diverse modes of maternal provisioning even among congeneric species. Here, we investigated the embryonic growth curve across seven recently-described species of the highly diverse genus Phalloceros (Eigenmann, 1907). We also investigated possible intraspecific differences and whether other female characteristics affected embryo mass. We found that embryo mass decreased until around stage 20 and then increased, resulting in a 1.5 to 3-fold mass gain from fertilization to birth. Embryo mass changed non-linearly with stage of development and was affected by species identity (or locality) and female somatic dry mass. This initial loss then gain of embryonic mass during development is unique among other Poeciliidae species and was conserved across populations and species, even though size at birth can vary. Other species instead either lose mass if they lack placentas or gain mass exponentially if they have placentas. The Phalloceros mode of maternal provisioning could thus represent a different form from that seen in other species of Poeciliidae.

Length-weight relationships of 24 stream-dwelling fish species from the Atlantic Forest in Rio de Janeiro, Brazil

“…For instance, P. harpagos fecundity and size is affected by predation (Gorini-Pacheco et al 2018). Also, being a matrotrophic species (the mother provides nutrients to the developing embryos throughout pregnancy), pregnancy stage could affect the female weight (Zandonà et al 2021) and thus be a source of variation in the LWR.…”

“…Length-weight relationships (LWR) are very important for ecological, ichthyological and fisheries studies as it allows one to calculate weight by measuring length and to estimate the condition factor, providing important information on fish physiology (Le Cren 1951, Froese 2006). Data about LWR can also elucidate different aspects about fish health, individual growth and, once the fish weight is known, it is possible to estimate the biomass of an entire population or community and thus investigate ecosystem processes and how fish species can affect them (Froese 2006, Giarrizzo et al 2006, Joyeux et al 2009, Camara et al 2011, Zandonà et al 2021, Lima et al 2022.…”

Increased superfetation precedes the evolution of advanced degrees of placentotrophy in viviparous fishes of the family Poeciliidae

“…For instance, some poeciliid species exhibit moderate and extensive placentotrophy without superfetation (e.g. most Phalloceros and Pamphorichthys species; [37]). In these taxa, the evolution of advanced placentotrophy is hardly explained by the preceding evolution of advanced degrees of superfetation, because they all lack superfetation.…”