Is the Orton’s rule still valid? Tropical sponge fecundity, rather than periodicity, is modulated by temperature and other proximal cues

Abstract

Although the Orton’s rule was already questioned for several invertebrate lineages, it remains unchallenged in sponges. To assess its validity in Porifera, we investigated the reproductive cycle of five species of Demospongiae on a tropical rocky shore, aiming to determine the effect of some environmental variables (EVs) on the periodicity, density of reproductive elements, and population engagement in reproduction. All species reproduced continuously, with low percentage of reproductive individuals and low fecundity. Each species presented a set of models containing different EVs to explain their reproductive traits. In general, the relationship of the EVs on the percentage of reproductive individuals and density of gametes and embryos was delayed in 1–3 months. Temperature was amongst the factors that best explained the species reproduction, with a delay of 1 month being the factor most consistently found amongst the models. In addition, we carried out a meta-analysis and discovered that in temperate regions most species reproduced periodically, whilst in the tropics, a small percentage reproduced continuously. Our findings suggest that Orton’s Rule is partially sustained, as species reproduced continuously, but challenge the lack of the influence of the temperature and other EVs in the reproduction of tropical marine invertebrates.