Taxonomy, Phylogeny, and Size Evolution in the Spider Genus Megaraneus Lawrence, 1968 (Araneae: Araneidae)

TL;DR

This paper studies the evolution of extreme female size in a spider species from Africa and its placement in the spider family tree.

Contribution

The study provides the first phylogenetic placement of the spider genus Megaraneus and reveals that extreme female size evolved through female gigantism.

Findings

Megaraneus belongs to a spider clade previously known only from Asia and Australia, now found in Africa.

Extreme female size in Megaraneus evolved due to female gigantism, not male size reduction.

The clade's common ancestor already had extreme sexual size dimorphism, with further increases in females.

Abstract

Many spider species show large differences in size between males and females, but biologists still do not fully understand how or why this evolves. In this study, we focused on the systematics and evolution of a little-known African spider, Megaraneus gabonensis, in which females are about four times larger than males. We provided an overdue taxonomic description of the species and examined where the genus Megaraneus fits in the spider tree of life. Our findings show that this spider is a part of a clade that has previously been thought to live only in East Asia and Australia, but we now know it also occurs in Africa. We also found that in Megaraneus, the large size difference between the sexes resulted primarily because females became larger, not because males would become smaller. This research helps us better understand how spider body size evolves differently in males and females and highlights the importance of studying lesser-known species to fill in knowledge gaps about the natural world.

Among terrestrial animals, spiders exhibit the most striking examples of sexual size dimorphism (SSD) but better understanding of its evolution requires improved taxonomy and phylogeny. Many sexually dimorphic spiders lack adequate description, phylogenetic placement, and natural history observations. In South Africa, we documented the natural history of a poorly known spider, Megaraneus gabonensis (Lucas, 1858), with extreme, female-biased SSD (eSSD, female:male approximately 4:1). Here, we redescribe M. gabonensis, place Megaraneus Lawrence, 1968 phylogenetically for the first time, assess whether the observed eSSD represents an independent evolutionary origin, and test whether the macroevolutionary pattern is better explained by male dwarfism or female gigantism. The recovered phylogenetic placement of Megaraneus in the araneid ‘backobourkiines’, a clade previously considered as restricted to East Asia and Australasia, extends the range of this clade to the Afrotropics. We find that eSSD was present in the common ancestor of the ‘backobourkiines’, with further increases in female body length occurring independently in Megaraneus, Backobourkia Framenau, Dupérré, Blackledge & Vink, 2010, and the currently misplaced Parawixia dehaani (Doleschall, 1859). We conclude that the evolution of eSSD reflects a complex pattern of sex-specific size changes across spider phylogeny, but that in Megaraneus it results from female gigantism.

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