Remipedes are outstanding in regard to their habitat preferences but as well exceptional in respect to their phylogenetic position. Current data supports the hypothesis that insects shared a common ancestor with remipedes. However, how representatives of these marine, cave dwelling centipede-like crustaceans transformed into six-legged, insects or how an ancestor possibly could have looked like is still unclear. To understand the origin of insects and their success as the most species-rich arthropod group, remipedes resemble a so called key taxon.

Evolutionary scenario of insect evolution from remipede crustaceans. The red question marks represent nodes that are ambiguous based on different data. ©BMvR

Comparative genome data of remipedes in combination with early insect lineages allow us to address several still obscure questions related to the not fully understood origin and evolution of the insects from within crustaceans. One obstacle, which is now solvable due to the new sequencing technolgies, was so far the large genome size of remipedes, Xibalbanus tulumensis throughs 7.9 Gb into the ring. The genome structure is so far unknown as is its possible complexity. Morphologically it remains challenging to conclude how the last common ancestor of today’s modern remipedes is linked to todays early insects such as collembolans, diplurans and proturans – and how the body plans were modified or adapted during evolution. This and several other questions such as which gene sets compose the remipede’s genome and if it reflects rather a crustacean or insect habitus can be tackled with the successful sequencing and annotation of a high-quality genome in the present remipede genome sequencing project

However, remipedes have even one more fascinating feature in their limb sleeves. Utilizing transcriptomics, proteomics and functional 3D morphology the first crustacean venom and its delivery system was described from Xibalbanus tulumensis. It was revealed that X. tulumensis probably utilizes a complex venom cocktail to subdue prey including knottin-like putative neurotoxins similar to known spider toxins. From an evolutionary perspective the benefit of using venom for predation is quite intriguing for a blind, cave dwelling organism that lives in habitats that mostly lack of species diversity and abundancy – so once a prey is grapped all chances for an escape are better eliminated by employing venom. In the best case a cocktail that includes a neurotoxic component



Profile of expressed and secreted (P) peptides and proteins in the venom glands of Xibalbanus tulumensis (von Reumont etal. 2017, Toxins).

Interestingly, venoms evolved in almost all animal groups, however, the mechanisms that drive venom compositions as one of the most important evolutionary adaptations are not well understood. One reason is that whole genome data of venomous species is generally sparse and only few distant taxa are covered (snakes, spiders, scorpions, platypus). The genome of remipedes enlightens the mechanisms and origin of toxin gene evolution in so far neglected venomous crustaceans. In addition, remipedes contribute further to answer questions on insect venom evolution as venomous crustacean group that shared a common ancestor with early hexapods.

Which processes of toxin evolution are similar to insects? Are venom proteins rather single copy, or multi copy genes? How did multi-domain toxins evolve in this lineage and how originated the remipedes unique, lineage specific toxins? What are potential applications based on the toxin activity? These are only few of the venom related questions.