Alexandra S. Savchenko and Rony Huys, from the Natural History Museum in London, will present a talk about their work, entitled “Radically Divergent Body Plans in the Crustacea: How to Fit Them in the Tree of Life.”

The presentation will begin at 4:00 PM via Zoom, preceded by an informal Q&A from 3:45 – 4:00 PM. Click here to join the Zoom meeting: https://bit.ly/3uEBr02.

Meeting ID: 918 1335 1440
Passcode: 170744

No group of plants or animals on Earth exhibits the range of morphological diversity as seen among the extant Crustacea. This structural disparity is best demonstrated by the symbiotic Copepoda and the Thecostraca (barnacles and allies).

The copepod fossil record extends back to the late Carboniferous period, but extrapolation of available divergence dates suggests that this pancrustacean clade had already originated in the Cambrian, providing ample time for endless experimentation with form and function. Hundreds of millions of years of evolution, and their successful colonization or utilization of virtually every metazoan phylum, have generated an astounding diversity in body morphology and life cycle strategies, both of which are arguably unparalleled in the Crustacea. Copepods occupy virtually every imaginable aquatic habitat, are arguably the most abundant metazoan clade in the Tree of Life and have been mistaken for molluscs, worms, and other distantly related animals due to their morphological plasticity. Many adult parasitic copepods exemplify a major divergence from the typical copepod body plan and lack any external evidence in support of their arthropod identity.

Tantulocaridans, together with epicaridean isopods, rhizocephalan barnacles and nicothoid copepods, are marine parasitic crustaceans that exclusively utilize other crustaceans as hosts. Among the Crustacea they can be regarded as non-conformists, deviating from the typical ontogenetic sequence, moulting cycle and basic body plan. Unlike other crustaceans there is as yet no published substantiated evidence for moulting between the successive stages of their unique dual life cycle. Tantulocaridan larvae lack antennules – a basic crustacean attribute – and hold the position of pre-eminence of being the smallest segmented larvae known in the Crustacea. Attempts to homologize their highly specialized feeding apparatus with that of other crustaceans proved largely futile.

The divergent body plans and life cycle strategies of both tantulocaridans and many highly derived copepod lineages defy any attempts to place them in a higher-level classification on morphological grounds alone. Secondly, some copepod families, such as the Monstrillidae and Thaumatopsyllidae, demonstrate how extremely powerful natural selection can be in shaping morphology to meet functional needs so that distantly related taxa may appear uncannily similar. During this presentation we will use a few examples to show how the use of molecular sequence data (1) can help resolving some of the controversial issues that had reached a temporary impasse in the phylogeny and classification of some crustaceans with radically divergent body plans, (2) is not the Holy Grail but has limited usefulness in placing other taxa, and (3) can lead to the discovery of previously overlooked morphological characters.