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Multiple Origins of Feeding Head Larvae by the Early Cambrian Canadian Journal of Zoology Multiple origins of feeding head larvae by the Early Cambrian Journal: Canadian Journal of Zoology Manuscript ID cjz-2019-0284.R1 Manuscript Type: Review Date Submitted by the 15-Apr-2020 Author: Complete List of Authors: Strathmann, Richard; Friday Harbor Laboratories and University of Washington Department of Biology, Is your manuscript invited for consideration in a Special Zoological DraftEndeavors Inspired by A. Richard Palmer Issue?: Cambrian, DEVELOPMENT < Discipline, EVOLUTION < Discipline, LARVAE Keyword: < Discipline, marine, planktotrophy © The Author(s) or their Institution(s) Page 1 of 66 Canadian Journal of Zoology 1 Multiple origins of feeding head larvae by the Early Cambrian1 Richard R. Strathmann Friday Harbor Laboratories, 620 University Road, Friday Harbor, WA 98250 USA [email protected] 1This review is one of a series of invited papers arising from the symposium “Zoological Endeavours Inspired by A. Richard Palmer” that was co-sponsored by the Canadian Society of Zoologists and the Canadian Journal of Zoology and held during the Annual Meeting of the Canadian Society of Zoologists at the University of Windsor, Windsor, Ontario, 14–16 May 2019. Draft © The Author(s) or their Institution(s) Canadian Journal of Zoology Page 2 of 66 2 Abstract: In many animals the head develops early, most of the body axis later. A larva composed mostly of the developing front end therefore can attain mobility and feeding earlier in development. Fossils, functional morphology, and inferred homologies indicate that feeding head larvae existed by the Early Cambrian in members of three major clades of animals: ecdysozoans, lophotrochozoans, and deuterostomes. Some of these early larval feeding mechanisms were also those of juveniles and adults (the lophophore of brachiopod larvae and possibly the ciliary band of the dipleurula of hemichordates and echinoderms); some were derived from structures that previously had other functions (appendages of the nauplius). Trochophores that swim with a preoral band of cilia, the prototroch, originated before divergence of annelids and molluscs, but evidence of larval growth and thus a prototrochal role in feeding is lacking for molluscs until the Ordovician. Feeding larvae that definitelyDraft originated much later, as in insects, teleost fish, and amphibians, develop all or nearly all of what will become the adult body axis before they begin feeding. On present evidence, head larvae, including feeding head larvae, evolved multiple times early in the evolution of bilaterian animals and never since. Key words: Cambrian, development, evolution, larvae, marine, planktotroph © The Author(s) or their Institution(s) Page 3 of 66 Canadian Journal of Zoology 3 Introduction Marine larvae differ in form from later juvenile and adult stages and differ among major clades of animals. Origins of feeding larvae are of special interest because feeding requires more elaborate structures than does motility and because larval feeding enables development from a smaller egg or to a larger juvenile or both (Pernet 2018). Diverse kinds of feeding larvae are of ancient origin, but inferences on their antiquity have varied widely. The times of origin, frequency of origin, and derivation of structures for larval feeding are related issues. Most recent studies of early larval origins have emphasized molecular genetic evidence for homology, convergence, or divergence in development (e.g., Raff 2008; Arenas-Menas 2010; Hejnol and Vellutini 2017; Marlow 2018; Gąsiorowski and Hejnol 2019). Fossils also provide evidence on early evolution of larvae (e.g., Nützel 2014;Draft Zhang et al. 2010, 2018). This discussion adds functional requirements for larval feeding and therefore focuses on groups with both a fossil record and living representatives. Some feeding larvae of bilaterian animals develop much of the larval body from what will become the anterior part of the later juvenile and adult. Some of these larvae have been called head larvae. Walossek and Müller (1993) used the term head larva in reference to the crustacean head specifically, but the term has since been applied more broadly to larvae that swim or feed with anterior structures before the trunk has formed (Lacalli 2005; Gonzalez et al. 2017) (Fig. 1). However, what is called a head and the extent of posterior development of the larva differs among animals. Although a more general name would be front-end larva, here I have used the term head larva in a broad sense. Though differing in extent of anterior to posterior development, head larvae nevertheless contrast with larvae that have formed all or nearly all of the eventual adult body axis when they initiate feeding. Head larvae develop with little expression of Hox genes because they begin mobility and © The Author(s) or their Institution(s) Canadian Journal of Zoology Page 4 of 66 4 feeding before posterior body parts have developed (Lacalli 2005; Hejnol and Vellutini 2017; Gąsiorowski and Hejnol 2019, 2020). The absence of posterior parts accounts for these larvae being “the other body plan” (Raff 2008). Head larvae occur in all three major branches of bilaterian evolution, the ecdysozoans, lophotrochozoans, and deuterostomes (Walossek and Müller 1990; Lacalli 2005). Where head larvae and larvae composed of more of the body axis occur in the same clade, the head larvae are more broadly distributed among lineages, which suggests an earlier origin. Estimated times of origin of feeding head larvae can be especially early, but how early? In four clades the evidence for early origins of feeding larvae includes informative fossils as well as extant larval forms. These clades are the Crustacea, Brachiopoda, Ambulacraria (Echinodermata + Hemichordata), and Mollusca.Draft Extant animals provide evidence for traits of their common ancestor (the common ancestor of the crown group), but the traits of that common ancestor originated in earlier ancestors prior to the crown group (in the stem group). Inferences for origins of larval traits therefore require evidence beyond traits of existing larvae. The evidence and therefore the basis of inferences for origins of feeding larvae differs for each clade (Table 1). For crustaceans, evidence includes a body fossil. For brachiopods there are indications of larval growth in the larval shells remaining at the umbos of fossil brachiopod shells. For the Ambulacraria, there are inferred homologies of extant larvae that are necessarily feeding larvae and the first appearances of fossils of adults in a clade inferred to have had such a larva. For molluscs, one of the ciliary bands used for feeding is also used for swimming; homology is uncertain for the additional bands required for feeding; larval shells are not preserved in the early fossils; and larval feeding might have originated somewhat later. The times of origin that are indicated by this evidence are contrasted with hypotheses for later or earlier origins of feeding larvae. Also discussed are apparent similarities and differences © The Author(s) or their Institution(s) Page 5 of 66 Canadian Journal of Zoology 5 for origins of feeding larvae in several other taxa, including those that are not head larvae. Crustacean nauplius Fossil evidence of an early origin of feeding larvae is most direct for crustaceans and their stem group. The nauplius of crustaceans is a head larva. The nauplius can begin feeding when only the three most anterior pairs of appendages are functional: the antennules, antennae, and mandibles (Fig. 1A) (Sanders 1963a,b). With addition of appendages posteriorly, but continued use of naupliar structures for feeding, the larva becomes a metanauplius. With further development, the anterior appendages change form and function, and food is no longer acquired by the naupliar feeding mechanism (Martin et al. 2014). Antiquity of feeding nauplii. Fossil larvae from the Early Cambrian include a metanauplius stage; structures that indicateDraft that this larva fed like a nauplius are the labrum and basal medially directed spines on the second antenna and mandible (Fig. 2) (Zhang et al. 2010). Although the hypostomal spine and pair of eyes are not features of extant nauplii, extant nauplii possess similar structures for feeding (Sanders 1963a; Ferrari et al. 2011). This early eucrustacean larva is from the Yu’anshan Formation, which may correlate with late Atdabanian (Cambrian Stage 3) (Zhang et al. 2010; Yuan et al. 2011). Other fossil evidence for an early origin of feeding nauplii is from the feeding head larvae of the extinct phosphatocopines, if the phosphatocopines are related to eucrustaceans as a sister group rather than nested within them. The phosphatocopine larvae had naupliar structures for feeding (Maas and Waloszek 2005; Zhang et al. 2012; Haug et al. 2013; Eriksson et al. 2016). The phosphatocopine larvae differ from the eucrustacean nauplius in having initially four pairs of appendages instead of three, but the labrum and the spines at the base of second antennae and mandibles of their head larvae are inferred to be homologous (Siveter et al. 2001; Maas and Waloszek 2005). Fossils, inferred homologies, and inferred relationships therefore indicate that © The Author(s) or their Institution(s) Canadian Journal of Zoology Page 6 of 66 6 the feeding mechanism of the crustacean nauplius had evolved before the common ancestor of eucrustaceans. Derivation of structures for naupliar feeding. Initiation of motility and feeding with only the anterior appendages present may
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