Abstract
Polyclad flatworms offer an excellent system with which to explore the evolution of larval structures and the ecological and developmental mechanisms driving flatworm and marine invertebrate life history evolution. Although the most common mode of development in polyclads might be direct development (where the embryo develops directly into a form resembling the young adult), there are many species that develop indirectly, through a planktonic phase with transient larval features, before settling to the sea floor. In this review, I introduce polyclad life history strategies, larval diversity and larval anatomical features (presenting previously unpublished micrographs of a diversity of polyclad larvae). I summarize what is known about polyclad larval development during the planktonic phase and the transition to the benthic juvenile. Finally, I discuss evolutionary and developmental scenarios on the origin of polyclad larval characters.The most prominent characters that are found exclusively in the larval stages are lobes that protrude from the body and a ciliary band, or ciliary tufts, at the peripheral margins of the lobes. Larvae with 4–8 and 10 lobes have been described, with most indirect developing species hatching with 8 lobes. A ventral sucker develops in late stage larvae, and I put forward the hypothesis that this is an organ for larval settlement for species belonging to the Cotylea. Historically, the biphasic life cycle of polyclads was thought to be a shared primitive feature of marine invertebrates, with similarities in larval features among phyla resulting from evolutionary conservation. However, our current understanding of animal phylogeny suggests that indirect development in polyclads has evolved independently of similar life cycles found in parasitic flatworms and some other spiralian taxa, and that morphological similarities between the larvae of polyclads and other spiralians are likely a result of convergent evolution.
Highlights
The Platyhelminthes are an extremely diverse spiralian clade (Figure 1A, B) that includes the catenulids and rhabditophorans but excludes the acoels and nemertodermatids [2]
This review examines polyclad larval diversity, development and evolution by summarizing classical literature, synthesizing recent work and presenting new data
There is still much that we do not know about polyclad larval diversity, development and evolution
Summary
The Platyhelminthes (flatworms) are an extremely diverse spiralian clade (over 100,000 known species [1]) (Figure 1A, B) that includes the catenulids and rhabditophorans but excludes the acoels and nemertodermatids [2]. The transition from a slightly laterally flattened planktonic larva to the dorso-ventally flattened benthic adult involves changes in the mode of feeding and locomotion, from ciliary to muscular in both cases This transition in polyclads is gradual [14,37], and involves the development of the gut diverticula, a plicate pharynx, and diagonal body wall, dorso-ventral, parenchymal and sucker muscles [37,52]. Ciliary bands for locomotion and feeding and sensory apparatus for orientation in the water column are found in many spiralian and metazoan larvae Morphological similarities in these larval features led to the suggestion that they are evolutionarily conserved and that the biphasic life cycle is a shared primitive feature of marine invertebrates [11,69,70]. Understanding the intra-specific variation of larval phenotypes on which contemporary selective pressures and developmental, functional and physiological constraints act, will allow us to gain a better picture of the microevolutionary processes that generate macroevolutionary patterns in mode of development
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