Ontogeny of the holothurian larval nervous system: evolution of larval forms
- 214 Downloads
Echinoderm larvae share numerous features of neuroanatomy. However, there are substantial differences in specific aspects of neural structure and ontogeny between the dipleurula-like larvae of asteroids and the pluteus larvae of echinoids. To help identify apomorphic features, we have examined the ontogeny of the dipleurula-like auricularia larva of the sea cucumber, Holothuria atra. Neural precursors arise in the apical ectoderm of gastrulae and appear to originate in bilateral clusters of cells. The cells differentiate without extensive migration, and they align with the developing ciliary bands and begin neurogenesis. Neurites project along the ciliary bands and do not appear to extend beneath either the oral or aboral epidermis. Apical serotonergic cells are associated with the preoral loops of the ciliary bands and do not form a substantial commissure. Paired, tripartite connectives form on either side of the larval mouth that connect the pre-oral, post-oral, and lateral ciliary bands. Holothurian larvae share with hemichordates and bipinnariae a similar organization of the apical organ, suggesting that the more highly structured apical organ of the pluteus is a derived feature. However, the auricularia larva shares with the pluteus larva of echinoids several features of neural ontogeny. Both have a bilateral origin of neural precursors in ectoderm adjacent to presumptive ciliary bands, and the presumptive neurons move only a few cell diameters before undergoing neurogenesis. The development of the holothurian nervous systems suggests that the extensive migration of neural precursors in asteroids is a derived feature.
KeywordsOntogeny Neural development Echinoderm Evolution
This study was supported in part by a discovery grant from NSERC (Canada) to RDB and an NSERC post-doctoral fellowship to CDB. Mark Martindale (University of Hawaii) and Michael Hadfield (University of Hawaii) are gratefully acknowledged for supporting aspects of this work. Thurston Lacalli provided helpful comments on the manuscript.
MOV 57 MB
MOV 52.5 MB
- Burke RD, Angerer LM, Elphick MR, Humphrey GW, Yaguchi S, Kiyama T, Liang S, Mu X, Agca C, Klein WH, Brandhorst BP, Rowe M, Wilson K, Churcher AM, Taylor JS, Chen N, Murray G, Wang D, Mellott D, Olinski R, Hallbook F, Thorndyke MC (2006b) A genomic view of the sea urchin nervous system. Dev Biol 300:434–460PubMedCrossRefGoogle Scholar
- Chen CP, Tseng CH, Chen BY (1995) The development of the catecholaminergic nervous system in starfish and sea cucumber larvae. Zool Stud 34:248–256Google Scholar
- David B, Mooi R (1997) Major events in the evolution of echino-derms viewed by the light of embryology. In: Mooi R, Telford M (eds) Echindoerms: San Francisco. Balkema, Rotterdam, pp 21–28Google Scholar
- Lacalli TC (1994) Apical organs, epithelial domains, and the origin of the chordate central nervous system. Am Zool 34:533–541Google Scholar
- Nakajima Y (1988) Serotonergic nerve cells of starfish larvae. In: Burke R, Mladenov PV, Lambert P, Parsley RL (eds) Echinoderm biology. Balkema, Rotterdam, pp 235–239Google Scholar
- Strathmann R (1978) The evolution and loss of feeding larval stages of marine invertebrate larvae. 32:894–906Google Scholar
- Strathmann RR, Eernisse DJ (1994) What molecular phylogenies tell us about the evolution of larval forms. Am Zool 34:502–512Google Scholar