Enteropneusts or acorn worms are marine deuterostomes that have retained many plesiomorphic characters. Thus, enteropneusts are of prime interest in evolutionary comparisons between deuterostomes and protostomes. In the present study, the larval eyes of Glossobalanus marginatus were reconstructed and described based on serial sectioning for transmission electron microscopy. The everse eyes of the late Metschnikoff/early Krohn-stage tornaria larvae of G. marginatus are epidermal structures consisting of two rows of in total 13 shading pigment cells and another two rows of 13 photoreceptor cells. The pigment cells form a shallow cup with a relatively wide opening, making the cup-shaped eye optically unsuitable for picture generation. We demonstrate that the photosensitive cells possess numerous enlarged microvilli and an unmodified apical cilium. Our ultrastructural studies thus corroborate the photoreceptor cells in the eye of G. marginatus to be of a clearly rhabdomeric type. Preliminary immunohistochemical experiments support those findings by demonstrating immunopositive reaction of the tornarian eye photoreceptors with an antibody designed against rhabdomeric sea urchin photopigment (Sp-Opsin4). Observations of living animals indicate that Late Metschnikoff/early Krohn-stage tornaria larvae are negatively phototactic, probably concordant with imminent metamorphosis.
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We thank Maria I. Arnone for kindly providing the anti-Sp-Opsin4 antibody. Financial support by the Deutsche Forschungsgemeinschaft (DFG) is gratefully acknowledged.
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Animation of 3D reconstruction of eye of a Late Metschnikoff/early Krohn-stage larva of Glossobalanus marginatus. Only one row of pigment cells is shown; compare to Fig. 3c. (MPG 20350 kb)
Movie showing movement of a Late Metschnikoff/early Krohn-stage larva. Fairly straight swimming movement with the apical plate forwards is propelled by the beating of the multiciliary telotroch. The two dark eyes are clearly visible. (AVI 40033 kb)
Movie showing movement of an Agassiz-stage larva. Crawling movement, with the proboscis probing the substrate is at least partially propelled by the beating of the multiciliary telotroch. Towards the end of the movie the larva speeds up due to an increase in beat rate of telotroch cilia. Contraction of muscles in the proboscis can be seen. The two dark eyes are still visible. (AVI 36849 kb)
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Braun, K., Kaul-Strehlow, S., Ullrich-Lüter, E. et al. Structure and ultrastructure of eyes of tornaria larvae of Glossobalanus marginatus . Org Divers Evol 15, 423–428 (2015). https://doi.org/10.1007/s13127-015-0206-x
- Ciliary photoreceptor