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Phototaxis

Phototaxis, directional swimming towards the light, is a widespread behaviour in marine plankton. About 80% of all marine invertebrates with a dispersing larval stage show positive phototaxis at the beginning of the dispersal phase. Following this positive phototactic stage, older larvae often become negatively phototactic. We study the mechanisms of both positive and negative phototaxis in Platynereis and other marine invertebrate larvae.

We have a comprehensive understanding of the mechanism of early larval positive phototaxis in Platynereis. Positive phototaxis is mediated by a pair of very simple eyes, or eyespots. These eyespots are composed of two cells only: a photoreceptor and a shading pigment cell, and resemble Opens external link in current windowDarwin's 'proto-eyes', considered to be the first eyes to appear in animal evolution. Many planktonic larvae possess these simple eyespots.

Eyespots cannot form images but enable the animal to sense the direction of light, because the pigment cell shades the photoreceptor from one side. In Platynereis, selective illumination of one eyespot changes the beating of adjacent cilia by direct cholinergic innervation, resulting in locally reduced water flow. Computer simulations of larval swimming show that these local effects are sufficient to direct the helical swimming trajectories of larvae towards the light. The computer model also shows that axial rotation of the larval body is essential for phototaxis and that helical swimming increases the precision of navigation. Our findings in Platynereis larvae provide a general mechanistic understanding of phototaxis in marine zooplankton larvae and its regulation by simple eyespots. It is possible that a similar direct coupling of light-sensing with ciliary locomotor control was a principal feature of the first animal eyes.
Phototaxis of 35 hour old Platynereis larvae. A blank frame indicates when the diffuse directional white light source was switched on at the right side of the chamber (1x1 cm). This movie is 2x accelerated.

Anterior view of a young Platynereis trocophore larva with the eyespot pigment shown in red. The large yolk droplets which sustain the early larva are also visible.

SEM image of a Platynereis trocophore larva, lateral view. The larvae swim with the prominent ciliary band, consisting of thousands of cilia, emanating from 24 large multiciliated cells. The eyes (not visible in the picture) are located above the ciliary band, in the upper half of the larva (episphere).

Serial section transmission electron microscopy (TEM) reconstruction of the eyespot photoreceptor showing the direct innervation of a ciliated cell.

Computer simulation of Platynereis phototaxis. Download the executable of the phototaxis simulation program PlatySwim Opens external link in new windowhere.

References

Jékely, G., Evolution of phototaxis. Opens external link in new windowPhilos Trans R Soc Lond B Biol Sci, 2009. 364(1531): p. 2795-808.

Jékely, G., et al., Mechanism of phototaxis in marine zooplankton. Opens external link in new windowNature, 2008. 456(7220): p. 395-9.
 
Randel N, Bezares-Calderón LA, Gühmann M, Shahidi R, Jékely G. Expression dynamics and protein localization of rhabdomeric opsins in Platynereis larvae. Integr Comp Biol. 2013 May 10. [Epub ahead of print]