Fig 1.
The Schwabe organ in Leptochiton asellus and the larval eye.
A, Schwabe organ in a fresh glutaraldehyde-fixed specimen. B, Several months after death. Note the persistence of a concentrated dot at the anterior edge (chevron). Scale bar, 500 μm. C, The larval eye in Lepidochitona cinerea in ovo (chevron). Scale bar, 200 μm. D, The ventral migration of the larval eye of Leptochiton asellus. Left to right; dorsal view of developing trochophore with visible shell fields; lateral view with larval eye visible; ventral view with larval eyes having migrated to position lateral to the tip of the foot; metamorphosed larva with larval eyes in final position, reminiscent of the Schwabe organ. Adapted from Christiansen, 1954. E, The position of the larval eye in the anterior nervous system of Lepidozona mertensii (purple, adapted from Heath, 1904). Buccal ganglia and nerves are highlighted in green. Scale bar, 1 mm. F, The position of the Schwabe organ in the anterior nervous system of adult Leptochiton asellus. Scale bar, 1 mm. ac, anterior commissure; bg, buccal ganglia (in green); srg, subradular ganglia.
Fig 2.
A, Chitons were placed in circular arenas with two blackened and two lit quadrants of equal size. Scale bar, 1 cm. B, Arenas were placed on a horizontal lightbox, illuminating the lit quarters from below. Scale bar, 5 cm.
Fig 3.
Anatomy and ultrastructure of the adult larval eye within the chiton Schwabe organ.
A, Semi-thin section showing pigmented epithelium and secondary pigmented structure (chevron) in the Schwabe organ in Leptochiton asellus; the pallial cavity is to the right as shown. lnc, lateral nerve cord; m, mouth; ner, nerve. Scale bar, 100 μm. B, Semi-thin section showing epithelial and secondary pigmented structures observed the region of the Schwabe organ. Scale bar, 50 μm. C, Ultrastructure of the larval eye (from Fischer, 1980). Scale bar, 2 μm. D, Ultrastructure of the pigmented subepithelial structure. Insets from main image; i, putative ciliary roots (chevron); ii, nerve bundles; iii, membrane folding (chevron). lu, lumen; me, mesoderm; mt, mitochondrion; ner, nerve; p, pigment; pb, large pale bodies; asterisk, elongated dark structure. Scale bar, 10 μm.
Fig 4.
Behavioural experiments to test response of the Schwabe organ to upwelling light.
Percentage of time spent in the darkened quadrants by the four subject groups. S indicates innate presence of Schwabe organ (Leptochiton asellus), X indicates innate absence of Schwabe organ (Lepidochitona cinerea); + indicates subject intact,–indicates ablation surgery. Error bars indicate standard error and are intended as visual guidelines only. Letter codes above bars denote statistically equivalent time spent in the darkened quadrants between treatment groups (based on KW post hoc pairwise comparisons).