Leech Vision | neurobiology

Medicinal leeches (genus Hirudo) have surprisingly complex visual systems. The eyes are found in 2 major groups. First are cylindrical pigmented eye cups (B) at the head, cephalic eyes (A). There are 5 pairs of eyes, each having about 50 individual photoreceptor neurons (C). These photoreceptors are called "phaosomal." They depolarize in light and generate action potentials conducted into the CNS.

These 5 pairs of eyes sample a wide field of the visual surroundings

Jellies and Groves 2015

The midbody also has another set of eyes. There are 7 pairs of sensilla that sample light from dorsal (#7) to ventral (#1) and these are iterated in each of the 21 midbody segments for a total sensillar array consisting of at least 294 simple eyespots.

So, the leech can obtain visual image information front to back and dorsal to ventral over 360 degrees.

In earlier work we demonstrated that sensillar axons pioneer the MA nerve and within the CNS they segregate into 3 distinct fascicles. Knowing now that there are at least 3 different sensory responses it is tempting to suggest that these are functionally organized. Click here for a look at those fascicles.

To explore just a bit more about what leeches can see and get a sense of how we address these questions, click the leech face at the right.