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Postembryonic Eye Growth in the Seashore Isopod Ligia exotica (Crustacea, Isopoda)

¹ Department of Biology, University of Oulu, P.O. Box 3000, SF-90014 Oulu, Finland
² Department of Developmental Genetics, National Institute of Genetics, Mishima 411-8540, Japan
³ Department of Biology, School of Engineering and Science, International University Bremen, D-28725 Bremen, Germany
⁴ Department of Biology, Hamamatsu University School of Medicine, 1-20-1 Handayama, Hamamatsu 431-3192, Japan

* To whom correspondence should be addressed. E-mail: B.Meyer-Rochow{at}iu-bremen.de

The eye of Ligia exotica is of the apposition type and has open rhabdoms. The facets are hexagonal, and the dioptric apparatus consists of a flat cornea and a spherical crystalline cone placed in the center of two large cone cells. Each ommatidium has seven regular retinula cells and one eccentric cell; a basement membrane forms the proximal boundary of the retina. With increases in body size from 0.6 to almost 4.0 cm, facet numbers and ommatidial diameters increased from 800 to 1500 and 35 µm to 100 µm, respectively; eye length and width grew from 1.2 to 3.2 and 0.9 to 2.5 mm, respectively; and length of dioptric apparatus and width of retinal layer changed from 70 µm to 180 µm and about 70 µm to 120 µm. Visual angles and interommatidial angles of centrally located ommatidia remained constant at about 30 and 6.9 degrees, respectively. An almost perfect linear relationship was found when eye length was plotted against the product between the square root of the total number of ommatidia and the ommatidial diameter. No difference between males and females was observed in any of the relationships, but the results suggest that, compared with smaller specimens, larger ones possess increased absolute sensitivity in single ommatidia, increased sensitivity to point sources, and overall larger angular visual fields for the eye in its totality. This means that larger individuals of L. exotica (which are also faster) have an advantage over smaller individuals at night, but that smaller individuals may cope better with bright lights. Vision in L. exotica seems useful not only in detecting potential danger, but also in locating and approaching cliffs from a distance of 2–4 m when swimming in seawater.

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