THE LARVAL COMPOUND EYE OF BARNACLES

Abstract

The compound eye of the cypris larva of Balanus crenatus comprises about 10 ommatidia. The ommatidia possess a crystalline cone formed by three cone cells, two distal pigment cells, and six retinular cells that together form the rhabdom. The compound eye of the cypris larva of Balanus exhibits similarities to that of branchiurans and ostracodes, but the latter two groups share more common characteristics. Adult barnacles have reduced nauplius eyes (Fahrenbach, 1965; Krebs and Schaten, 1976). Their sessile life requires little visually stimulated behaviour, the shadow reflex being the best known (Gwilliam, 1963). The planktonic larval forms, known as nauplii, have well-developed nauplius eyes (Kauri, 1962) capable of serving in light/dark perception. Between the planktonic nauplii and adult sessile forms, a short-lived cypris appears (Bookhout and Costlow, 1959), adapted for settlement. In contrast to all other stages, the cypris larva has a pair of small but morphologically well-differentiated compound eyes (Fales, 1928; Walley, 1969). The settlement behaviour has been analysed and the role of light guidance has been established (Barnes et al., 1951). Nothing is, however, known about the fine structure of the eyes of the cypris larva. Further, the kinship of the barnacles (Cirripedia) with related groups is poorly known as regards eye structure. The structure of the nauplius eye places the barnacles in the maxillopod-ostracode group (Elofsson, 1966). The present study was undertaken to elucidate the above problems.