The Evolution of Eyes in the Bivalvia: New Insights*

Abstract

Two types of multi-cellular eyes have been identified in the Bivalvia. Paired cephalic eyes occurring internally above the anterior end of the ctenidia are seen only in representatives of the Arcoidea, Limopsoidea, Mytiloidea, Anomioidea, Ostreoidea, and Limoidea. These eyes, comprising a pit of photo-sensory cells and a simple lens, are thought to represent the earliest method of photoreception. Many shallow-water marine, estuarine, and freshwater bivalves also possess simple photoreceptive cells in the mantle that enable them to respond to shadows. In some other marine, shallow-water taxa, however, a second type of more complex photoreceptors has evolved. These comprise ectopic pallial eyes that can be divided into three broad categories, in terms of their locations on the (i) outer mantle fold in representatives of the Arcoidea, Limopsoidea, Pterioidea, and Anomioidea, (ii) middle fold in the Pectinoidea and Limoidea, and (iii) inner fold in the Cardioidea, Tridacnoidea, and Laternulidae (Anomalodesmata). Eyes do not occur in deep-sea bivalve taxa. Where ectopic pallial eyes occur, they measure amounts of light and integrate intensities from different directions, thereby supplying information to the individual possessing them about the distribution of light in its immediate environment. This does not mean, however, despite broad, phylogenetically related advances in pallial eye complexity, that any bivalve can perceive an image. A revised picture of the independent evolution of ectopic pallial eyes in the Bivalvia is provided. In bivalves, pallial fold duplication has resulted in improvements to the peripheral visual senses, albeit at different times in different phylogenies and on different components of the mantle margin. This has been achieved, it is herein argued, through: (i) selective gene-induced ectopism; (ii) pigment cup evagination in Category 1 eyes; (iii) invagination in Categories 2 and 3; and (iv) natural selection. The invaginated distal retina in representatives of the Pectinidae and Laternulidae provides the potential for image formation and the detection of movement. In the absence of optic lobes capable of synthesizing such information, however, these complex eyes must await matching cerebral sophistication.

Molecular Phylogeny and Morphological Distinctions of Two Popular Bivalves,Ctenoides scaberandCtenoides mitis

“…For example, a diverse spectrum of visual abilities can be found in the family, ranging from eyeless (Limaria) to pit eyes (Lima) to closedlens eyes (Ctenoides) [29][30][31]. Members of the family also show distinct defense mechanisms, including nest-building behavior [20], swimming, tentacle autotomy [14,18,20,21], and, potentially, defensive chemicals [30].…”

“…They are well-known to SCUBA divers [9] and shell collectors, and several charismatic species are routinely collected for the aquarium trade [10]. Numerous studies have been conducted on their behavior [11][12][13][14][15][16], morphology [17][18][19], habitat [20][21][22], reproduction [23,24], physiology [25], and vision [26][27][28][29][30][31]. However, systematics studies have only been done on a small number of species in this family [10,19,[32][33][34][35].…”

Toward Developing Models to Study the Disease, Ecology, and Evolution of the Eye in Mollusca*

“…There is an incredible amount of morphological variation in eyes of the Bivalvia (review in Morton 2001). Most bivalve eyes are not cephalic as bivalves do not have a distinct head.…”

“…Much work has been done on the scallop eye including recent work on optics (Land 1965(Land , 1966a, comparative anatomy (Morton 2000(Morton , 2001 and references therein, Speiser and Johnsen 2008a), electrophysiology (Gorman andMcReynolds 1969, Gomez andNasi 1994), neurophysiology (Spagnola andWilkens 1983, Wilkens 2006), visualmediated behavior (Wilkens and Ache 1977, Hamilton and Koch 1996, Wilkens 2006, Speiser and Johnsen 2008b, phototransduction (Kojima et al 1997), and lens formation and protein evolution (Carosa et al 2002, Piatigorsky 2008.…”

Biomonitoring Climate Change and Pollution in Marine Ecosystems: A Review onAulacomya ater

“…Its dispersion is likely to have been favored by kelp-rafting of juvenile and/or adult individuals and not by an anthropogenic dispersal mechanism [2]. mechanical reception of water currents [10]. They are also known to have light-sensitive cephalic and (in more evolved species) pallial eyes that are used to measure proximal light intensities emitted from different directions [11].…”