二枚貝の外套膜に現れる眼について，指導教官と話すことがあり少し調べてみることにした．改めてみるととてもきれいで面白い形質だが，意外ときちんとした発生過程の記載はすぐに見つからなかった．もっと古い文献も探してみるべきだろうが，見つかった形態のレビュー論文を中心に文献を挙げておいた．要旨が長いので先にコメントをまとめておく．
コメント
文献一つ目が二枚貝の外套膜にできる眼の多様性について解説したもの．二つ目は孵化後の幼若個体と成体について、眼の後胚発生過程を詳しく調べた論文．三つ目は眼に着目したものではないが，二枚貝の幼生から幼若個体まで発生過程を記載した論文で，眼の形成が始まるタイミングや形成初期の形態についても述べられている．
そもそも二枚貝の眼の形態が系統ごとにかなり多様化しているのが面白い．発生過程なども見ているとどうも外套膜の辺縁にできるrentacleと相同な器官のようだ．二枚貝の外套膜は2，3重に重なっており，そのうち外側の部分にできるのか，内側の部分にできるのか等形成される場所がかなりバラバラなのは興味深い．tentacleから眼が進化したというのもとても面白いのだが，その形成過程の差が生じ始めるところの記載がまだ見つけられず，非常に気になるところ．今自分が行っている研究にもヒントが得られそうなんやけど…．自分で観察してみようか．
リファレンス-1
The evolution of eyes in the Bivalvia: new insights.
Morton, B. (2008). American Malacological Bulletin, 26(1/2), 35-45.
https://doi.org/10.4003/006.026.0205
要旨-1
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.
リファレンス-2
Development of the pallial eye in Nodipecten nodosus (Mollusca: Bivalvia): insights into early visual performance in scallops.
Audino, J. A., Marian, J. E. A., Wanninger, A., & Lopes, S. G. (2015). Zoomorphology, 134(3), 403-415.
https://doi.org/10.1007/s00435-015-0265-8
要旨-2
Scallop pallial eyes have been the most studied optical system in bivalve mollusks. Despite recent advances in our understanding of the function and evolution of scallop eyes, little attention has been focused on eye development and early visual performance. Here, the anatomy and development of pallial eyes were investigated in the scallop Nodipecten nodosus (Linnaeus, 1758) by means of integrative microscopy techniques (i.e., light, electron, and confocal microscopy). After metamorphosis, juvenile scallops bear small papillae that rapidly transform into minute ocular organs on the middle mantle fold. The distal epithelium gradually becomes pigmented, except for the cornea formed at the distal center of the eye. Internally, the optic vesicle comprises undifferentiated cells in the distal region, while mirror plates are secreted at the base of the eye, next to pigmented cells. Within the undifferentiated cell mass, the proximal retina is the first to be formed, followed by the distal retina and then by the lens. In this respect, the late development of the scallop lens from retina precursor cells may represent a unique condition among animal eyes. Adult eyes are characterized by large pigment distribution in the epithelium, tall columnar cornea, and lens above a slightly curved double retina. Whereas the pallial eyes from adult scallops are a complex visual system based on a mirror mechanism to form a focused image on the retina, early eye condition suggests a simple degree of directional photoreception, with no spatial vision.
リファレンス-3
The development and external morphology of pelagic larval and post-larval stages of the bay scallop, Aequipecten irradians concentricus Say, reared in the laboratory.
Sastry, A. N. (1965). Bulletin of Marine Science, 15(2), 417-435.
https://www.ingentaconnect.com/content/umrsmas/bullmar/1965/00000015/00000002/art00006
要旨-3
Bay scallops, Aequipecten irradians concentricus Say, are reared from fertilized eggs to preadults in the laboratory at 24.0 ± 1.0°C. The external morphology of veliger larval and post-larval stages are described. After ten days of pelagic life the larvae settle and crawl on the bottom before metamorphosis and attachment on the thirteenth day. The metamorphosis of scallop larvae involves: complete loss of velum; movement of mouth through 90° to an anterior and dorsal position; loss of anterior adductor muscle and the development of posterior adductor muscle; and development of gills. The larvae after metamorphosis attach to the substratum with byssus threads.
After attachment, the post-larval (dissoconch) shell rapidly grows to reach the adult form by the 29th day. During post-larval development, the gill filaments increase in number and size. The long tentacles and the eyes are developed on the mantle by the 23rd day and the 27th day respectively. The posterior adductor muscle and the heart enlarge by the 25th day. The post-larval scallops can free and reattach to the substratum and also show crawling movements when free on the bottom. The plicacations (ribs) appear on the 29th day and extend over the entire shell by the 35th day. The plicated scallops crawl on the vertical sides and attach at the level of water surface. The swimming ability in adult manner is developed in the plicated scallops. The young scallops also float on the water surface with extended foot and tentacles held by surface tension. The sequence of developmental changes and the functional morphology of larval and post-larval organs appear to have preadapted to the habits of young scallops.
