Collin S P, Potter I C, Braekevelt C R
Department of Zoology, University of Western Australia, Nedlands, Western Australia.
Brain Behav Evol. 1999 Aug;54(2):96-118. doi: 10.1159/000006616.
This paper describes the ocular morphology of young adults of the southern hemisphere lamprey Geotria australis, the sole representative of the Geotriidae, and makes comparisons with those of holarctic lampreys (Petromyzontidae). As previously reported for the holarctic lamprey Ichthyomyzon unicuspis [Collin and Fritzsch, 1993], the lens of G. australis is non-spherical and possesses a cone-shaped posterior that may be capable of mediating variable focus. The avascular retina of G. australis is well differentiated, containing three retinal ganglion cell populations, three layers of horizontal cells and three photoreceptor types. In contrast to petromyzontids that contain only two photoreceptor types (short and long), G. australis possesses one rod-like (R1) and two cone-like (C1 and C2) photoreceptors. Although the rod-like receptor in G. australis may be homologous with the short receptors of holarctic lampreys, the two cone-like receptors have morphological characteristics that differ markedly from those of the long receptors of their holarctic counterparts. The features which distinguish the two cone-like receptors from those of the long receptor type in holarctic lampreys are the characteristics of the mitochondria and the presence of large amounts of two different types of stored secretory material in the endoplasmic reticulum of the myoid (refractile bodies). The endoplasmic reticulum of each receptor type has a different shape and staining profile and is polymorphic, each showing a continuum of distension. It is proposed that the presence of two cone-like photoreceptors with different characteristics would increase the spectral range of G. australis and thus be of value during the parasitic phase, when this lamprey lives in the surface marine waters. The irideal flap, present in G. australis but not petromyzontids, would assist in reducing intraocular flare during life in surface waters. The results of this study, which are discussed in the context of the proposed evolution of lampreys, emphasise that it is important to take into account the characteristics of the eyes of southern hemisphere lampreys when making generalizations about the eyes of lampreys as a whole.
本文描述了南半球七鳃鳗澳洲石纹鳗（Geotria australis）幼体的眼部形态，它是石纹鳗科的唯一代表，并与全北区七鳃鳗（盲鳗科）进行了比较。如先前关于全北区七鳃鳗单尖鱼七鳃鳗（Ichthyomyzon unicuspis）的报道[科林和弗里茨施，1993年]，澳洲石纹鳗的晶状体不是球形的，具有一个锥形后部，可能能够调节可变焦距。澳洲石纹鳗的无血管视网膜分化良好，包含三个视网膜神经节细胞群、三层水平细胞和三种光感受器类型。与仅含有两种光感受器类型（短和长）的盲鳗科不同，澳洲石纹鳗拥有一种杆状（R1）和两种锥状（C1和C2）光感受器。尽管澳洲石纹鳗中的杆状感受器可能与全北区七鳃鳗中的短感受器同源，但两种锥状感受器的形态特征与它们全北区同类的长感受器明显不同。将这两种锥状感受器与全北区七鳃鳗中的长感受器类型区分开来的特征是线粒体的特征以及肌样内质网（折射体）中存在大量两种不同类型的储存分泌物质。每种感受器类型的内质网具有不同的形状和染色特征，并且是多态的，每种都显示出连续的扩张。有人提出，存在两种具有不同特征的锥状光感受器会增加澳洲石纹鳗的光谱范围，因此在寄生阶段（当这种七鳃鳗生活在表层海水中时）具有价值。澳洲石纹鳗中存在但盲鳗科中不存在的虹膜瓣，将有助于减少在表层水域生活期间的眼内眩光。本研究结果在七鳃鳗进化提议的背景下进行了讨论，强调在对七鳃鳗的眼睛进行整体概括时，考虑南半球七鳃鳗眼睛的特征很重要。
