Studies On Topographical Distribution Of Rhabdom In The Retinato Estimate The Visual Function Of Three Species Of Teuthida

[Objective] Previous studies of retinal histology of fish have established that the distribution of visual cell density often reflects both their habitats and feeding behaviors,while there are few studies concerned about the visual function related to the visual cell distribution of squid.The Japanese common squid Todarodes pacificus,Spear squid Heterololigo bleekeri and Firefly squid Watasenia scintillans are all important commercial squids in Japan with quite landing amounts every year and their habitats are different.However,the distributions of visual cell in relation to the habitat of these three species have not been fully examined or even unknown.In this study,the arrays of rhabdoms among three species of Teuthida were compared,the densities of visual cells were analyzed,the distributions of the rhabdoms were examined and the topographic maps of visual cell density were drew.Besides the visual axes in the horizontal and in the vertical plane were estimated,the visual acuities were calculated and comparisons of visual functions among the 3 species of Teuthida were also made in this study.[Materials and Methods] The Todarodes pacificus,Heterololigo bleekeri and Watasenia scintillans were sampled in Wakasa bay Fukui prefecture in Jun 2015,Izu Shizuoka prefecture in Jan 2016 and in Toyama Bay during April 2016 in Toyama prefectures respectively.After fixed in the Bouin's fluid,the retina of each sample was divided into 37,57 or 81 pieces according to the size of specimen,then made into the paraffin sections.The distributions of rhabdoms across the retina for each piece were observed using a light microscope OLYMPUS BX41 N,the pictures were taken by the OLYMPUS E-330 camera.Calculated the visual cell density per square millimeter,tested and analyzed all the data by software of IBM SPSS Statistics 22.0.Then built iso-density contour maps of the visual cell densities by using the software of Golden Software Surfer 11.Visual axes were estimated from the visual cell density contour maps and visual acuities were estimated from the visual cell densities and the focal distance of the lenses.[Results] The rhabdom arrays of Todarodes pacificus and Watasenia scintillans were similar,showed an orthogonal mosaic-like pattern array while the rhabdom array of Heterololigo bleekeri appeared to be in disorder,the adjacent two rhabdoms connected together tightly.Visual cell density differed in different parts of one retina and differs among different species.Visual cell densities were all higher in the posterior retina in all 3 species.For the Todarodes pacificus,which had a higher cell density than the other 2 species,the highest density region was ventral to the midline of the body,while the region of highest density was dorsal to the midline of the body in Heterololigo bleekeri and Watasenia scintillans.Visual axes showed difference for different species and suited for their habitat and feeding behaviour respectively.The visual axis of Todarodes pacificus was estimated to be upwards and forwards which might be advantageous in detecting prey items of above.While the downward and forward directed visual axes in Heterololigo bleekeri and Watasenia scintillans might to be advantageous in detecting items of its underside.The distributions of visual acuity were almost the same as the distributions of visual cell,the visual acuities were all higher in the posterior retina in all 3 species.In the Todarodes pacificus and Heterololigo bleekeri,the visual acuities were estimated to be 0.70,0.74 which were higher than ordinary fish species,this may enhance their visual abilities to recognize distant objects and to promote a higher resolution.While there were no significant difference in visual acuities(around 0.48)among different specimen in Watasenia scintillan.