QUICK SEARCH:   [advanced] Author: Keyword(s):
Home     Help     Feedback     Subscriptions     Archive     Search     Table of Contents

First published online March 31, 2007
Journal of Experimental Biology 210, 1303-1310 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.003418

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Hyodo, S. Articles by Toop, T. Search for Related Content PubMed PubMed Citation Articles by Hyodo, S. Articles by Toop, T.

Osmoregulation in elephant fish Callorhinchus milii (Holocephali), with special reference to the rectal gland

Susumu Hyodo^1,*, Justin D. Bell^2,3, Jillian M. Healy², Toyoji Kaneko⁴, Sanae Hasegawa¹, Yoshio Takei¹, John A. Donald² and Tes Toop²

¹ Laboratory of Physiology, Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo 164-8639, Japan
² School of Life and Environmental Sciences, Deakin University, Victoria, Australia
³ Primary Industries Research Victoria, Queenscliff, Victoria, Australia
⁴ Department of Aquatic Bioscience, Graduate School of Agricultural and Life Sciences, University of Tokyo, Bunkyo, Tokyo, Japan

^* Author for correspondence (e-mail: hyodo{at}ori.u-tokyo.ac.jp)

Accepted 5 February 2007

Osmoregulatory mechanisms in holocephalan fishes are poorly understood except that these fish are known to conduct urea-based osmoregulation as in elasmobranchs. We, therefore, examined changes in plasma parameters of elephant fish Callorhinchus milii, after gradual transfer to concentrated (120%) or diluted (80%) seawater (SW). In control fish, plasma Na and urea concentrations were about 300 mmol l^–1 and 450 mmol l^–1, respectively. These values were equivalent to those of sharks and rays, but the plasma urea concentration of elephant fish was considerably higher than that reported for chimaeras, another holocephalan. After transfer to 120% SW, plasma osmolality, urea and ion concentrations were increased, whereas transfer to 80% SW resulted in a fall in these parameters. The rises in ion concentrations were notable after transfer to 120% SW, whereas urea concentration decreased predominantly following transfer to 80% SW. In elephant fish, we could not find a discrete rectal gland. Instead, approximately 10 tubular structures were located in the wall of post-valvular intestine. Each tubular structure was composed of a putative salt-secreting component consisting of a single-layered columnar epithelium, which was stained with an anti-Na⁺,K⁺-ATPase serum. Furthermore, Na⁺,K⁺-ATPase activity in the tubular structures was significantly increased after acute transfer of fish to concentrated SW (115%). These results suggest that the tubular structures are a rectal gland equivalent, functioning as a salt-secreting organ. Since the rectal gland of elephant fish is well developed compared to that of Southern chimaera, the salt-secreting ability may be higher in elephant fish than chimaeras, which may account for the lower plasma NaCl concentration in elephant fish compared to other chimaeras. Since elephant fish have also attracted attention from a viewpoint of genome science, the availability of fish for physiological studies will make this species an excellent model in holocephalan fish group.

Key words: holocephalan fish, osmoregulation, rectal gland, elephant fish