Journal of Fish Biology (1999) 54, 1326–1328 Article No. jfbi.1999.0948, available online at http://www.idealibrary.com on BRIEF COMMUNICATIONS Evidence for partitioning of physiological functions between holobranchs: protein synthesis rates in flounder gills A. R. L* J. G. B† Department of Biological Sciences, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, U.K. (Received 24 December 1998, Accepted 8 February 1999) Fractional rates of protein synthesis were significantly greater in the first than in the second and third, but not the fourth, holobranchs of seawater-adapted flounders. These differences may reflect partitioning of physiological functions between holobranchs. 1999 The Fisheries Society of the British Isles Key words: gills; branchial function; Pleuronectes flesus; protein turnover; variation. Fish gills have been studied intensively for a long time, from both morphological and physiological perspectives (Houlihan et al., 1982; Hoar & Randall, 1984a, b; Lyndon & Houlihan, 1998), on account of their crucial, multifunctional role in homeostasis. However, in spite of acknowledgement that they constitute a highly heterogeneous organ system (Hughes, 1984), the separate holobranchs, eight of which constitute the gills in teleosts, are often considered as identical units within the gill system (Perry & Farrell, 1989; Lyndon, 1994). This assumption remains to be tested. The present paper reports the results of a study comparing protein synthesis rates in different holobranchs of the flounder Pleuronectes flesus L. Protein synthesis is a fundamental biological process underlying many physiological functions. Therefore, if holobranchs constitute identical branchial sub-units, significant variation in their protein synthetic activity would not be anticipated. Flounders [mean weight 218 68 (..) g] were caught by a local inshore fishing boat in Aberdeen Bay (5710 N; 200 W) during April 1995. They were kept alive on board the vessel in plastic dustbins of oxygenated sea water and were transferred as soon as possible to a filtered, recirculating seawater aquarium (34‰; 12 C) at the Zoology Department in Aberdeen where they were kept for at least 3 weeks (fed chopped squid ad libitum) before use in experiments. The flooding dose method was used for the determination of protein synthesis, as described elsewhere (Houlihan et al., 1994). Briefly, fish were anaesthetized in MS-222 (100 mg l 1 ) 24 h after the last feed and injected in the caudal vessels with 1 ml 100 g 1 wet weight of a solution containing -2,6- 3 H-phenylalanine (Amersham International) at 3·7 MBq ml 1 and 135 mmol l 1 -phenylalanine in Cortland saline (pH 7·8). They were transferred immediately to a recovery bucket of vigorously aerated sea water (12 C) where they recovered rapidly and remained for the incubation period of 60 min. At the end of this time the fish were killed by a sharp cranial blow and transection of the spine and the entire branchial basket was excised and frozen immediately in liquid nitrogen. The individual holobranchs (I–IV) *Author to whom correspondence should be addressed. Tel.: +44 (0)131 451 3462; fax: +44 (0)131 451 3009; email: a.r.lyndon@hw.ac.uk †Present address: Department of Zoology, University of Aberdeen, Tillydrone Avenue, Aberdeen AB24 3TZ, U.K. 1326 0022–1112/99/061326+03 $30.00/0 1999 The Fisheries Society of the British Isles