Osmotic and ionic performance of the anadromous sea lamprey, PetromyzonMarinus

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OSMOTIC AND IONIC PERFORMANCE OF THE A N A D R O M O U S SEA LAMPREY, P E T R O M Y Z O N MARINUS F. W. H. BEAMISH,P. D. STRACHANand E. THOMAS College of Biological Science, Department of Zoology, Univerisity of Guelph, Guelph, Ontario, Canada (Received 14 October 1977)

Abstraet--l. Ammocoetes maintained a hyperosmotic blood serum in salinities between 0 and 16°~. The salinity at which one-half of the animals died within 24 hr was approximately 14.8~oo. 2. Anadromous feeding adults of all sizes were able to osmoregulate between 0 and 34Y,,o,in contrast to the landlocked form in which only the larger individuals could regulate above 1 6 ~ 3. In the anadromous feeding adult a given change in serum chloride was accompanied by a smaller shift in sodium, this control being particularly tight in the lower salinities. Regulation of sodium by the landlocked sea lampreys was not as precise as that exhibited by the anadromous individuals. 4. The capacity of anadromous lampreys for marine osmo- and iono-regulation deteriorated with the approach of sexual maturity. 5. Gill (Na-K)-ATPase was found consistently only in feeding adults, particularly those animals in saltwater.

INTRODUCTION

The anadromous sea lamprey, Petromyzon marinus, twice during its adult life migrates between fresh and saltwater. Subsequent to a larval stage which is spent in the substrate of rivers and streams, and prior to migrating downstream to the ocean as an adult, sea lampreys undergo a period of metamorphosis during which marked morphological and physiological alterations occur. Ammocoetes actively transport ions from the water, across the gill epithelium, particularly sodium and chloride and excrete large volumes of dilute urine (Morris, 1960). Following metamorphosis and their entry into saltwater, young adults modify their osmoregulatory processes. The passive exchange of ions and water created by the ion-rich medium are offset by the active outward transport of sodium and chloride and near cessation of urine production (Pickering & Morris, 1970). That the full osmoregulatory capacity of small adult sea lampreys may not be realized for some time after their downstream migration was suggested by Mathers & Beamish (1974) on the basis of observations made by Mansueti (1962) of over-wintering, recently transformed P. marinus in Chesapeake Bay where the salinity is about 16~o~ Recently, Beamish & Potter (1975) and Potter & Beamish (1977) observed that some young adult sea lamprey over-wintered in the fresh waters of the St. John River while others apparently migrated to at least the brackish waters of the estuary. Following a period of active feeding sea lampreys ascend rivers, reproduced, and shortly thereafter, die. At the time of the spawning migration, sea lampreys again undergo anatomical and physiological changes (Hardisty & Potter, 1971), one major change being a reduction of the marine osmoregulatory capacity. Sea lampreys have become landlocked in a number of freshwater lakes in North America (Gage, 1893; Davis, 1967; Lark, 1973). In the Great Lakes it is generally assumed that sea lampreys first invaded the

lower Lake Ontario after the Wisconsin glacial ice receded approximately 8000 years ago and subsequent to their migration to the upper lakes (Smith, 1971). The osmoregulatory capabilities of the landlocked sea lamprey suggest that ammocoetes are exclusively fresh water animals, being unable to osmoregulate in salinities as low as 10°/oo (Mathers & Beamish, 1974). Only adults greater than 250mm in length could osmoregulate for long periods in salinities between 16 and 34~o~ The observations by Mathers & Beamish (1974) provided some evidence for the earlier suggestion by Hardisty (1956) that differentiation of the landlocked P. marinus involved selection for adults of smaller size and reduced osmoregulatory performance. However, measurements of osmoregulatory capacity of the anadromous sea lamprey are few and generally confined to the spawning phase of the life cycle (Fontaine, 1930; Pickering & Morris, 1970). Marine lampreys and teleosts share the mechanical processes of osmoregulation such as swallowing seawater, uptake of water and monovalent ions across the gut wall and excretion of those ions via the gills (Morris, 1972). However, the question of a common biochemical mechanism of osmoregulation has not been addressed. In the gill tissue of a number of teleost species (Epstein et al., 1967; Kamiya & Utida, 1968; Pfeiler & Kirschner, 1972; Zaugg & McLain, 1976} a sodium-potassium-dependent ATPase enzyme (Na-K ATPase) has been identified at the site of the "chloride cells" (Karnaky et al., 1976) and is believed Io constitute the pump responsible for the outward transport of sodium and inward movemeent of potassium across the gill epithelia (Adams et al., 1975). The present study was undertaken to measure osmotic and ionic regulation as well as gill (Na-K)ATPase for the life cycle stages of the anadromous sea lamprey in relation to ambient salinity.

435

436

F. W. H. BEAMISH,P. D. STRACHANand E. THOMAS MATERIALS AND METHODS

The St. John River in New Brunswick with its lake-like extensions and many tributaries was the collecting area (Beamish & Potter, 1975; Potter & Beamish, 1977). Ammocoetes were collected by electrofishing from the Nashwaaksis and Keswick Rivers in late June and mid May 1977. Small adults which had only just begun feeding {Potter & Beamish, 1977) were removed from gaspereau, Alosa pseudoharenyus, American shad, Alosa sapidissima, and white sucker, Catostomus commersoni which had been captured in trap nets set by commercial fishermen in WashadomoaL Grand, and French Lakes. These small adults were believed to have overwintered in the St. John River after the completion of metamorphosis. Large feeding adults were removed from upstream migrant Atlantic salmon, Salmo salar, captured at the fishway near the Mactaquac hydroelectric installation and dam between early July and late August. The earliest of the non-feeding adult upstream migrants {early upstream migrants) were captured in early to midMay, again in trap nets set in Washadomoak Lake. From the bluish skin coloration which was characteristic of many of these migrants it was presumed that they had only recently left the sea {Potter & Beamish, 1977). Nearly mature upstream migrants were captured towards the end of June from the fishway at the Mactaquac Dam (Beamish & Potter, 1975; Potter & Beamish, 1977). Towards the end of June and in early July spent adults were taken by d!pnet from the Keswick River approximately l km upstream from the point where it discharges into the St. John River. Generally, all lampreys were shipped by air to the University of Guelph within six days of capture. Spent adults were transported immediately after their capture to the university of New Brunswick where they were sampled. Adult lampreys were held in tanks supplied with non chlorinated well water ( I I -4-_ 1 C) prior to experimentation. White suckers and carp, Cyprinus carpio, were provided as hosts for feeding adult sea lampreys. Ammocoetes were maintained in tanks with a sandy silt substrate in which they burrowed and were fed weekly a suspension of yeast [Hanson et al.. 19741. In all tanks oxygen was kept at or near air saturation and ammonia below 0.1 mg Lampreys which were to be exposed to saline water were transferred to separate tanks of recirculated water and the salinity increased 27~,oper day by the addition of artificial sea salt {Instant Ocean). One group of small feeding adults was transferred directly from freshwater to 347~ and held for 2 weeks. Food was withheld from lampreys in saltwater (Mathers & Beamish, 1974). All life cycle stages except early migrants and spent adults were exposed to saline water. Salinity was checked daily with a conductivity meter {Radiometer model CDM-3 with CDC 304 probe) which had been calibrated against the silver nitrate titration method described by Barnes {1959). Before lampreys were sampled, they were killed by severing the spinal cord immediately posterior to the last branchial aperture and the total length measured to the nearest l mm. Blood was collected from ammocoetes and small adults in non-heparinized capillary tubes from an incision made immediately ventro-posterior to the last branchial opening. Blood of the larger adults was withdrawn from either the anterior portion of the dorsal aorta or by cardiac puncture with a non-heparinized syringe. Blood was held at room temperature in non-heparinized capillary tubes, for 15 rain to permit complete clotting. It was then centrifuged at 15,000 0 for 5 min. Serum was separated from red blood cells and stored, refrigerated, for not longer than 3 days (Mathers & Beamish, 1974). Serum osmolality was determined using a vapour pressure osmometer (Wescor Inc. model 5100). Serum sodium concentration was measured by flame emission spectrophotometry (Radi-

ometer model FLM-2) and chloride by automatic titration using a Buchler-Cotlove chloridometer (model 4-2000). Triplicate samples were measured where serum quantities permitted. Gill tissue for (Na-K)-ATPase assays was taken from some of the individuals whose blood was analyzed as described above. Within approximately 2 rain of severing the spinal cord, gill tissue was excised and transferred to a 0.0% w v i NaC1 solution held in an ice bath. At least 100-200mg of gill tissue was removed from the ammocoetes and small adults. Two grams of tissue was taken from the larger animals. Each gill tissue sample was homogenized in a 10°., w v solution containing 0.25 M sucrose. 5raM EDTA in Tris-HCl buffer, pH 7.4 at 4°C. Large gill samples were homogenized first in a Virtis ~'23" homogenizer at maximum speed for 90see then in a tissue grinder at 1000rpm with 10 strokes using either a glass or teflon pestle. Sodium deoxycholate was added to the tissue grinder contents to a concentration of 0.1~,w v a. Small gill samples were homogenized with 10 strokes of the tissue grinder followed by the addition of sodium deoxycholate to the above concentration and an additional 10 strokes. Homogenates were centrifuged at 900 0 for 10rain. 10,000 g for 30min, and for 105,000 o for 60min at 0-4°C. The supernatant was removed by suction and the final pellet rinsed once in I ml of the original homogenizing medium {0.5 ml for small samples) and resuspended in the same volume of that medium by gentle vortexing. The microsomal suspension was used in all assays. Protein concentration in this suspension was determined by the method of Lowry et al. (1951) and diluted, if necessary, with homogenizing medium so that the protein concentration did not exceed 2 mg ml-~. Incubation of the enzyme-catalyzed reaction was carried out in a water bath at 11 +__ I~C for 30 min in 1.8 ml of incubation medium to which was added 0.1 ml of enzyme suspension and 0.1 ml of 5mM disodium ATP {Sigma Chemical Co.). Incubation media consisted on the following concentrations: 40 mM Tris-~HCl pH 7.4, 3 mM MgCI 2, 100mM NaCI and 20mM KCI or the above minus the NaCI and KCI. Reaction was stopped by addition of 0.5 ml of 30~, trichloroacetic acid. The products were centrifuged at 30000 for 10min and the amount of inorganic phosphate liberated from ATP was determined by the method of Fiske & Subbarow (1925) on aliquots of 2 ml. Each result was expressed as the mean of duplicate determinations.

RESULTS Serum osmolality of ammocoetes in freshwater, 2 2 5 m O s k g -1, was not significantly different from that in 8%~ 233 m O s kg-1 (P < 0.05, Fig. 1). Osmolality in 8%° was measured daily for each of two individuals over a 5 day exposure period. While variation did occur there was no indication of a trend of increasing or decreasing concentrations over this period. An abrupt and significant increase in osmolality occurred on exposure to 10%o (Fig. 1). Osmolality of ammocoetes in 10%o for 24hr, 280 + 28.2 m O s k g - t (n = 8) was less than that after 8 days, 299 + 23.6 m O s k g - 1 (n = 8); however, significant differences were not demonstrable. Of the 7 a m m o coetes sampled after 2 4 h r in 16%,, blood was obtained in sufficient quantity from only 2 individuals which had a mean osmolality of 336 m O s kg-1. At each salinity, serum osmolality was above ambient which for 0, 8, 10 and 16%o was 7, 208, 256 and 397 m O s k g - 1 respectively.

Performance of the anadromous sea lamprey

437

Mortality was not observed among ammocoetes in from that of nearly mature upstream migrants o/ 0 or 8o~ and restricted in 10/0o to one on each of 259 mOs kg- ' (Fig. 3). In both stages of the life cycle the sixth and seventh days in that salinity. Among osmolality of the serum was above that of freshwater. the ammocoetes destined to an exposure of 16% Osmolality of nearly mature migrants did not change (n = 35), one died in 12~, 11 in 14% and 16 in 16%o. significantly between 0 and 8%0 (Fig. 1). Further, Probit analysis of the mortality rates indicated the exposure to 8%0 for 4 days did not alter significantly salinity at which 50% of the ammocoetes died within the osmolality found after 1 day in this salinity. In 16%o serum osmolatity rose precipitously to 24 hr to be approximately 14.8To~ Small feeding adults did not exhibit a significant 358 mOs k g - ' , a significant increase over that found at the lower salinities. The increase in osmolality conchange in osmolality between freshwater and ]6oo, o~ 238 and 234 mOs kg-~ respectively (Fig. 1). Osmolatinued to 24°,00at which it was 460 mOs kg- ~. Mortality of nearly mature adults occurred in salinities lity increased significantly in 26~°~ to 256 mOs kg-~, the upward trend continuing to 263 mOskg-~ in , as low as 8%0 where one lamprey died during the 3 4 0 At each of the salinities examined there was second day of exposure. Two individuals died on the no evidence of a significant change in osmolality with second day of exposure to 16°,;o. All three migrants exposure periods of 1-8 days (Fig. 2). Serum osmolain 24°,° appeared to be near death when they were lity was above ambient at 0 and 8%0 and below at sampled after an exposure of less than 8 hr. Probit o/ 16, 26, and 34~oo in which environmental osmolalities analysis of the mortality rates suggested the salinity were 397, 613, and 919 mOs kg- a respectively. Mortaat which 500'0 of the animals died within 48 hr to lities were not observed over the range of salinities be approximately 15.2°,o. Osmolality of the serum from spent adults in freshapplied. Further. of 10 small adult lampreys transferred directly from freshwater to full strength sea water was 190mOskg f, significantly lower than water no deaths occurred within a period of 2 weeks. that for all other life cycle stages except the large Serum osmolality of large feeding adults increased feeding adults (Fig. 3). continuously between 0 and 26%, the highest salinity Serum sodium concentration of ammocoetes inapplied (Fig. 1). However, for comparable salinities, creased significantly from 101 to 139m-equivkg -~ significant differences in osmolality between the two between 0 and 10%o (Fig. 4). Sodium concentration size classes of feeding adults were demonstrable only did not vary appreciably over the 5 day exposure in freshwater. Large feeding adults maintained a period at 8"~,,. with an overall mean of hyposmotic blood serum in 0 and 8%0 and hyperos113 m-equiv kg- ~. Similarly, sodium concentration of o/ motic in 26/0o. Mortalities were not observed among ammocoetes in 10%o for 24hr was not significantly large adults at any of the salinities applied. different from that after 8 days. At no salinity did The osmolality of early upstream migrants in freshsodium exhibit a consistent pattern of change with water was 285 mOs kg-~ which differed significantly exposure time. Large feeding adults had a mean

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Fig. 1. Serum osmolality of different life cycle stages of the sea lamprey in relation to salinity. Vertical bars around the mean denote 95% confidence limits. The mean body length (mm) and sample size. in brackets, is indicated for each osmolality. The values for the landlocked individuals, closed circles. were taken from Mathers & Beamish (1974).

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Fig. 2. Osmolality of small feeding adult sea lampreys in relation to exposure time in a given salinity. Each point represents the mean (+9570 confidence) for the sample size indicated. The overall mean is indicated by the heavy line with the 95% confidence limits represented by the area shaded by small dots on a white background. abrupt and significant rise in sodium occurred in 16%o, 177m-equivkg -1, which continued to 24%0 where the mean was 226 m-equiv k g - 1 (Fig. 4). Spent adults in 0Yoo had a sodium concentration of 93 m-equiv k g - t which like their osmolality, was significantly lower than that of early migrants or nearly mature adults in fresh water (Fig. 3). Chloride concentration of ammocoetes increased significantly from 76 to 106 m-equiv k g - 1 between 0 and 8Yoo and remained unchanged to 10~/oo (Fig. 4~ Among small feeding adults chloride concentration

sodium concentration of 94 and 96 m-equiv kg- t in 0 and 8",., respectively (Fig. 4). In 26",.... sodium concentration increased significantly to 127 m-equiv kg -1. Sodium was significantly higher in small than large feeding adults in 0 and 8",,,,; however, significant differences in 26",,,, were not demonstrable. Sodium concentration of early upstream migrants in freshwater was 156m-equivkg -~, significantly above that of nearly mature adults, 137 m-equiv k g - ' (Fig. 3). No significant change in sodium concentration of nearly mature adults occurred in 8°~. An 300

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* Significant at P < 0.05. The multiple R 2 was 42.08 and the F-value, 23.25, significant at P < 0.05. Calculated changes in sodium for the range of serum chloride levels observed in each salinity are given in Fig. 5. The pattern of change in the chloride concentration of large feeding adults with salinity parallelled that for sodium, the former being the lower although at no salinity were significant differences demonstrable (Fig. 4). Early upstream migrants in fresh water maintained a significantly higher serum chloride concentration than that found for the nearly mature adults (Fig. 3). Serum chloride of the nearly mature lampreys decreased sharply and significantly between 0 and 8%0 but increased in 16 and 24~oo (Fig. 4). Spent adults

had the lowest chloride concentration of any of the life cycle stages in freshwater. Gill (Na-K)-ATPase showed consistent measurable activity only among the feeding adults. Among small feeding adults (Na-K)-ATPase increased in activity from <1 to 3 + 5 . 6 to 4 + 8 . 5 # M m g - l h r - ' in salinities of 0, 16 and 26"~,,, respectively. Enzyme activity in large feeding adults was absent in 0",,,, and < 1 #M m g - 1 h r - ~ in 26Too. However enzyme activity of the large feeding adults in 26Voo increased with exposure time from 1 + 4.5 to 2 _+ 2.2 M m g - ~ hrbetween 1 and 15 days. Of 6 early migrant adults and a similar number of nearly mature lampreys assayed for N a - K ATPase only 2 and 1 animals respectively exhibited measurable activity levels. DISCUSSION

The osmoregulatory mechanism in ammocoetes of the anadromous sea lamprey appears to function in the same direction in both fresh and saltwater. Thus the hyperosmotic blood serum of ammocoetes in freshwater continued to increase in osmolality with ambient salinity. This is illustrated in Fig. 6 by decreasing ratios of serum osmolality in freshwater relative to that in the experimental salinities. Presumably hyperosmoticity is maintained by the unidirectional activity of the "ion transport" cells located in the gill lamellae in concert with the excretion of a dilute urine (Morris, 1957; Pickering & Morris, 1970; Morris & Pickering, 1975). The difficulty experienced in blood collection from anadromous ammocoetes in 16Too together with the dry appearance of the body cavity is clear evidence of the continued efficiency of the kidney in water excretion. Elevation of serum osmola-

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landlocked sea lamprey. The regressions for the landlocked sea lamprey were re-drawn from Mathers & Beamish (1974). lity with increasing salinity appears to be primarily the result of increases in sodium and chloride ions. The inability of anadromous ammocoetes to cope with saline water is reflected by the mortalities which were suffered in salinities as low as 109~o~ Ammocoetes of both the landlocked sea lamprey and the non parasitic river lamprey, Lampetra planeri, also maintain a hyperosmotic serum in relation to ambient salinity (Hardisty, 1956; Bull & Morris, 1967; Mathers & Beamish, 1974). Osmolality of the landlocked sea lamprey ammococtes was consistently above that exhibited by the armdromous larvae (Fig. 1). However, the ratio of serum osmolality in freshwater relative to that in each of the experimental salinities, 0 and 109/~ was remarkably similar for the two forms of Petromyzon marinus. Differences in osmolality between the landlocked and anadromous ammocoetes may be attributable to season, the former being sampled during the summer and autumn, the latter in the autumn and winter (Mathers & Beamish, 1974). Certainly, both forms are well adopted to a freshwater life and unable to survive in salinities as high as one half full seawater. If the transition from an anadromous to a landlocked existence was characterized by a change in osmolality in the ammocoete stage its adaptive significance remains to be determined. Changes in integument thickness (Downing & Novales, 1971) and in the gut (Hardisty et al., 1970) as well as the replacement of the anterior pronej~hric and mesonephric kidneys with a posterior mesonephric kidney (Youson, 1970) occur during transformation from larva to adult and assist in the regulation of both internal ions and water content. Further, "ion transport" ceils in the gills of adult lampreys are reported to assume an excretory role in the removal of excess monovalent ions from the blood (Pickering & Morris, 1970). Additionally, the feeding adult undoubtedly gains osmoregulatory a~istance by the ingestion of blood and other body f l t l ~ whose ionic concentrations are not appreciably different from that of the lamprey serum.

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ambient lalinity for several life cycle stages of the anadromous sea lamprey. Values for landlocked small feeding adults were calculated from data in Mathers & Beamish (1974).

Performance of the anadromous sea lamprey

441

Feeding anadromous adults were able to osmo- the regulatory ability of nearly mature adults in a regulate in all salinities, the larger individuals main- hyperosmotic environment is illustrated by the detaining lower serum osmolalities, and sodium and creasing ratio of serum osmolality in freshwater to chloride concentrations than the smaller animals. that in the experimental salinities (Fig. 6). CompariOsmolality remained virtually unchanged between 0 son of the ratio in 24%o for nearly mature lampreys, and 16~oo among small feeding adults. Even in 34?/o0 0.57, with that estimated for small feeding adults at serum osmolality relative to that in freshwater in- the same salinity, 0.94, demonstrates the reduction in creased by less than 10~o (Fig. 6). In contrast only capacity for marine osmoregulation. Loss of the swalthe large feeding adult landlocked sea lampreys were lowing mechanism and a reduced capacity to absorb able to osmoregulate for long periods in salinities ions and water across the gut wall were suggested by Pickering & Morris t1970) as factors contributing above 16°~ (Mathers & Beamish, 1974). This trend to the breakdown on the marine osmoregulatory is illustrated in Fig. 6 by the rapid decline in the ability in upstream migrating sea lampreys and L. ratio of serum osmolality in freshwater relative to ambient salinity. Between 0 and 26~00 the ratio for fluviatilis. Freshwater osmoregulation deteriorated in spent adults. Osmolality and the concentration of landlocked adults decreased from 1.00 to 0.76 whereas sodium and chloride dropped below those of any that for the anadromous individuals declined only to 0.93 over the same salinity range. Small feeding other stage in freshwater indicating the tissues responsible for maintaining hyperosmoticity had ceased to anadromous adults of 135 mm were able to osmorefunction efficiently perhaps coupled with a leakage gulate in full seawater without mortality. In sharp of ions or water across the integument. An increase contrast over 50~o of the landlocked feeding adults in integument permeability to water was reported by of 127-188mm died within 10 days at 26°~.o. Only Morris (1958) and Pickering & Morris (1970) in L. when landlocked adults reached almost 280 mm did fluviatilis and P. marinus on the spawning migration. they survive in full seawater. In support of the size effect noted for landlocked Hardisty (1956) found increased body water in L. sea lampreys Mathers & Beamish (1974) suggested planeri and L. fluviatilis as each approached sexual that the full osmoregulatory capacity of adult sea maturity. Massive degeneration of the kidneys in lampreys might not be realized for some time after spawning landlocked sea lampreys was observed by their downstream migration. The observation of the Youson t19701. Gill (Na-K)-ATPase activity declined with age of overwintering of recently transformed anadromous sea lampreys in Chesapeake Bay where the salinity adult lampreys, being highest in feeding adults and is approximately 16oo o/ by Mansueti (1962) was cited effectivel2~ absent from those individuals which had in support of the argument. However, in the light entered the St. John River on their spawning migraof the results of this study a more probable explana- tion. However, at no stage was the enzyme activity tion is that the small adults remained in Chesapeake as high as values found for teleosts (Adams et al., 1975; Giles & Vanstone, 1976; Zaugg & McLain, Bay not because of the salinity but rather, on account 1970; 1971; 1976). Almost all measurements of branof the abundance of Atlantic menhaden, Brevoortia tryrannus on which they fed. Adjustment to full sea- chial Na--K ATPase have been performed at 37°C water by adult lampreys soon after transformation irrespective of its ecological relevance. Measurements of ATPase activity in sea lampreys were made at was reported also for the anadromous L. fluviatilis 1 I°C, the same temperature at which they were mainby Potter & Huggins (1973). In both the landlocked and anadromous feeding tained in the holding tanks. Branchial (Na K~adult sea lampreys changes in chloride ion were not ATPase in rainbow trout, Salmo gairdneri was almost accompanied by an equivalent shift in sodium sug- 4-fold higher when measured at 37 than lilac (T. A. gesting the relationship between the two ions is not Watson, personal communication). An additional passive. For a given change in chloride that by reason for the discrepancies between lampreys and sodium was less, although the control was not as tight teleosts may lie in the widespread practice of deriving in the higher salinities. The close control held over net activity values from the difference between results sodium has been demonstrated also for several obtained in the presence of sodium, potassium, teleosts (Houston et al., 1968; Byrne et al., 1972; Lutz, and magnesium--{Na-K-Mg)-ATPase--and those 1972). Lutz (1972)suggests that the physiological sig- obtained in the presence of these ions plus the cardiac nificance of sodium is in the uptake of metabolites glycoside, ouabain. Ouabain is known to inhibit (Naby the cell as well as in the regulation of potassium. K)-ATPase (Dahl & Hokin, 1974) producing baseline Regulation of sodium by the landlocked sea lampreys activities below those obtained in the absence of was not as fine as that exhibited by the anadromous sodium, potassium, and ouabain (Mg ATPase). Tobin individuals particularly in the higher salinities which et al. (1972) reported that the degree of suppression may have contributed to the difficulties they experi- of N a - K ATPase by ouabain varies among mamenced in saltwater. However, it should be emphasized malian species making comparison of activities that the equation describing the relationship for land- among species difficult even when the same experilocked sea lampreys had a much higher multiple mental conditions, enzyme preparation methodology, R2--86.75--than that obtained for the anadromous and assay procedures are used. individuals--42.08--making absolute comparisons of Evidence supporting the involvement of N a - K ion shifts tenuous. ATPase in gill ion excretion in salt water has been Upstream migrant adults had a serum osmolality demonstrated for several teleosts (Epstein et al., 1967; in freshwater which was above that of feeding adults. Kamiya & Utida, 1968; Zaugg & McLain, 1970; This rise apparently signals the restoration of the Pfeiler & Kirschner, 1972; Sargent & Thompson, freshwater osmoregulatory apparatus. The decline of 1974). Higher enzyme activities were found in teleosts

442

F . W . H . BEAMISH.P. D. STRACHANand E. THOMAS

exposed to salt- than freshwater, usually with greater activity occurring in the higher salinities which is consistent with the observations on adult sea lampreys. Higher enzyme activity might have been obtained had sea lampreys exposed to 34%o been examined. Further, most of the exposures to saltwater were only of 24 hr duration. The higher (Na-K)-ATPase activities obtained from large feeding adults after 15 days in 26%o compared to their values after 1 day in that salinity suggest the importance of the exposure period. The functioning of (Na-K)-ATPase in small adult sea lampreys presumably enhances the efficiency with which they are able to make the transition from fresh to saltwater. Once lampreys begin feeding, the body fluids ingested from the host fish will facilitate osmoregulation making the role of the enzyme less important. Further, as lampreys increase in size, the corresponding reduction of the surface area in relation to volume lowers the osmotic stress exerted by the external medium. The results of this study support the suggestion that in the evolution of the landlocked sea lamprey much of the capacity for osmoregulation in saltwater has ~,'~en lost. Hardisty (1956) and Mathers & Beamish (1974) also proposed that differentiation of the land~locked P. marinus involved selection for individuals of smaller size and lower potential fecundity. The apparent plasticity of the sea lamprey has enabled the species to successfully adapt to both fresh and saltwater.

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Rex Bd Can. 33, 54-62. HANSON L. H., KING E. L. Jr. HOWELL J. H. & SMITH A. J. (1974) A culture method for sea lamprey larvae. Prog. Fish. Cult. 36, 122-128. HARDISTY M. W. (1956) Some aspects of osmotic regulation in lampreys, d. exp. Biol. 33, 431-447. HARDISTY i . W. & POTTER 1. C. (1971J The general biology of adult lampreys. In The Biology of Lampreys, Vol. I (Edited by HARDISTY M. W. & POTTER I. C.L pp. 127 206. Academic Press, New York. HARDiSTY M. W., POTTER I. C. & STURGE R. (1970) A comparison of the metamorphosing and macrophthalmia stages of the lampreys Lampetra fluviatilis and L. planeri. J. Zool., Lond. 162, 383-400. HOUSTONA. H., REAVESR. S., MADDENJ. A. & DE WILDE Acknowledoements--Special appreciation is extended to M. A. (1968) Environmental temperature and the body Mr R. Robinson of Queens County, New Brunswick, and fluid system of the freshwater teleost--1. 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