J . Fish Biol. (1986) 29,649-662

Renal glomerular evolution in Antarctic notothenioid fishes

J. T. EASTMAN* AND A. L. DEVRIES~ Department of Zoology and College of Osteopathic Medicine, Ohio University, Athens,

Ohio 45701-2979, U.S.A. ?Department of Physiology and Biophysics, University of Illinois, Urbana, Illinois 61801,

U S . A . and Portobello Marine Laboratory, Portobello, New Zealand

(Received 24 October 1985, Accepted 23 April 1986)

Light and electron microscopy were used to document the degree ofglomerular development in 10 species of Antarctic notothenioid fishes. When combined with results of previous studies, data revealed that 16 of 20 species inhabiting subzero sea water were aglomerular. One subantarctic and two temperate species were pauciglomerular, and an additional temperate species had a moderate number of glomeruli. Renal corpuscles were variable in number and diameter among the pauciglomerular species, and most had few patent glomerular capillaries. Radiolabelled markers indicated that the glomerular filtration rate was low in the pauciglomerular Nororheniu ungusruru, ranging from 0.005 to 0.124 ml h-' kg-' in eight specimens. Arterial perfusion of Microfil demonstrated that arteries supplying aglomerular and pauciglomerular kidneys were confined largely to the periphery of the organ, and glomerular capillaries were absent or few in number. As ancestral notothenioids probably had glomerular kidneys, data from 2G25% of the fauna suggest that there has been an evolutionary loss of glomeruli in many species. The pattern of glomerular reduction is consistent with the hypothesis that the selective advantage of aglomerularism is in the urinary conservation of small molecular weight antifreeze glycopeptide compounds that are vital to survival in sub-zero Antarctic waters.

I. INTRODUCTION

The nephron is the structural and functional unit in the kidneys of all vertebrates. The evolution of the aglomerular nephron in a few species of teleost fishes is probably the most striking departure from the basic renal pattern. As of 1969, only 23 of the approximately 20 000 species of teleosts were known to be aglomeru- lar (Hickman & Trump, 1969). There is a general trend toward the reduction of glomerular filtration area in marine teleosts, presumably resulting in a saving of energy that would otherwise be expended on the tubular reabsorption of fil- tered substances (Hickman & Trump, 1969). The adaptive significance of aglomerularism, however, is difficult to assess, as the 23 species represent seven unrelated families, both freshwater and marine.

Subsequent to Hickman & Trump's (1969) compilation, Dobbs et al. (1974) and Dobbs & DeVries (1975a, b) identified 10 aglomerular species of Antarctic notothenioid fishes from two families. They related aglomerularism to the urinary conservation of small molecular weight antifreeze glycopeptides that are necessary for survival in subzero sea water. Dobbs & DeVries (1975~) noted that the Antarctic Ocean contains a disproportionately high percentage of aglomerular species.

Previous studies of aglomerular kidneys have dealt with the histology or ultra- structure of single species or of a few unrelated species (Edwards, 1928; Grafflin,

*Send correspondence to: J. T. Eastman at the above address. 649

0022-1 I12/86/120649+ 14 $03.00/0 0 1986 The Fisheries Society of the British Isles

650 J. T. E A S T M A N A N D A. L. D E V R I E S

1931, 1937; Bulger, 1965; Olsen & Ericsson, 1968; Ericsson & Olsen, 1970). The study reported here is the first to consider the pattern of glomerular development in a large, related group of primarily aglomerular teleosts. This report includes renal histology for an additional 10 notothenioid species including representatives of three families not examined by Dobbs & DeVries (1975~). With documentation for 20 species (20-25% of the fauna) from all five notothenioid families, it is now possible to draw conclusions about the evolution of aglomerularism in these fishes. More specifically, data are presented on: (1) the incidence of aglomerularism in Antarctic, subantarctic and temperate notothenioids; (2) glomerular histology, ultrastructure and function as indicated by clearance of radiolabelled markers in the pauciglomerular temperate notothenioid Notothenia angustata; and (3) arterial blood supply to aglomerular and pauciglomerular kidneys as demonstrated by injection of Microfil.

11. MATERIALS AND METHODS

Field work was conducted near the U.S. McMurdo Station on Ross Island in the south-western Ross Sea. Most fishes were collected from McMurdo Sound using methods outlined by Eastman & DeVries (1982). Notothenia larseni, N . kempi and N . coriiceps neglecta were captured at a depth of 100 m near Sabrina Islet, Balleny Islands in a bottom trawl pulled by an ice breaker. Kidneys of Dissostichus eleginoides were removed from a specimen (SIO 72-155) in the collection of the Scripps Institution of Oceanography at La Jolla, California; this fish was trawled off Chile (2Y2 l’S, 70’45’W). Bovichthys variegatus was netted in tide pools and N. microlepidota and N. angustata were caught in traps near Portobello, New Zealand. Some specimens of N. angustata were transported by plane to McMurdo Station. They were held in an aquarium facility and acclimated to seawater temperatures of 2-6” C. Clearance experiments were begun after specimens recovered from the stress of capture and transport.

All samples used for microscopy were taken from the caudal (trunk) kidney of adult specimens. Samples for light microscopy were fixed in Bouin’s solution, embedded in paraf- fin, sectioned at 5-7 pm and stained with hematoxylin and eosin, hematoxylin and phloxine, hematoxylin-phloxine-safran or Masson’s trichrome. Samples for electron microscopy (specimens NAN-1 9 and NAN-20) were fixed in Karnovsky’s paraformaldehyde- glutaraldehyde, postfixed in 1 % osmium tetroxide, dehydrated in ethanol and propylene oxide and embedded in Epon 812 according to conventional procedures. Thick sections were stained with methylene blue. Sections with silver interference colours were stained with uranyl acetate and lead citrate before viewing in a Zeiss 109 electron microscope.

Both kidneys of a specimen of N. angustata (NAN-9, body wt = 629 g) were serially sectioned at 15 pm. The number of renal corpuscles in each of 614 sections was counted and the number divided by five as it was determined that the average corpuscle was 77 pm in diameter in this specimen (meaning each corpuscle extended for five sections). Estimates of numbers of renal corpuscles were made on three additional specimens of N . angustata and one specimen of D. eleginoides. One-fifth of the kidney was sectioned and corpuscles were counted. The total number of corpuscles was estimated by extrapolation from the number in this fraction of the kidney.

After calibration with a stage micrometer, an ocular reticle was used to measure the diameter of renal corpuscles in histological sections. The point of measurement was the maximum diameter on the periphery of the parietal layer of Bowman’s capsule.

Microfil silicone rubber injection compound (Canton Bio-Medical Products, Boulder, Colorado) was used to demonstrate arterial blood supply to the aglomerular kidneys of Dissostichus mawsoni, Pagothenia borchgrevinki and Trematomus bernacchii and to the pauciglomerular kidney of N . angustata. The bulbus arteriosus was cannulated with PE-50 tubing and the circulatory system was then flushed with heparinized nototheniid Ringer’s solution (Eastman et al., 1979). The sinus venosus was cut to facilitate wash-out of blood.

NOTOTHENIOID KIDNEYS 65 1

White, orange or yellow Microfil was gravity-perfused through the cannula. Pressure during the perfusion was 48 mmHg as measured by a pressure transducer. After 90 min perfusions were terminated and specimens were fixed in 10% formalin. Subsequent treat- ment included skinning to remove opaque pigment layers, dehydration in ethanol and clearing in methyl salicylate.

Glomerular filtration rate was assessed in the pauciglomerular N . angustata by intra- muscular and intravenous injections of a total of 15 pCi of the glomerular markers 14C- polyethylene glycol or 3H-polyethylene glycol (mw = 5000 daltons; New England Nuclear). In another series of experiments, 15 pCi of the marker 14C-inulin was administered intra- venously. Fish were fitted with urinary catheters for continuous urine collection and 100 p1 aliquots were counted daily. Using a PE-10 cannula in the caudal vein, 150 pl of blood was withdrawn daily and 50 pl of serum was counted. Radioactivity in urine and plasma was detected by liquid scintillation counting.

111. RESULTS INCIDENCE OF AGLOMERULARISM

Table I summarizes work of Dobbs et al. (1974), Dobbs & DeVries (1975a) and the present study. With documentation of 16 completely aglomerular noto- thenioids, the number of aglomerular teleosts has been increased by 70% since the summary of Hickman & Trump (1969).

All 13 species from McMurdo Sound were aglomerular. Three species of Notothenia from the Balleny Islands were also aglomerular; however, two species of the same genus from New Zealand had a few glomeruli. Dissostichus eleginoides from subantarctic waters off South America showed a similar degree of glomerular development. These three species were considered pauciglomerular. Based on the glomerular classification of Marshall & Smith (1 930), the pauciglomerular notothenioids belong to Group 111, fishes having infrequent, small glomeruli.

Bovichthyids are the most generalized notothenioids, and their morphology may approximate that of the basal percoid stock (Eakin, 1976; Iwami, 1985). Accordingly, the glomeruli may reflect the ancestral condition. The temperate Bovichthys variegatus had more and better developed glomeruli than the pauciglo- merular species. Bovichthys had a moderate number of average sized glomeruli and, according to Marshall & Smith’s (1930) classification, belongs to Group I1 in common with many other marine teleosts.

LOCATION OF RENAL CORPUSCLES IN PAUCIGLOMERULAR SPECIES In N . angustata, N . microlepidota and D . eleginoides, renal corpuscles were

usually clustered in groups of two or three [Fig. l(a)] immediately beneath the connective tissue capsule of the kidney, near the walls of major collecting ducts or adjacent to the opisthonephric duct [Fig. l(b)]. In these locations the glomerular capillaries were supplied by branches of capsular and ductular arteries. Corpuscles were rarely found in the interior of the kidney as this portion had a poor arterial blood supply (see below).

NUMBER OF RENAL CORPUSCLES

species. Counts were low and, in N . angustata, they were also variable.

RENAL CORPUSCLE DIAMETERS Figure 2 summarizes data on corpuscle diameters. Diameters were variable,

especially in N . angusrara. According to the classification of Marshall & Smith

Table I1 presents counts of renal corpuslces for two of the three pauciglomerular

652 J. T. EASTMAN A N D A. L. D E V R I E S

TABLE I. Habitat, biological antifreezes and glomerular development in notothenioid fishes

Ice-laden Glycopeptide Glomerular water antifreezes kidney

Location, latitude,

water temp. Species

Antarctic species Nototheniidae

Trematomus: bernacchii, hansoni, lepidorhinus, loennbergii, centronotus, nicolai, newnesi Pagothenia borchgrevinki Dissostichus mawsoni Pleuragramma antarcticum

Gymnodraco acu t iceps

Pogonophryne scotti

Pagetopsis macropterus Antarctic circle and sub-Antarctic species

Bath ydraconidae

Harpagi feridae

Channichthyidae

Nototheniidae Notothenia: larseni, coriiceps neglecta

Notothenia kempi Dissostichus eleginoides

McMurdo 78" S - 1.9" C

Yes Yes No*t

- 109°C - 1-9"C - 1-9°C

Yes Yes Yes

Yes Yes 9

No * No * No

- 1.9"C Yes Yes

?

3

No * - 1'9°C Yes No

- 1.9"C Yes No

Balleny Is. 67"s

- 1 to + 1°C - 1 to + 1°C

Chile 25"s

> + 5 " C

Yes Yes No

NOS No

No No

No Yes

(pauciglomerular)

Temperate species Nototheniidae

Notothenia: angustata, microlepidota

Portobello, N.Z. 46"s

No No Yes (pauciglomerular)

+ 5 to 19°C Bovichthyidae

Bovichthys variegatus + 5 to 19°C No No Yes (moderate number)

*Dobbs et al. (1974) tDobbs&DeVries (1975~) $Near the Balleny Is. N . kempi lives at a depth of 100 m where the water temperature remains above 0" C throughout

the year

(1930), corpuscles less than 39 pm in diameter (their Group 111) were considered small. In most marine teleosts (Group 11) corpuscles ranged from 33 to 94 pm in diameter.

With the exception of D. eleginoides, Fig. 2 demonstrates that samples approached statistical adequacy in terms of the ratio of dispersion (s.D.) to relia- bility (s.E.M.) (Hubbs & Hubbs, 1953). For sample sizes larger than 30, twice the S.E.M. approximates the 95% confidence interval for the mean (Sokal & Rohlf, 1981). When the confidence intervals (black rectangles in Fig. 2) overlapped,

NOTOTHENIOID KIDNEYS 653

FIG. I . (a) Cluster of three renal corpuscles in kidney of Notothenia microlepidota. Glomeruli have few patent capillaries. Widest point at base of cluster is 75 pm. Hematoxylin and eosin. x 342.3. (b) Small (40 pm) renal corpuscle adjacent to wall of opisthonephric duct in Notothenia microlepidota. Masson’s trichrome. x 360. (c) Small (30 pm), dense renal corpuscle typical of those in kidney of Notothenia microlepidota. There are a few patent glomerular capillaries. Hematoxylin and eosin. x 352. (d) Unusually well developed renal corpuscle from Notothenia angustata (specimen NAN- 19). Although this corpuscle appears functional, the specimen has a low glomerular filtration rate (Table 111). A large arteriole (A), the vascular pole and patent glomerular capillaries are evident. Long axis of corpuscle is 80 pm in diameter. Hematoxyiin and eosin. x 307-2.

differences among mean corpuscle diameters were not significant. When means were significantly different, as indicated by inspection of Fig. 2, we used Student’s t-test to obtain exact probability values for these comparisons: N . microlepidota v. B. variegatus ( t = 7.513, P<O*OOl), N . microlepidota v. N . angustata ( t = 10.316, P<O-OOl), D . eleginoides v. N . angustata ( t = 2.733, P<O.OOl) and B. variegatus v. N . angustata ( t = 2.389, P<0.02) .

Among notothenioids, corpuscle diameter is not a reliable criterion of glomerular function. For example, N . angustata, known to be functionally

654 J. T. B A S T M A N A N D A. L. D E V R I E S

TABLE 11. Numbers of renal corpuscles in two pauciglomerular notothenioids

No. Of corpusc'es in one kidney Method of counting Species & specimen no.

Dissostichus eleginoides DEL- 1

Notothenia angustata NAN-3

NAN-7

NAN-8

NAN-9

77 Estimate based on partial serial sections

192 Estimate based on partial

1224 Estimate based on partial

195 Estimate based on partial

453 Complete serial sections

serial sections

serial sections

serial sections

Species

Notothenm inmolepidoto 3 6

Dissostichus elegmoides I

Bovichlhys voriegofus 6 25

Nofothenio ongustoto 6 64

FIG. 2. Dice-Leraas diagram (modification of Hubbs & Hubbs, 1953) comparing renal corpuscle diameters among four species ofnotothenioids. Heavy horizontal line is range; short vertical line is mean; black rectangle is two S.E.M. on each side of the mean; one-half of each black rectangle plus white rectangle is one S.D. on each side of the mean. See text for additional explanation.

aglomerular from clearance studies (see below), had the largest mean corpuscle diameter.

RENAL CORPUSCLE HISTOLOGY AND ULTRASTRUCTURE Corpuscle histology was similar in the three pauciglomerular species [Figs 1,

3(a)]. The parietal layer of Bowman's capsule was thick and there was frequently a large capsular space. The glomerulus was dense with few patent capillaries [Fig. 1 (a)-(c)]. Occasional hyalinized glomeruli were encountered in N . angustata and D. efeginoides. Some corpuscles in N . angustata showed no evidence of glomerular vascularization as evidenced by either patent capillaries with red blood cells or the presence of afferent and efferent arterioles. In addition, serial sections indicated that some corpuscles were not continuous with tubules in N . angustata and D . eleginoides [Fig. 3(a)].

NOTOTHENIOID KIDNEYS 65 5

FIG. 3. (a) Large (87 pm), non-functional renal corpuscle from Dissostichus eleginoides adjacent to two veins (V). Serial sections indicate that the corpuscle lacks a vascular pole and is not continuous with a tubule. Hematoxylin and eosin. x 332.8. (b) Renal corpuscles are more numerous in Eovichfhys variegafus. Patent glomerular capillaries are present in each of these two corpuscles. Largest corpuscle is 63 pm in diameter. Hematoxylin and phloxine. x 338.4. (c) Left lateral view of the kidney of Trematomus bernacchii. Arteries were perfused with yellow Microfil and the specimen was cleared in methyl salicylate. Relatively few arteries supply the interior of the kidney and glomerular capillaries are not present in this aglomerular species. Dark parietal peritoneum covers the ventral surface of the kidney. Urinary bladder (B) is ventral to kidney and stomach (S) is to the left. Microfil reached vessels as small as 7-10 pm in diameter. Major branches from dorsal aorta to left half of kidney are: (1) left renal artery, (2) four left segmental arteries and (3) midline renal artery. x 3.03.

Although larger and more numerous, corpuscles in B. variegalus were generally similar to the pauciglomerular species. They differed, however, in having greater numbers of patent capillaries, and these were usually located at the periphery of the glomeruli [Fig. 3(b)].

Figures 4 and 5 show the ultrastructure of corpuscles from two specimens of N. angustata (NAN- 19 & 20) that were utilized in the clearance experiments described below. The glomerular filtration membrane was substantial (Figs 4,5), averaging 0.45 f 0.024 pm (s.E.M.) thick in a series of 20 measurements from both specimens.

656 J. T. E A S T M A N A N D A. L. D E V R I E S

FIG. 4. Portions of two glomerular capillaries from Notothenia angustata (specimen NAN-19) showing the arrangement of the pedicles, thick basal lamina and cellular (possibly phagocytic) debris (CD) in capillary lumen. Uranyl acetate and lead citrate. x 6600.

Podocytes had large processes connected to irregularly shaped pedicles that adjoined the basal lamina. Desmosomes connected adjacent processes and pedicles had filtration slit membranes (Fig. 5). A thickened basal lamina com- pletely encircled the glomerular capillaries. The capillary endothelium was attenuated and lacked fenestrae. Features suggesting sluggish or intermittent blood flow included irregularly-shaped red blood cells and phagocytic debris (Fig. 4) in the capillary lumina.

GLOMERULAR FILTRATION RATE IN NOTOTHENIA ANGUSTATA Three radiolabelled markers were used to study glomerular function in N .

angustata (Table 111). Since inulin and polyethylene glycol are filtered in the glomeruli but are neither secreted nor reabsorbed by the renal tubules, the clear- ance of these substances equals the glomerular filtration rate (Prosser, 1973).

NOTOTHENIOID KIDNEYS 657

FIG. 5. Components of the glomerular filtration membrane in Notofhenia angustata (specimen NAN-19) include pedicles (PD) and adjacent filtration slit membranes (FS), a thick basal lamina (BL) and glomerular capillary endothelium (E). Glomerular filtration membrane is 0.39 pm thick at double headed arrow. Also visible are capillary lumen (C)containing red blood cell, desmosome (D) between processes of a podocyte and mesangial matrix (M) between endothelium and basal lamina. Uranyl acetate and lead citrate. x 24 800.

Although there may be some absorption of markers, the bladder is least permeable to polyethylene glycol, and this is consequently the most reliable marker in teleosts (Beyenbach & Kirschner, 1976).

Clearance values were constant and low over the course of the experiments, indicating little filtration except possibly in the case of specimen A. In spite of the presence of some corpuscles with patent glomerular capillaries in specimen NAN-19 [Figs l(d), 41, clearance values were not notably higher than in other specimens. Values were considerably lower than those reported for other marine teleosts (Hickman & Trump, 1969).

658 J. T. EASTMAN A N D A. L. D E V R I E S

TABLE 111. Mean glomerular filtration rates for clearance of ''C-inulin, 3H-polyethylene glycol or ''C-polyethylene glycol in Notorhenia angustata.

Water temperature + 6" C except for specimen B1 (+ 2" C )

C A B1 NAN-2 1 NAN-19 NAN-20 BC 1 BC2

I4C-inulin 489 4C-inulin 1022

14C-inulin 1000 3H-PEG 1130 3H-PEG 1555 3H-PEG 1645 14C-PEG 1420 14C-PEG 2266

0.036 0.124 0.047 0.03 1 0.013 0.005 0.008 0.025

81.5 81.5

403 395 435 435 101 120

RENAL ARTERIAL BLOOD SUPPLY IN NOTOTHENIOIDS Although the renal blood supply in most marine teleosts is primarily venous

(Hickman & Trump, 1969), glomeruli, if present, must have an arterial supply. Microfil effectively fills glomerular capillaries in those species possessing these structures (Hentschel, 1977a, 6) . Microfil preparations of the kidneys of D . rnawsoni, P . borchgrevinki, T. bernacchii and N . angustata were used to identify the renal arterial blood supply. The first three species were aglomerular, whereas N . angustata was pauciglomerular . Measurements with an ocular micrometer indi- cated that Microfil reached vessels 7-10 pm in diameter. Arteries were more pre- valent near the periphery of the kidney, with the interior being poorly vascularized. Glomerular capillaries containing Microfil were not evident in these four species; however, patent capillaries were identified histologically in N . angustata.

The arteries to the notothenioid caudal kidney, as exemplified by T. bernacchii [Fig. 3(c)], originated from three sets of branches off the dorsal aorta. First, one pair of large renal arteries entered anteriorly and supplied most of the interior of the kidney. Second, four pairs of small segmental arteries were confined largely to the capsule of the kidney. Third, a midline renal artery travelled near the posterior margin of the kidney; this vessel supplied most of the posterior aspect of the kidney and continued to the dorsal and posterior aspects of the urinary bladder.

IV. DISCUSSION

GLOMERULAR PLASTICITY The teleost nephron exhibits remarkable structural and functional plasticity in

both phylogeny (Nash, 1931) and ontogeny. Grafflin (1929,1933) first discovered ontogenetic glomerular degeneration in the aglomerular Lophius americanus and in the pauciglomerular Myoxocephalus scorpius. A variety of factors contribute to intraspecific variability in glomerular morphology. De Ruiter (1981) found that testosterone altered the structure of several glomerular components in Gasterosteus. The effect of salinity on glomerular development in euryhaline species is well documented. For example, Colville et al. (1983) observed that the

NOTOTHENIOID KIDNEYS 659

glomeruli of seawater-adapted Salmo gairdneri were significantly smaller and fewer in number than those of a comparable group of freshwater-adapted rainbow trout.

For unknown reasons some taxa exhibit extreme variability in glomerular morphology. Forster (1975) noted that sculpins of the genus Myoxocephalus show marked inter- and intraspecific variation in the degree of glomerular development, ranging from aglomerular to fully glomerular. Appreciation of renal plasticity may help to explain the pattern of glomerular development in notothenioids.

EVOLUTIONARY AGLOMERULARISM Notothenioids dominate the fish fauna of Antarctica. Numbering 8CL100

species, they constitute two-thirds of the species and 90% of the individuals in the Antarctic region (DeWitt, 1971). It has been hypothesized that the ancestral notothenioid stock was associated with the continent from the time the waters began to cool down 38 million years ago (Regan, 1914; Norman, 1938; DeWitt, 1971). As notothenioids have been long separated from their ancestors, it is diffi- cult to identify sister groups among the Perciformes. In summarizing hypotheses of higher-level systematic relationships, Andersen (1984) indicated that trachi- noids and blennioids might have shared a common ancestor with notothenioids.

Trachinids and blenniids have glomerular kidneys (Hickman &Trump, 1969) as does the mugiloidid Parapercis colias (Eastman, unpubl. data). The Bovichthyidae is the most primitive notothenioid family (Eakin, 1976; Iwami, 1985) and, as mentioned previously, Bovichthys variegatus also has glomeruli. Another bovichthyid, Pseudaphritis urvilli, enters rivers well above the tidal influence and can be transferred directly from sea water to fresh water without adverse effects (Berra, 1981). Glomeruli are probably well developed in this species. Thus the pattern of glomerular development in both extant primitive notothenioids and possible sister groups suggests that the ancestral kidney was glomerular and that there has been an evolutionary loss of glomeruli in many species. Sixteen of 20 species examined to date are aglomerular. Thirteen of those 16 species are from McMurdo Sound, one of the coldest bodies of water in the world with a mean annual water temperature of - 1.9" C.

Most notothenioids living north of the Antarctic Convergence or in the temper- ate waters around New Zealand are pauciglomerular. The few glomeruli present are probably vestigial, reflecting the ancestral condition rather than the functional necessity of these structures. This idea is substantiated by our clearance exper- iments on N . angustafa which did not produce significant filtrate at the glomeruli.

Hickman & Trump (1969) regard loss of glomeruli in marine teleosts as an energy-conserving mechanism. They note that when there is a reduction in the number of ions and molecules escaping from the plasma at the glomeruli, energy need not be expended in reabsorbing these components from the filtrate. The aglomerular kidneys of notothenioids have the additional advantage of preventing urinary loss of small ( < 34 000 daltons) glycopeptides with antifreeze properties (Dobbs et al., 1974). These compounds are vital to survival in subzero sea water (DeVries, 1982). If filtered into the urine at the glomeruli, recovery of antifreezes would involve degradation into amino acids in the lysosomes of the kidney tubules. Resynthesis of antifreezes would then require metabolic energy. For example, during the activation of each amino acid in protein synthesis, one ATP molecule is

660 J. T. E A S T M A N A N D A. L. D E V R I E S

hydrolyzed to AMP and two high-energy phosphate bonds are consumed (Stryer, 1981). By eliminating glomeruli during the course of their evolution, many noto- thenioids may save energy by not having to degrade and reabsorb compounds necessary for the resynthesis of antifreezes.

All 13 McMurdo Sound species examined to date are aglomerular. Ten of these 13 have antifreezes (DeVries & Lin, 1977; Ahlgren & DeVries, 1984; DeVries, unpublished data). Definitive identification of antifreezes has not been attempted in Pleuragramma, Pogonophryne and Pagetopsis, as these species are difficult to collect in ice-covered McMurdo Sound. Most notothenioids inhabiting sub- antarctic and temperate waters do not have antifreezes because the water never drops below zero and they therefore have no need for protection. As far as we know there are no pauciglomerular species that have antifreezes. Selection pressure for evolutionary loss of glomeruli may not have been as great in these pauciglomerular species. A similar pattern of glomerular development is evident in the genus Dissostichus, with two closely related species having non-overlapping distributions (Yukhov, 1972). Dissostichus eleginoides lives north of the Antarctic Convergence (water temperature 2-6" C), lacks antifreeze and is pauciglomerular. Dissostichus mawsoni, on the other hand, lives in the subzero waters south of the Convergence, has antifreeze and is aglomerular.

While it cannot be definitively proven that aglomerularism in notothenioids is causally related to the presence of antifreezes, the correlative evidence is consistent with this hypothesis. Energetic efficiency confers an adaptive advantage to an organism and is therefore an important feature in the evolutionary design and modification of all body systems. This is especially true in notothenioids inhabit- ing waters devoid of sunlight and productivity for long periods of the year. Aglomerularism may be a component in the spectrum of cold adaptation and energy conservation displayed by this group.

This research was supported by NSF grants DPP 79-19070 to J.T.E. and DPP 84-15266 to A.L.D. J.T.E. was also supported by funds from the Ohio University College of Osteopathic Medicine. For assistance in the field and lab we thank Wayne Van Voorhies, Bob Boyd, Fred Whoriskey, Terry Miller, Joe Schrag, Scott O'Grady, Jeff Turner, Martha Wolfe and Beth Marks. Jack Jenkins and Prof. John Jillette of the Portobello Marine Laboratory kindly provided specimens of Notothenia angustata. Richard H. Rosenblatt of Scripps Institution of Oceanography gave us kidneys from Dissostichus eleginoides.

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Berra, T. M. (1981). An Atlas of Distribution of the Freshwater Fish Families of the World. Lincoln: University of Nebraska Press.

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NOTOTHENIOID KIDNEYS 66 1

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