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ADAPTATION TO HIGH HYDROSTATIC PRESSURES OF ABYSSAL GAMMARIDS FROM LAKE BAIKAL
IN EASTERN SIBERIA
R. W. BRAUER’, M. Y. BEKMAN’, J. B. KEYSER’, D. L. NESBITT’, G. N. SIDELEV’ and S. L. WRIGHT’
‘Institute of Marine Biomedical Research, University of North Carolina at Wilmington, Wilmington, NC, U.S.A. and
‘Institute of Limnology, Siberian Branch, U.S.S.R. Academy of Sciences, Listvyenichnoye na Baikale, Irkutsk Oblast, U.S.S.R.
(Received 5 April 1979)
Abstract-l. Compression of freshwater gammarids from Lake Baikal elicits a series of 9 distinguishable response stages reflecting successive phases of hyperactivity, convulsions and immobilization.
2. Convulsion threshold pressures are much greater for abyssal than for shallow water gammarids and average at a pressure near the maximum existing in the lake.
3. Threshold pressures for the other two phases are substantially greater than can possibly be encoun- tered in Lake Baikal and differ little between deep and shallow water gammarids.
4. The relation of these observations to the problem of the role of hydrostatic pressure as a selection factor in the evolution of deep water fauna is discussed.
INTRODUCTION
Lake Baikal in Eastern Siberia is one of the excep- tional sites on earth where evolutionary processes going on in relative isolation for a very long time have produced a fauna characterized not only by an extraordinarily high endemism rate, but also by an extraordinary species diversity within many of those genera which have occupied this lake for long periods of time (Kozhov, 1963, p. 291). Low mean annual tem- peratures, and an unusual horizontal and vertical cir- culation pattern (Verbolov, 1977), assure a high oxygen content throughout the entire water column (Votyntzev, 1961) so that, unlike the few other lakes of comparable depth, the fauna of Lake Baikal extends to all depths of this deepest among the world’s lakes.
Taken together, these characteristics render the fauna of Lake Baikal of extraordinary interest from the point of view of the study of the physiological and evolutionary problems associated with the relatively late penetration of aquatic animals into abyssal depths. In marine crustaceans, evidence of adaptation to high pressure regimes has been found in animals captured at depths as shallow as 800 m, about half the maximum depth of Lake Baikal (George, 1979; Mac- Donald & Teal, 1975). As a general rule, animals cap- tured at depths in excess of 3000 m fail to survive retrieval to the surface unless provision is made to maintain habitat pressure throughout the capture and recovery process (MacDonald & Gilchrist, 1978). The role which pressure plays as a selection factor in the evolution of such deep water species has remained controversial. In particular, the relative importance of genetic factors as opposed to acclimatization to a high pressure regime during the life span of individual deep water animals has not been resolved to date, nor is there any conclusive evidence which would allow one to point to any specific biophysical or biochemi-
cal differences as causally associated with tolerance to high pressures.
The existence of multiple interrelated species of crustaceans of the family gammaridae at all depths of Lake Baikal (Kozhov, 1963, pp. 109-117) should provide an excellent opportunity for approaching these problems by comparative physiological studies. The present report deals with experiments seeking to establish whether high pressure adaptations are de- monstrable in freshwater gammarids, which are members of an abyssal fauna, and to make a prelimi- nary assessment of the relative importance of species differences in this respect. The results presented were obtained during 3 weeks of collaborative study in July, 1978.
METHODS
Collection procrdures
Gammarids were captured in baited traps positioned directly on the bottom. Two similar trap designs were deployed: (1) commercially available steel traps, 42cm long, 23 cm in diameter at the center and tapering to 17 cm at the ends, with a 0.5 cm wire mesh covering; and (2) traps built at the Institute of Limnology, 80 cm long, 30 cm in diameter. constructed of bin. steel bar framing, covered with 0.5 cm mesh cloth netting. Access to both types of traps was through cone-shaped segments at both ends. leading to either a 3 cm or 5 cm diameter opening. Traps were fished 4 to 8 at a time, attached to the ends of a metal cross-piece, 1 x I m, and were stabilized by placing small, 12.5 cm, metal flotation spheres above them on the cross- piece.
These traps were baited either with pieces of fish. beef or chicken without any major differences in capture results being detectable. Deployment of the traps was either as a component of a free vehicle assembly or at the end of a line supported by a surface buoy. In either case, traps were left on the bottom for periods which ranged from
109
I IO R w. BK4WH t’, ,I/
4 to 71 hr. with I1 15 hr the most common. More detailed analyses of the trapping operatlom uill be presented m another context. Trapping of amphipods was conducted during the month of July, in the southern basin. No attempt was made to provide thermal protection during retrieval from either shallow or deep stations. Spring turn- over of open Lake Baikal waters. i.e. homothermy. nor- mally occurs during the middle to end of June in the south- ern basin of Lake Baikal (Kozhov. 1963. p. 291) During homothermq, surface to hottom temperatures arc uni- forml! 3 4.3 <‘ in open waters. From the end of June through July. due to temporary warming in open water areas following extended periods of calm. surface tempera- ture may reach a maximum of I2 15 C: however, the ther- mocline 1s usually quite shallou and typically located at 4 5 m depth. At a depth of S-6 m. temperatures have already dropped to only 6 5 C. Thermal ihock to animals collected from these open waters \\as mimmized during retrieval by arranging the free vehicle system \o that the trap assembly would float below, the thermocline, at a depth of approx 12 m.
Shallow collection sites. such as on the Selenga Avan- delta. have surface temperatures of from 15 I6 C during July.
The traps were brought on board one at a time and the contents were transferred immediately into pans con- taining lake water at 4.5H1.0 C. Captured gammarids were counted at once in these pans, separated by species and swiftly transferred to flat trays. These were placed in a refrigerator, hopefuully operating at it 6 C. and maintained there without oxygenation. Water aas replaced at 24 hr intervals to minimize the effects of accumulation of excrc- tory products or possible oxygen depletion. Animal densit? was held below 2OOg/m’ of tray surface. a number u hich. in vie& of the expected metabolic rate of these annnals. should permit adequate oxygenation h! din‘uslon through the surface.
Pressure tests were performed in a small vertical cylm- drical pressure chamber, 5 cm in. id. and 9 cm deep. For temperature control. this chamber was surrounded by coohng coils supplied by a recirculating pump which fed water by way of a heat exchanger immersed in an ice bath to the cooling coils or to a bypass and reservoir. the whole being surrounded by a thermal lagging envelope. Chamber
temperature uas regulatea h> .tdJusttng the prop~~rtlc~n (~1 cooling v,ater passed through the coohng coils. At equllih- rium, chamber temperature wa\ found to be within 2 0.5 C‘ of the coil effluent temperature. For purposes ,)I” ob\er\,r- tlon. the chamber was provided with conical Lucite win- dows at either end. A microscope illuminator tilth Infrared filter was placed above the upper \\rndou. ;md .I front silvered mrror placed at :L 1’ ,inglc helo~ the Iower w tn- do* permitted \afe ,lb\er\ att~>n ‘)i clnematographlc recordmg. The aybtem pressure V.,IL contrc>lled hq a manually operated pump .tnd registered h\ mean5 of ,I Bourdon tube precision hlsh pl-e,\urc gauge Manual pumpmg controlled by the gauge permItted obtalnlng con- stant compression rates wIthin clo\e tolerances For a11 comparative studiea. :I compression rate of 11 5 atm mln (l.c. 200 psi ‘min) wiih employed
All gammarld\ were c~hser\ed ~lngl> A restratnmg net was emplolzd 10 ctTecrlvci! I onfinc animals rmaller than 0.7 g to the Io\+cr 5 cm 01 the pressurlted compartment. Since ‘tI1 ob4er\atlons \tere performed \+ith only one sn- mal in the pressure chamber. the bloma.\s in the teat con- partment was always held helou 3.5 g .At a comprcsblon rate of 11.5 atm min. a11 experiments were completed within 30 mm Thus. oxygen content of the chamber eater would not be depleted to an> hipnificant extent
The response of gammarId crustaceans to compresamn conslsts of a sequence of behavior changes, which can be subdivided into nme stage\. the onbet pressure for each of which can be established with a rea\onablc degree of conlidence. The ldentiticatlon of these stages and the nu- merical results indicating onset pressures for each stage and each species will hc diszusscd under Results.
III all. x species of gammarld crustaceans were retrieved and subjected to hehavlor test\ hy this method. The distri- butlon of the species. with regard to depth of capture. will be dircussed In conjunction aith Table I. Identification of the species was effected b! one of us (M.Y.B.) based on the classification scheme currently in use at the Institute
of Limnology.
E xptvimrrirtrl dctkyti imd rltlltl r~~lf~~~l~~~~~
Experimental design ~.a\ circumscribed b, the success of trapping operations as well as by the total time available for laboratory observations. With one exception. each
Table I. (‘haracterlzation of gammarid hpecies utilized
Capture depth Reported vertical Body weight (g)
Group No. Name (ml range (mi mean and range
S I
s 7
S 3
S 4
S s
D 1
D 2
D 3
D 4
Crany0ny.x sp.
Pul/asctr cur1ccllu.5 any.
Eulimnogummurtr.s wri-wow\
Pallusec~ yruhei
Acanthoyammarus u/bus
Odonrogummtrrus muryurrrucrus
Parupullasrci IugowskI
Ommutogummurus ulhinu.5
,4urnrhogummurus grewinyki
Beach
IO m
50 m
50 m
935 and 1OOOm 935 and
1000m 935 and
1OOOm 1400 m
0 100
Littoral
Littoral
I 175
5~ X20
500 I 300
200 1350
50 1300 140 1400
0.034 (0.010 0.045)
1.70 11.6@ 1.X0)
0.460 IO.38 I-O.53 1
0.30 (0.20@0.?00)
I.56 CO.X?- 2.10)
0. I x2 IO.O’)H) 73 J
0.476 (0.3@ I .‘tO)
0. I x0 ~0.15~0.20)
3.50 (0.79 4.X)
Gamm~ids adapt to pressure in Lake Baikal 111
species reported upon was represented by at teast four indi- viduals subjected to compression studies (cf. Table 2). Nu- merical data refer to threshold pressures in atmospheres absolute (ata) for the various behavioral endpoints selected.
A normal distribution of threshold pressures was assumed for each separate species, allowing the use of small number statistics for mean, standard deviation, and for assessment of significance of interspecies differences by Student’s t-test. For comparisons involving groups of species (as, in particular, deep water vs shallow water species), normality of distribution could not be assumed, and analyses were performed on the basis of rank order using Wilcoxson’s ranking statistic to derive significance estimates for differences between group d~strib~ltions (Wilcoxson, 1945).
RESULTS
Taxonomic and vertical distribution of capture sites
Table 1 shows the 8 species of Baikalian gammarids utilized in the present series of experiments, together with the one species of North American freshwater gammarid utilized in some of the baseline studies as well as for comparison. The species are grouped in the table in order of the depth at which they were captured (Column 3). They fall into a shallow water (S) group consisting of four Baikalian and the one North American species, all from depths of 50 m or less, and a deep water (D) group of 4 species from depths of 9351400m. Column 4 shows the range of depths over which each species has been captured in previous work by Soviet investigators (Bekman, per- sonal ~rnrnu~i~ation~. For four of the species used here, a wide vertical distribution has been reported, crossing the 250m depth which tends to separate deep and shallow water fauna in Lake Baikal. Column 5 shows the mean body weight range of the individuals of each species obtained and underscores the remarkably large size of Baikalian gammarids.
Pattern of response to cotnpression
Careful ob~rvation of the locomotor activity of gammarids during exposure to progressively increas- ing pressures revealed a series of nine recognizable stages marked by successive changes in locomotor be- havior.
Stage I. This pattern is poorly marked in those species which normally display a considerable amount of active swimming, but rather pronounced in species like P. grubei or 0. albinus, which spend much of their time on the bottom or which normally indulge in continuous, but tranquil, swimming ac- tivity. When exposed to pressures of the order of a few tens of atm, in each of these cases the rate of swimming and the percentage of time spent in active swimming becomes greatly enhanced.
Stage II. This stage is marked by a general increase in locomotor activity including both extremities and Aexion and extension of the entire body.
Stage III. This stage is probably identical with Stage II, but finds its representation most notably in species in which increased swimming activity is least marked. This stage involves stretching and Sexing of the entire body and, in particular, of the telson area.
Stages identified as I, II and III represent different aspects of a common behavior pattern marked by a generalized increase in intensity and frequency of ac- tivities which form part of the normal repertoire of
undisturbed gammarids. Furthermore, they do not occur in any consistent order-in some species, III may precede I or II, et cetera. Together, however, they define the initial ~havioral reaction to compres- sion and will be designated as Phase A, the phase of hy~ractivity.
Stage IV. This stage is characterized by isolated jerking motions encompassing the entire body. These are isolated events recurring at several second inter- vals in the appropriate pressure range. In some of the animals, the frequency of body jerks increased progressively as the pressure was raised.
Stage V. This stage is marked by vigorous kicking of the teison region, propelling the animal back and forth across the chamber. These motions are typically grouped in a series of kicks followed by a brief pause and resumption of kicking.
Stages IV and V thus are the first stages where non-normal, pathological patterns of locomotor ac- tivity make their appearance. Together, they will be designated as Phase B of the response to compression, the con~lsion phase.
Stage VI is marked by extension of the telson and suspension of the kicking activity, but continued though very reduced flexion and extension motions of the thoracic portion. During this stage, as during the succeeding stages, the extremities retain their indi- vidual mobility but there is little displacement of the animals around the bottom of the chamber.
Stage VII involves increasing rigidity of the thora- tic segments as well as of the telson, so that the entire animal appears to be fully extended with dorsal flex- ion.
The transition between Stage VI and VII, in many cases, is not sharply marked. The two stages appear to occur, in most cases, in rapid succession and make the transition from convulsive activity to the well marked next stage.
Stage VIII is characterized by exaggerated rapid activity of the pleopods while the remainder of the animals remains immobile. This “fluttering” of the pleopods, in some cases, appears to represent a further development occurring at pressures higher than those giving rise to Stage VII.
Stage IX, finally, represents the suspension of all motor activity, pleopods included. The only activity remaining in some of these animals is occasional slow oscillatory motion of one or another extremity. There seems to be no selectivity as to which of the appen- dages are involved in this final residual activity.
Stages VI to IX comprise the gradual progression from the exaggerated locomotor activity of Phases A and B to final immobilization. Together, they consti- tute Phase C, the phase of immobilization as a resuit of compression.
Factors potentially af$ecting response to co~p~@.~s~on
Compression rate. In many vertebrates, compression rate affects the threshold pressures for various stages of the high pressure neurologic syndrome. Prelimi- nary studies were a~ordingly undertaken to deter- mine to what extent this factor also modifies the re- sponse of gammarid crustaceans to compression. Tests were conducted using species S-l of Table 1 to compare convulsion thresholds at compression rates of 1.8, 10, 20, 50, 100 and 16Oatm~min. Mean thresh-
Tab
le
2.
Thr
esho
ld
pres
sure
s fo
r va
riou
s st
ages
of
th
e co
mpr
essi
on
synd
rom
e in
fr
eshw
ater
ga
mm
arid
s
Nam
e C
aptu
re
dept
h h:
--
--_l
-
Mea
n th
resh
old
pres
sure
s (A
TA
) +
SE
fo
r st
age
Phas
e A
----
Hyp
erac
tivity
Ph
ase
IL-c
onvu
lsio
ns
Phas
e C
--Im
mob
iliza
tion
I Ii
II
I IV
V
V
I V
II
VII
I IX
.--
.
2%
4 2 5
NO
N
O
NO
X
2.6
i 0
(24)
N
O
0 24
.8
24.8
k
15.4
N
O
35,O
f
10.6
14.6
4
4.7
(1,5
) 21
.4
wr
41.6
14
.6-6
9.0
(214
1 17
9.2
75.8
82
.6
75.8
i:
23.1
105.
1 -t
_ 16
.7
87.1
*
4.1
96.2
&
3.4
96.2
rL
: 0
85.4
i
8.4
132.
5 i_
5.1
11
1.5
* 6.
7
NO
206.
3 _?
8.8
13!4
) 84
.7
* 10
.3
(l/2
) 19
7
(41%
18
4.7
li: 1
0.2
206.
3 *
X.8
(314
1 19
7.2
If:
11.5
N
O
117.
3 &
14
.0
86.9
_t
4.4
11
0.7
+ 8.
i f3
/4)
130.
3 &
17
.3
W)
231
225.
5-23
9.1
NO
(5i6
) (5
/6)
(l/S
) 15
X.X
$
24.0
22
4.1
):
23.9
23
9 18
1.3
k 28
.7
NO
N
O
247.
5 2
7.9
204.
3 *
12.8
205.
1 +-
10.
6
179,
2 j
4.3
321.
2 i.
8.4
268
rt:
3.8
300.
3 It
20
.3
3 (3
4 53
.7
f 10
.9
6.8
l--
14.6
N
O
(5 6
) 15
.x
f 19
.6
NO
(4
5)
75.8
*
21.2
(6’7
) (1
71
36.1
+
13.0
89
.4
234.
0 i
9.3
264
i_ 2
0.7
z.
c 15
1.x
i: 13
.0
166.
3 .?
c 16
.7
211.
9 +
26.7
242.
5 +
17.8
30
5.8
i_ 1
4.6
39.8
j
18.1
I3
,7)
(217
) 24
1.4
222.
1 17
0.1-
306.
1 19
0.5-
251.
7 (2
14)
(214
) 19
4.9
235.
1 16
3.3.
22
4.5
210.
9 25
8.5
NO
‘7
11
‘-t
17.6
_,
_.
334.
3 &
18.
4
(1.4
1 18
.7
14.6
- 14
3.9
186.
3 2
16.3
25
11
* 17
.1
‘l N
O
N 0
NO
N
ot
obsc
r~ed
. f’
ract
ions
in
pa
rcnt
hexs
pr
tqw
rtio
n of
an
imal
s in
gr
oup
show
ing
resp
onse
s in
dica
ted.
~ammarids adapt to pressure in Lake Baikal 113
old pressure for Stage IV at a compression rate of I, 8 or 10 atm/min was 86.2 & 5.3; at 20atm/min, 71.3 f 1.4; at SOatm/min, it was 74.3 f 4.5, and at 160 atmimin, it was 95.0 _t 9.3 atm. The differences between these values are not statistically significant (P > 0.2 for all combinations). Linear regression of convulsion threshold pressure (P,) on compression rate (P) is represented by the equation (P, - 88) = 0.11 ,(P - 63.5) with a correlation coefficient of 0.30. The regression coefficient, 0.11, does not differ from 0 (P > 0.90).
We con&de that compression rate changes within the range tested do not affect the convulsion thresh- old pressures of gammarids to any recognizable extent. The compression rate chosen for subsequent work (13.5 atm/min) was selected to permit optimum opportunity for observation without running the risk of significant oxygen depletion of the system during the time of the experiment.
excellent. Most species appeared to survive for at least 6 days under the conditions established. The possibi- lity was nonetheless considered that either the trauma of capture or deterioration during survival at low pressure might modify the compression response of the animals. Pooling all animals of species D-l---D-5, it was possible to explore the effect of tests conducted as early as 1 hr after capture, and as bdte as 130 hr. The results (Fig. 1) are represented by the regression equation (P, = 169.5) = -O.l2(t, - 40.5). with a correlation coefficient of 0.20. The slope, -0.12, does not differ significantly from 0 (P = 0.40).
We conclude that within the limits tested, convul- sion threshold pressures (Stage IV) in Baikalian abys- sal gammarids do not vary with the time of sojourn at 1 atm.
Threshold pressures for key stages of the compression .Qyndrome in gam~rids
~e~~er~ir~re. The effect of compression tempera- ture (T) was explored in preliminary studies with
Sum~ry of t~r~sha~d pressures for all stages. The
species S-l over a temperature range from 8 to 23°C response patterns of the various gammarid species tested are described in terms of the mean threshold
using animals acclimatized to 15°C. The regression of P, on 7” tias represented by the equation
pressure for the general stages described above. These
(P, - 86.1) = 1.15.(T - 12.7) with a correlation coef- values are shown in Table 2, the several species being
ficient of 0.06. The regression coefficient, 1.15, was arranged as in Table 1 in the order of the depth at
found not to differ signi~cantly from 0 (P > 0.2). which the specimens tested were collected. It should
While most experiments with Baikalian specimens be observed that not all of the stages listed provide
were performed at or close to habitat temperature, equaily satisfactory endpoints for comparison: In-
practical exigencies compelled us to conduct eleven deed, not all stages were observed in all species. Gaps
compressions at higher temperatures (including two are especially noticeable for responses of Phase A,
at 14°C and one at 18”Q permitting a regression the hyperactivity phase: Stage I is absent in $ of the
analysis for P, and T also for the group made up shallow and 2 of the deep species; Stage II is incom-
by species D-l-D-5. The regression equation is pletely represented or absent in 4 of the shallow water
(P, - 169.5) = 1.81 (T - 6.48), with a correlation forms and f of the deep water forms; and Stage III
coefficient of 0.22. The slope, as in the previous series, is general for only one species-a deep water one. There is no recognizable relation of the distribution
is not significantly different from 0 (P = 0.2). Taken together, the data fail to indicate any signifi-
of any of the voids in Table 2 with depth of capture
cant dependence of P, in gammarids on compression in the Baikalian species. In the North American fresh- water gammarid, all three stages of the hyperactivity
temperature in the range tested. Nonetheless, com- pression temperatures were held between 4 and 8’C
Phase A (I, I1 and III) were absent, or so poorly
for all but 3 of the tests on deep water species expressed as not to be identifiable with confidence.
D-l-D-5, and between 6 and 15°C for the shallow By contrast, Stages IV, VIII and IX were recognized in all experiments and hence will be made the basis
water species S-l-S-5. for further analysis of the data. Time ufter capture. The survival of Baikalian deep
water gammarids in our aquaria was on the whole Comparison of shallow and deep water gammarids.
Perusal of the threshold pressures for Stage IV-the convulsion threshold-suggests a separation of species into two groups, one with mean convulsion threshold pressures between 85 and 90 atm, and com- prising all species designated as S, and a second group with mean convulsion threshold pressures between 150 and 190 atm, and comprising all species desig- nated as D. Reference to Column 2 of Table 2 shows that this division corresponds to the division in terms of capture depth, the shallow species having the low convulsion thresholds and the deep species the higher ones. This division is not reflected in Stages VIII and IX for each of which the ranges of mean threshold pressures for shallow and deep water species coincide completely.
- I
0 ‘lo 80 izo 60
TIME AFTER CAPTURE (h)
Fig. 1. Convulsion threshold pressures, P,, (Stage IV) for Baikalian gammarids measured at various times after cap- ture. The regression equation of P, on T after capture (tp,) is (P, - 169.5) = -0.12~(tp~ - 40.3, with r = 0.20, and t (for h = 0) = 0.87, corresponding to P = 0.40 for
the null hypothesis.
Figure 2 compares the frequency distributions of threshold pressures for Stages IV, VIII and IX for all representations of each of the two groups of Baika- lian gammarids. Mean values for threshold pressures in atm for each group, for Stages IV, VIII and IX, respectively, are shown in Table 3.
c R,P, 65',A--H
PffESSlJRE (ATA:
Fig. 2 Frequency distribution iix rhresl~old pressures for
Stages IV. VIII and IX for shall~~\v water tspecies S-2-S-5) and deep water (qwies II-I D-5) speciv~ of Raikalian
gammarid\
Before attempting any further analysis of these results, a matrix analysis was performed for the threshold pressures for Stages IV. VIII and IX to determine whether significant species differences could be detected within either of the two subgroups S and D. Despite the small numbers of specimens available for several of the species, the results of this analvsis suggest that within each group at least two species show differences which could be considered statistically probable (P < 0.05): Among the shallow water species, this is true for Prrllrrsucl c~ntmdlarin my. and Acat~rhopmn~~mo ulhus (Stage IX); for the deep water forms. the same applies to O~~~lnt,,ycmlmLtru.s mrrrycrrirurrus and 0mttwto~umtturrn.s dhitzus (Stage IV and IX). Thus, although the data do not permit any generalizations with respect to in-group species differ- ences. they indicate that neither of the two sub-popu- lations can be treated as internally homogeneous. Figure 2 suggests that with respect to convulsion threshold pressures, each of the two populations shows a well defined central tendency so that it is meaningful to compute group means as in Table 3. Even this procedure appears questionable for the stages of immobilization (Fig. 2h and c). the data for which suggest more than a single modal value in each case .
Further analysis of these results w/as accordingly undertaken by methods which do not assume a nor- mal frequency distrib~ltion but analyze rank order of experimentally determined values for each group in pooled samples of all values in terms of a model assuming random distribution. The results of this analysis in terms of Wilcoxson’s statistic indicate that in all three cases illustrated in Fig. 7, the distribution of ranks differs from a random distrib~~tion with a high degree of probability P < 0.001 for Stage IV, P < 0.01 for Stages VIII and IX. Thus. the percentile differences between the general mean threshold pres- sures computed above do appear to reflect real differ- ences between the responses to compression of deep water and shallow water species. It should be noted that by far the greatest and the most clear cut differ- ence is shown by the mean convulsion threshold pres- sures, the deep water species tolerating nearly twice as high pressures as shallow water species (88’5; differ- ence). By contrast, group mean threshold pressures for the two stages of immobilization differ by only 9 and 19”,, respectively. from one another.
~‘~~~ir~~~~~,~~)~~ ‘!f ,~ltr.fh ~l?Wic’ii!l (~~JJJfJJ?~J~~~~ With
~~Jj~~J~;fJJ7 ~~ZfJ~~~J~~’ d~UtCJ’ ~;~JJ?ZJJJ~J~j~~.~. ,A hii PBiJlt
worth noting is that the mean v;tlu~’ of convulsion threshold pressures l’or all Baikahan shallow water gammarids, 89.1 f 3.1 atm. is substantially identical with that for the North American freshwater gam- marid, 86.1 + 3.9 atm. Comparison of threshold pres- sures for Stages VIII and IX, on the other hand. shows that immobilizat~(~il in C r~~~~~~(~~i~,.~ sp. occurs at substantially higher pressures than in the shallow water Baikalian gammarids 247 + 8 vs 203 5 7 loi Stage VIII and 320 i 8 vs 259 -t 7 for Stage IX. Together with the absence of any observable stages of the hyperactivity phase (I, II or 111) in Crwgon~.u sp., this suggests that details of the compression syn- drome may vary signi~cantly between the North American and the Baikalian gammarids, while susccp- tjbility to the convulsion stage is regulated within closer tolerances and appears identical in all shallow water forms tested.
Baikalian amphipods can be separated into two major groups according to differences in both. their internal osmotic pressure and their osmoregulatory ability (Basikalova et ~1.. 1946a,b). One group con- tains more conservative forms, such as ~I~~~~FJJz~)~~J~JI-
mums and Odotzrogc2mnurnts which are rn~~rph~~l~~gi- tally most closely related to the widespread marine and fresh water genera Grmtnaru.~ and Mtrrinoymw tmrus. These animals have a relatively high internal osmotic pressure and have retained the ability to osmoregulate within certain limits of surrounding salt concentration, In this respect. these forms are similar to marine euryh~iline ~~rnphip~lds and are felt to hc genetically most closely allied to them.
The second group contains genera such as Prrrctptri- I~~setr and A~~~nlhogtrrFlnlr~rl~\. which are thought to have adapted to the specific conditions of Lake Baikal and which are morphologically unique. These animals have a characteristically low internal osmotic pres- sure. and have lost their ~~sm~)re~ul~ltory ability. Whiie, in general. the abyssal gammarid fauna ol Lake Baikal is composed of large numbers ol species from the more specialized second group, complete segregation of these groups along lines of vertical dis- tribution does not exist. with both shallow and abys- sal fauna containing species representative of both groups.
Against that background, it is of interest to note that in terms of response to compression, the abyssal gammarids of Lake Baikal are seen to form a group which is clearly distinct from the shallow w’ater gam- marids. No further distinction between groupmgx.
Gammarids adapt to pressure in Lake Baikal 115
such as suggested by the results of Basikalova et al. (1946a,b), is possible on the basis of our criteria using the present data for the deep water gammarids. The shallow water gammarids appear to form an even more homogeneous group on the basis of their mean or individual convulsion thresholds, and the limited data available for Eulimnogammarus fail to distinguish this group even from the Acanthogammarus. These results thus suggest that pressure prevailing in the habitat, rather than taxonomic position or osmotic make-up, is correlated with the pattern of response to pressure in Baikalian gammarids. The present data do not suffice to determine whether within the abyssal group a further subdivision according to depth of residence is possible or whether the distinction between “deep” and “shallow” corresponds to a more or less sharply defined boundary depth beyond which all species show comparable degrees of pressure toler- ance. In marine crustaceans, the limited data available do suggest some correlation of pressure tolerance with depth of capture, but the increase in convulsion threshold appears to progress considerably slower than the capture depth (MacDonald, personal com- munication). No data are available as yet concerning compression tolerances of representatives of any species of relatively eurybathial amphipods from opposite ends of their vertical distribution range.
Before assessing the implications of these findings, it is necessary to examine in greater detail the signifi- cance of the several response phases described in the present communication. Most clearly understood among these would appear to be the convulsion Phase B. Both in vertebrates (Brauer, 1975) and in inverte- brates (Kendig, 1978; Spyropoulos, 1957, as com- pared to Campenot, 1975) there is evidence to support the view that this type of compression-induced behav- ior reflects changes in central nervous system activity. In vertebrates, the precise nature of these changes remains to be defined, but there is reason to presume that here the critical event reflects properties of the central neuropile rather than a catastrophic change in any single neuronal element (Brauer, 1975). In in- vertebrates and, in particular, among crustaceans, on the other hand, there is evidence that compression elicits repetitive discharges in giant synapses (Kendig, 1978; Spyropoulos, 1957) changes which could well account for the “whole body” flexing and “kicking” described above as the dominant pathological forms of locomotion corresponding to the behavior we are calling “convulsions.”
On this view, it seems plausible to hypothesize that the generalized catastrophic change in central nervous system function should be preceded by changes in central nervous system function of graded intensity, but below the critical level. Such changes would be in line with the graded character of the changes observed in vitro (Kendig, 1978; Spyropoulos, 1957) and would provide a plausible neurologic basis for the behavioral changes of Phase A, characterized by exaggerated activity which, however, is made up of locomotor components all of which also characterize the normal behavior of gammarids at 1 atm. The be- havioral significance of this stage, or of its partial or complete absence in various species, remains to be examined. It is conceivable that at this level compres- sion is perceived as an aversive stimulus. Preliminary
data suggest, among other things, that pressures in this range disrupt normal selection behavior in a tem- perature gradient (Kinney & Brauer, unpublished data).
Phase C, the phase of progressive immobilization, is less readily interpreted. Pressures in this range do appear capable of interfering not only with central nervous system function, but also with the functioning of contractile tissues (Arronet & Konstantinova, 1969; MacDonald, 1975). In the absence of specific experiments, one may conjecture that the ultimate im- mobilization of Stage IX may represent this type of action. Even here, however, one cannot yet dis- tinguish between a possible effect on myosin- actomyosin interaction (cf. Guthe, 1969; Josephs & Harrington, 1968) and an equally plausible effect on parts of the triggering mechanism which sets off this molecular sequence, such as the release of Ca’+ from the intracellular reticulum (Sandor, 1965 and e.g. Ponat, 1967).
Perusal of Table 2 shows that of the three phases, only Phase B shows what appear to be ecologically meaningful threshold pressures: Phase A endpoints are sporadic in occurrence and show only vaguely discernible correlation with habitat depth-the ear- liest manifestations rather consistently tend to appear at lower pressures in the shallow water gammarids than in the abyssal forms.
Phase C threshold pressures, and especially those for Stages VIII and IX, correspond to depths greatly in excess of any encountered in Lake Baikal and, hence, could hardly be of ecologic significance. This inference is borne out by the small differences between shallow and deep water species-197; and 9% for Stages VIII and IX, respectively-as well as by the lack of a central tendency and the wide range of variation of observed threshold values for these stages (Fig. 2).
By contrast, convulsion threshold pressures for the deep group of gammarids all fall at or close to that corresponding to the maximum depth of Lake Baikal and, indeed, their mean value of 168 atm is almost identical to that prevailing at the recorded maximum depth of the lake (162 atm) Kozhov, 1972).
One might question the ecological significance of observed convulsion pressures in the shallow water gammarids. Most of these forms do not ever penetrate to the depth of almost 9OOm, which corresponds to the mean convulsion threshold for this group. In answer, it is appropriate to recall that the convulsion thresholds here observed also correspond closely to values shown by many vertebrate (Brauer, 1975) as well as other shallow water invertebrate (MacDonald & Teal, 1975) forms. In vitro observations on crusta- cean axons and synapses (Kendig, 1978; Spyropoulos, 1957), likewise, indicate threshold pressures for repeti- tive firing of the same order of magnitude. Thus, the threshold pressure for convulsions in shallow water gammarids appears to reflect inherent pressure resist- ance of normal 1 atm acclimatized nervous tissue rather than reflecting any adaptive needs.
The depth corresponding to this threshold for shal- low water gammarids, however, is substantially less than the depths at which our various deep water gam- marid species were captured. Thus, an increased con- vulsion threshold pressure would appear to represent
a meaningful and indeed necessary adaptation for this latter group. In accordance with this interpretation. the experimental data summarized in Table 2 and 3 show a large and consistent difference in convulsion thresholds between deep and shallow water gam- marids. the former tolerating almost YO”,, greater pressures than the latter. The well defined central tendency and relatively modest spread of values char- acterizing convulsion threshold pressures in deep water species, likewise. are in accord with the view that. in contrast to Phase C responses. tolerance to convulsion inducing pressures is ;I criterion actively established or actively maintained in Baikalian deep water gammarids.
Reviewing the implications of the results obtained in the present study. it becomes clear that while they present a basis for much further investigation. they as yet leave many of the key questions unresolved. They do establish&as the comparable data for mar- ine forms could not (because t’, here is usually much greater than the capture deptht that adaptation to high pressure regimes is an essential characteristic of deep water Baikalian gammarids. They identify one
particular phase of the comples sequence of compres- sion elicited responses as the one most clearly in- volved. They make it seem highly probable that adap- tation in this sense involves changes in the properties of the central nervous system. And the) suggest that this is not a matter of generic differences, but rather a common characteristic shared to approximately the same degree by all gammarids inhabiting the deep waters of Lake Baikal. regardless of evolutionary his- tory or osmotic properties of tissue fluids.
On the other hand. theq do not resolve the question of the degree to which this propert) might vary within a given organism as a result of its individual history, or the degree to which pressure !olerance could be varied by prolonged exposure to various depths. They do not resolve the question t(l what extent the degree of adaptability to varied pressures is a genetically determined characteristic. And finally. they do not resolve the question of the price ;I given individual or a given species might pay for adaptation to a high pressure regime. Thus. the observation that deep water gammarids brought to the surface often appear to remain relatively sluggish compared to shallow water forms may reflect the inverse of the hyperexcita- bility stage in shallow water forms brought to 40 or 50atm. but equally well. it may reflect merely differ- ences in normal behavior between abyssal and shallow water gammarids (compare MacDonald & Gilchrist. 197X and Yayanos. 19%. for comparable phenomena in marine gammarids). Thus. the ecologi- cal and evolutionary significance of pressure as a selection factor must remain unresolved at present.
The results obtained here do. however, permit cast- ing of all of these questions in forms amenable to experimental test and, hence. provide a promising basis for future work which can hope. once and for all, to clarify the role of pressure as an environmental factor influencing the evolution of species inhabiting the upper 3000 or so meters of the hydrosphere.
It ma) not be amiss to recall that this limitation to 3000m is not fortuitous. Somewhere between 2500 and 3500 m would appear to be the boundary beyond which live retrieval of crustaceans without pressure
protection has so far procsd ~mp,~s~ble. Bc\ OIIU
200 atm. we already know that muscle t’unctlor becomes impaired (MacDonald. 19751: that cell &\I- sion is affected (Zimmerman, 1970) and that. In short changes other than those in central nervous \h\trn: function become increasingl! prominent. Thu\. Ctclap- tation in the sense here explored uould appear I\‘)
become inadequate when one ib dealing with those species living beyond the 2500 i5M m houndar>. :Inci other methods. and other question\ are Ilkel! 10 become dominant in dealing with this. more trill\ deep sea fauna.
A final word may be in order ulth regard IL) the relation born by the observations here recorded to high pressure responses reported bq other\ lor marine amphipods (George & Marum, lY74: MacDonalJ X: Gilchrist, 197X; Menzies CI cl/.. lY72). Both. the sta+ we designate as the “convulsion phase” and :he
sequence of events we characterize as the “immobill-
zation phase” do appear to be held m common h!
all of these forms while the hyperexcitabilitj phahe seems the most variable both from species to ,pecieb
and. perhaps. in terms of the extent to u-hicll 11 ~;m
be modified by varying compression conditlvns. SIILY
detailed descriptions of the successl\c evolution 01’ hc-
havioral responses to pressure, comparable t,, those
summarized in Table 2. arc not yet ;~vailable 11~ mar-
ine gammarids, a more precise comparlsau betuec,n marine and Baikalian forms can hardly be mean;ngful
at this point. Quantification in terms of ~IIC‘II p;lr- ameters as pleopod beat frequent! or oxygen CXIII-
sumption rate. as achieved by se\er;il worker\ 1;~
marine forms during compresslon. ~\hilc intcre\tlng
seems to us to be conditioned by beha\loral ,I$ ~zeil
as. perhaps. by biochemical factors and. thuh. aid ic>
the complexities of description ulthnut clar~l’~~n~ either biochemical or physiologic mcchanlsm ;It pl,~~, These seem to us to call for work .It the IIWIL~ cv
cellular level and can onl! bc attacked with hope 01
success after the basic facts of adaptation or ,1cclim.l- tization of the several species ha\u become fully defined and suitable capture methods dciclopcd to wholly eliminate the possibilit! th;lt xomr ol the’ changes observed are artifacts due I,: tr,tuma $11’ w-
trieval.
.-1(,~norc,/ur/geme,lts~ This v,orl\ \~as \upported ,,I parr b> grants from The National Geographic Society and from The Griffis Foundation.
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