Marine Biology 33, 57-66 (1975)
? by Springer-Verlag 1975
Invertebrates of the Upper Chamber, Gatun Locks,
Panama Canal, with Emphasis on Trochospongilla leidii (Porifera)
M. L. Jones and K. Ri.itzler
Department of Invertebrate Zoology, National Museum of Natural History, Smithsonian Institution; Washington, D. C., USA
Abstract
Observations were made on the horizontal distribution of certain invertebrates on
the walls and floor of the upper chamber of the Gatun Lock system of the Panama
Canal. The hydroid Cordylophora caspia (Pallas) and the gastropod Neri tina usnea Roding
extended the full length of the chamber; the oligochaete Marionina sp. was confined
to the upper half of the chamber (adjacent to Gatun Lake), and the amphipod Gitanop?
sis tortugae? Shoemaker, the isopod Munna sp. and the bivalve Mytilopsis sallei (Recluz)
to the lower half; the sponge Trochospongilla leidii (Bowerbank) was distributed from
the upper end to near the lower end of the chamber. A peculiar wedge-shaped distri ?
bution of T. leidii is thought to be a response to the influence of slight increases
in salini ty. Morphological observations on T. leidii are presented.
Introduction
During the past 60 years since the open?
ing of the Panama Canal there has been
but one published survey of the fauna
and flora of the lock chambers. Hilde?
brand (1939), although concerned mainly
with the fish fauna and the role of the
canal in the transisthmian migration of
this fauna, observed the distribution of
certain invertebrate animals in the var?
ious chambers of the lock systems of the
canal. Hildebrand's observations were
carried out during periods of "dewatering"
of the lock chambers for routine mainte?
nance and modification of the chambers.
In March 1974 one of us (M.L.J.) was
afforded a similar opportunity to col?
lect biological specimens during the
dewatering of all three of the east
chambers of the Gatun Locks. For the
most part, these collections are still
being processed, but observations on the
distribution of the sponge Trochospongilla
leidii (Bowerbank) have proven to be of
more than passing interest and have
prompted the present paper.
Trochospongilla leidii was described by
Bowerbank (1863) as Spongilla leidii from
the Schuylkill River (Pennsylvania, USA).
The synonymy and a redescription, based
on the type and on additional material
from the type locality, were given by
Penney and Racek (1968).
Numerous fragments scraped from four
sites of the highest Gatun Lock chamber
are deposited in the National Museum of
Natural History, Smithsonian Institution,
under Catalog No. USNM 24373.
Materials and Methods
As mentioned elsewhere (Jones and Dawson,
1973), chambers in the Panama Canal lock
system are about 1000 ft (304.8 m) long,
110 ft (33.5 m) wide, and can accommo?
date vessels drawing 40 ft (12.4 m), in
seawater. The three chambers of the
Gatun Locks provide a descent of about
85 ft (25.9 m) from Gatun Lake, directly
to the Caribbean Atlantic through Limon
Bay. Lockage water for each of the three
28-foot (8.6 m) lowerings of the three
chambers comes from Gatun Lake, either
directly or from the next higher chamber.
The previous dewatering of the east
chambers of the Gatun Locks was carried
out in January, 1968; at this time the
chambers were dry for about 10 days, and,
as is usual, neither the walls nor the
floors of the chambers were scraped.
Thus, the epifauna of the chambers was
recruited some time in the previous 6
years.
An attempt to accompany biological
collections with salinity and tempera?
ture observations in all of the chambers
58 M.L. Jones and K. Rutzler: Invertebrates of the Upper Gatlin Locks
of the Gatun Locks, was abortive due to
the loss of the sensor cell. However,
before this loss, salinity and tempera?
ture records were made at 2-m intervals
at the two ends of the upper chamber and
at two stations outside the lock system
in Gatun Lake; these 4 stations represent
re-occupations of the sites of Jones and
Dawson (1973) shown in their Figs. 2A-D
and 4A-D. Surface and bottom salinities
in April, 1972 were both a.1~ at the
upper end of the upper chamber and a.1
and a.3~, respectively, at the lower end;
November, 1972 observations were a.2~ at
both surface and bottom at the upper end,
and a.3~ at the surface and a.4~ at the
bottom at the lower end. As before, the
present observations were made with a .
Beckman Model RS5-3 portable salinometer,
which also records temperature; all
readings were rounded to the nearest
tenth and factory specifications of ac?
curacy were ~ a.3~ and ~ a.50 C.
The original intent was to take a
series of quantitative samples from both
the chamber walls and floors, but the
rough texture of these surfaces made
this impossible. Thus, qualitative sam?
ples of the biota were taken from the
walls at a level of about 6 ft (2 m)
above the floors and from the floors, at
each of 5 sites along the length of each
of the three chambers, as well as on the
Limon Bay-side of the lower gates of the
lowest chamber. In the upper chamber the
5 collection sites, A-E, were at 13, 34,
56, 76, and 93% of the chamber length
from the Gatun Lake end of the chamber
(Fig. 1).
Methods used for light and scanning
electron microscopy of Trochospongilla
Middle Chamber/Atlantic Ocean
I
UPPER CHAMBER, GATON LOCKS Gatun Lake
,
100% 90 80 70 60 50 40 30
I I I I I I I I
A .*
B ?
C ? ? ?
D
E ? ? ?
F
G ?
H
I
20 10
I I
?
?
?
CDE 90
0+--,--.------.----,---.-=:=..L.::c-.-4-~"r_:..:;;;;~-.,.....:.:...::::.:r;_.......;:.::..="'-'---'-,.....:.==
100%
~O[] ~OID~ salinity j Itemperature ~
?-10 ?_10
15 Of-I%_._o.:..1__3o_0_c ----'r- ---1\O- ,L.-+-~1t_---0-1-%.-0-~_---=3!....O'+C
!~ 10
..c
..
?cu 5
::c
Fig. 1. Distributional patterns of certain invertebrates in east upper chamber, Gatlin Locks, Panama
Canal. At bottom is a diagram showing distribution of Trochospongilla leidii (T) and a presumed blue?
green alga (BG) on eastern chamber wall. Also shown are locations of the 5 collection sites (A-E).
At top are horizontal distributions of 9 invertebrates along length of the chamber (A: T. leidii; B:
Cordylophora caspia; C: Neritina usnea; D: Marionina sp.; E: Paraoroides? sp.; F: Tanais stanfordi;
G: Gitanopsis tortugae?; H: Munna sp.; I: Mytilopsis sallei; dashed lines show distribution confirmed
by J. Rosewater). Two sets of salinity/temperature profiles are included, based upon observations
made just prior to draining of the water from upper chamber; arrows indicate location of these ob?
servations. Percentages of chamber length (top and bottom) are based upon usable length of the cham?
ber; these progress from Gatlin Lake end (right) to middle chamber end (left). Star: Trochospongilla
leidii was present in wall samples but not in floor samples at Collecting Site D; S: fresh-water
seeps
M.L. Jones and K. Rutzler: Invertebrates of the Upper Gatun Locks 59
leidii are those described by Rlitzler
(1974).
Observations
Only two species of invertebrate animals
were abundant in the highest chamber of
the Gatun Locks. The hydroid Cordylophora
caspia (Pallas) was found throughout the
length of the chamber, both on the cham?
ber walls and on the floor. Trochospongilla
leidii (Bowerbank) on the other hand,
nearly covered the walls and floor of
the upper (Gatun Lake) end (Fig. 3d),
and extended vertically to near the low
operational water level, i.e., the water
level of the upper chamber which allows
ship-movement between the upper and the
middle chambers, about 40 ft (12.2 m)
above the chamber floor (Fig. 2: LOWL).
The area between the upper distributional
boundary of the sponge and the low opera?
tional water level (about 2.4 m) was oc?
cupied by a dark band, presumably a blue?
green alga (Fig. 1 and 2, BG). The den?
si ty of T. leidii on the chamber walls
continued unchanged for about 63% of the
chamber length, where the lower distri ?
butional boundary departed from the cham?
ber floor (Fig. 3c). At approximately
73% of the chamber length, it was some
7 ft (2.1 m) above the floor (Fig. 3b),
and by 85% of the length, it was about
14 ft (4.3 m) above the floor (Fig. 3a).
The rather complete covering of the wall
by colonies of T. leidii commenced breaking
up at about 82% of the chamber length,
and, at 87%, gave way to isolated patches
about 25 ft (7.6 m) above the floor. The
patches then became even less numerous
until, at the lower gate of the upper
chamber, only a very few, small, isolated
colonies were observed (Figs. 1 and 2).
T. leidii was not present in the middle
or lower chambers of the Gatun Lock sys?
tem, while C. caspia was present through?
out the length of the middle chamber, but
was absent from the lower chamber.
Maximum area coverage by individual
colonies of Trochospongilla leidii is dif?
ficult to establish, but might have ap?
proached 300 to 400 cm2 . The total popu?
lation covered an area of more than
2400 m2 rather densely on the west cham?
ber wall (Fig. 2, T). Thickness of the
incrustations varied from 0.5 to 2.0 mm.
A number of other invertebrates were
also noted in the upper chamber (Fig. 1).
Neritilla usnea Roding, a gastropod, was
present throughout the length of the
chamber (J. Rosewater, personal com-
Fig. 2. View of eastern wall of east upper chamber, GatUn Locks. from lower end, toward GatUn Lake
end. Letters A-E indicate locations of collecting sites; LOWL: low operational water level; BG: band
of presumed blue-green alga; T: dried, white colonies of Trochospongilla leidii. Rungs of ladder at
left are 0.3 m apart
60 M.L. Jones and K. Rutzler: Invertebrates of the Upper GatUn Locks
Fig. 3. Direct views of eastern wall of east upper chamber, GatUn Locks, showing changes of density
and distribution of dried, white colonies of Trochospongilla leidii (al at about 87% of chamber lengtt
from lake end; (bl at about 80%; (cl at about 76%; (dl at about 53%. Low operational water level is at
upper margin of all 4 photographs; C and D: Collection Sites C and D (see Fig. 2l. Photographs taken
from top of western wall. Rungs of ladder in Fig. (cl are 0.3 m apart, and this scale applies to (al?
(dl. S: Fresh-water seeps
munication) although it was present in
only 4 of our collections. Marionina sp. ,
an enchytraeid oligochaete, was confined
to the upper half of the chamber, while
Gi tanopsis tortugae? Shoemaker, an amphi 10?
chid amphipod, and Munna sp., an asellote
isopod, were confined to the lower half.
A distribution similar to those of the
latter 2 species was also exhibited by
Mytilopsis sallei (Recluz), a bivalve, ac?
cording to Rosewater (personal communi?
cation), although it was present only in
the lowest two of our collections. Fi?
nally, so few specimens of Tanais stanfordi
Richardson, a tanaid, and Paraoroides? sp.,
an aorid amphipod, were obtained that
little can be said of their distribution
in the chamber. No living large crusta?
ceans were collected, although the cara?
pace of a decapod (Callinectes sp.) was
observed; in addition, some dead shells
of the bivalve Brachidontes exustus Linne
were also seen.
Observed temperatures (Fig. 1) varied
from 26.90 C at the surface to 27.00 c at
the bottom at the upper end of the cham?
ber, and from 26.7 0 C at the surface to
27.00 C at the bottom at the lower end.
Salinities varied from O.02~ at the sur-
face to O.3~ at the bottom at the upper
end of the chamber, and from O.2~ at the
surface to O.8~ at the bottom at the
lower end. These readings were taken
when the water in the chamber was at the
low operational level. The observations
in Gatun Lake, just outside the upper
chamber, indicated homogeneous salinities
in the water columns (O.2~ from surface
to bottom) and rather uniform tempera?
tures (from 27.20 C at the surface to
27.00 C at the bottom).
It was mentioned above that in the
upper part of the upper chamber Trocho?
spongilla leidii extended from the chamber
floor to near the low operational water
level. A number of exceptions to this
general upper boundary were noted coin?
ciding with fresh-water seeps in the
upper wall face from the culverts which
carry water from Gatun Lake along the
sides of the three lock chambers (Fig.
1, S).
In August, 1974, several cursory at?
tempts were made to determine the extent
of intrusion of Trochospongilla leidii into
Gatun Lake. Scrapings from the walls of
the approach to the Gatun Locks failed
to yield T. leidii, due, no doubt, to an
M.L. Jones and K. Rutzler: Invertebrates of the Upper Gatun Locks 61
inadequate sampler. Shore collecting
from submerged rocks and pieces of wood
near Gamboa (approximately 35 km from
the Gatun Locks or about 73% of the chan?
nel length of Gatlin Lake toward the Pedro
Miguel Locks, near the Pacific end of the
canal) failed to show the presence of the
sponge, but did provide numerous speci?
mens of Tanais stanfordi, a single colony
of Cordylophora caspia, and a colony of the
fresh-water bryozoan, Plumatella emarginata
Allman. Finally, scrapings from the walls
of the approach to the Pedro Miguel Locks
also failed to yield Trochospongilla leidii.
In February, 1975, the east chamber
of the Pedro Miguel Locks was drained
for routine maintenance. At this time a
series of 5 pairs of samples were made
(by M.L.J.) along the length of the cham?
ber, similar to those made in the Gatlin
Locks. Trochospongilla leidii was present in
the same abundance as in the upper part
of the upper chamber of the Gatlin Locks;
however, in contrast to the observations
recorded above, there was no vertical
diminution of the sponge anywhere along
the whole length of the Pedro Miguel
chamber. Further, the wall area between
a caisson which acted as a temporary dam
between Mirflores Lake, at the seaward
end of the Pedro Miguel Lock chamber,
and the gates at the lower end of the
chamber, was likewise covered with T. lei ?
dii. This, then, extends the distribution
of the sponge into Miraflores Lake, with
no indication of a wedge-shaped termina?
tion of distribution similar to that
observed within and at the lower end of
the upper chamber of the Gatlin Locks. -
Trochospongilla leidii
The color of the fresh sponge (observed
after several hours exposure to air) was
grayish to drab, and has not changed in
alcohol. The type material was described
as yellow, with a tint of green (Bower?
bank, 1863). It should be noted, however,
that the cellular material of our speci?
mens is reduced, most likely due to
being exposed to air after dewatering of
the lock chamber. For this reason no
data on ectosomal structures can be giv?
en. The consistency of the sponge is
firm but brittle.
The skeleton consists of plurispicular
strands of oxea oriented perpendicularly
to the surface; these are interconnected
by single or fasciculated spicules to
form a firm three-dimensional network.
The ends of the spicules are held to?
gether by small amounts of spongin. There
are no somal microscleres.
The gemmules (Fig. 4) are confined to
a single layer at the base of the sponge.
They are encased in a capsule of densely
packed oxea and connected to a solid
sheet of oxea. These capsular oxea are
slightly more stout and blunt than those
of the main skeleton, but are otherwise
identical. They also are cemented by
spongin. The foramina (Fig. 4a, b), are
directed toward the sponge surface and
are elevated on a conical mound about
30 to 40 urn high. The diameter of the
gemmules ranges from 380 to 520 urn. Their
walls (Fig. 4c, d) are about 16 urn thick,
and consist of an inner solid, striated
layer of spongin (6 urn) and an outer
pneumatic layer (10 urn). The perpendic?
ularly oriented gemmoscleres are em?
bedded with their lower halves in the
pneumatic layer. The upper portions of
the birotules are freely protruding. The
disks of the birotules usually overlap
those of the adjacent spicules, thus
greatly improving mechanical resistance.
There is only a single layer of gemmo?
scleres. In sponge fragments with a less?
er degree of tissue degradation many
developing stages of gemmulae were found.
The spicules of the main skeleton are
moderately curved, stout, more or less
fusiform oxea (Figs. Sa, 6a), with rath?
er abruptly tapered points. Their sur?
face appears smooth under the light mi?
croscope (Fig. Sa); however, the scanning
electron microscope (SEM) reveals that
the oxea are densely covered by minute
spines, about 0.1 urn tall (Fig. 6a). The
oxea of the gemmular capsule (Figs. 5b,
6b, c) are very similar, but tend to be
shorter, stouter and blunter. They also
tend to malformations, i.e., two or more
sharp bends, T-shapes, central annular
swellings and mammiform tips; they are
provided with a slightly coarser spina?
tion (0.2 to 0.3 urn) than those of the
main skeleton. The gemmoscleres (Figs. 5c,
d, 6d) are birotules with a short, thick,
smooth shaft which generally protrudes
beyond the outer surfaces of the rotules
and forms low mounds or spikes. Both
rotules have entire margins which are
exflected in the same direction, i.e.,
away from the surface of the gernrnula.
The lower rotule, embedded in the pneu?
matic layer is slightly (about 4%) larger
than the upper rotule.
The following sizes of spicules were
computed from 25 random measurements- of
each dimension. Ranges are given with
means ~ 1 standard deviation in paren?
theses. Oxea of the main skeleton (length
x width): 125.0 to 175.0 (149.6 ~ 10.8)
x 10.0 to 13.8 (12.6 ~ 0.8) urn. Oxea of
the gemmular capsule (length x width):
105.0 to 155.0 (141.9 ~ 14.2) x 11.3 to
16.3 (13.4 ~ 1.3) urn. Birotules, total
length: 11.5 to 13.2 (12.6 ~ 0.6) urn;
width of shaft: 4.2 to 6.9 (5.8 ? 0.8) urn;
62 M.L. Jones and K. Rutzler: Invertebrates of the Upper Gatun Locks
Fig. 4. Trochospongilla leidii. Gemmules. (a) Capsule of oxea (scanning electron microscope; 120 x);
(b) foramen and arrangement of birotules (SEM; 500 x); (c) light microscope se~tion through gemmular
wall showing embedded birotules and oxea of capsule (toluidine-blue stain; 500 x); (d) fractured
gemmular wall showing striated spongin and pneumatic layer with embedded birotules (SEM; )DQQ x)
M.L. Jones and K. Rutzler: Invertebrates of the Upper Gatlin Locks 63
b
c ..... ~_ ___l
d
Fig. 5. Trochospongilla leidii. Spicules; light microscope. (a) Oxea of main skeleton (600 x); (b)
oxea of gemmular capsule (600 x); (c) (d) birotules (1300 x)
Fig. 6. Trochospongilla leidii. Spicules; scanning electron microscope. (a) Oxeon of main skeleton
(800 x); (b) oxeon of gemmular capsule (800 x); (c) oxeon of gemmular capsule, tip and spination
(3300 x); (d) birotules (3300 x)
64 M.L. Jones and K. Rutzler: Invertebrates of the Upper Gatun Locks
Discussion
tral America, but has not as yet been
collected and reported upon. It is also
possible that this represents a local?
ized, unique introduction by ballast
water of ships transiting the Panama
Canal or by local aquarists.
rotule: 13.0 to
diameter of the
16.2 (14.8 :!: 1.2)
diameter of the lower
16.6 (15.4 :!: 1.0) lJ.!ll;
upper rotule: 12.0 to
lJ.!ll.
Hildebrand (1939, p. 22) recorded the
presence of a number of invertebrates in
the chambers of the Gatlin Locks, but is
explicit in noting that " ..? the collec?
tions of these lower forms must be con?
sidered far from complete" (p. 21). He
observed that Cordylophora lacustris Allman
[=C. caspia (Pallas)] was most abundant in
the upper chamber, as was a gastropod,
Neritina meleagris Lamarck [=N. usnea Reding];
two crustaceans, Callinectes sapidus acutidens
Rathbun [=c. sapidus Rathbun] and Macro?
brachium olfersii (Wiegmann) completed the
listing of invertebrates from the upper
chamber. In spite of Hildebrand's comment
concerning the incompleteness of his in?
vertebrate collections, the present dis?
tribution of Trochospongilla leidii is such
that there seems little doubt that it
was not present in the chambers of the
Gatlin Locks some 40 years ago. In fact,
Potts' (1887) observations on multilam?
inar growth of this species, if undis?
turbed for several successive seasons,
would suggest that the presumed popula?
tion explosion has taken place quite
recently. A similar event has been re?
ported by Bretschko 1 from Volta Lake,
Ghana. Investigation of the lake in
April, 1971 had not revealed sponges of
any kind. In April, 1972 mass develop?
ment of Corvospongilla boehmii (Hilgendorf)
had been observed in the study area. The
sponge, obViously introduced during 1971?
1972, encrusted wide areas of submerged
trees in the south of the lake (partic?
ularly dense in Amankwatonu). Its occur?
rence was correlated with high phyto?
plankton production in the area.
It would seem also that yet another
invertebrate has undergone a latter-day
introduction into the Gatlin Locks. In
his observations on the fauna of the
The geographical distribution of Tro- Miraflores Locks, at the Pacific end of
chospongilla leidii to date has been re- the canal, Hildebrand makes a special
stricted to the eastern United States.
.. point of noting the presence of Congeria
Apa::t from the ~ype local~ty ~n Pennsyl- (Mytilopsis) sallei Recluz [=Mytilopsis sallei
van~a, the spec~es has been reported from (R 1 )] b t d t t' 't
h ? 11" W tV' .. ec uz , u oes no men ~on ~ sNew Jersey, 0 ~o, I ~no~s, es ~rg~n~a, . th Gat' Lock D r'ng our
Kentucky, Arkansas, Florida, Louisiana presence ~n e un s. u ~
and Texas (Penney, 1960).
All previous detailed descriptions of
Trochospongilla leidii have been based on
material from or very near the type lo?
cality, Schuylkill River and reservoirs
near Philadelphia, Pennsylvania (Bower?
bank, 1863; Potts, 1887; Gee, 1931;
Penney and Racek, 1968). Growth form,
consistency, internal structure, and
spicular dimensions of our specimens
agree surprisingly well with these, in
spite of the wide geographical separa?
tion. The minute spination of the oxea
in our material cannot be considered a
significant difference and could well"
have been present, but beyond the limits
of light microscope resolution, in speci?
mens from other localities. A very
closely related species, T. horrida Welt?
ner, is distinct in having a very con?
stant, heavy spination of the oxea.
Although Trochospongilla leidii is a
fairly abundant species in eastern North
America, very little is known about its
biology or ecology. Potts (1887) observed
that it thinly coated timbers, pipes, and
the stone work of dam, reservoir, and
lock structures, covering "yards of sur?
face". It was commonly associated with
Spongilla lacustris (Linnaeus) and Eunapi us
[=Spongilla] fragilis (Leidy) but, in con?
trast to these and most other fresh-water
sponges, it grew to a depth of 3 m or
more, and was found even in the complete
darkness inside long iron pipes. Potts
also noted that T. leidii, more than any
other species, had been found in asso?
ciation with iron structures. Specimens
from heavily rusted pipes showed pro?
nounced erosion of the spicules. Potts
(1887) finally observed that this sponge,
although commonly encountered as a thin
incrustation, can develop thick sheets
(of about 2 em) after "many years un?
disturbed growth". In this case, however,
a series of thin laminae, each formed
on top of the other, can be distinguished
in cross-section, each representing a
successive season.
It is not known how Trochospongilla
leidii came to be in the freshwaters adjacent
to Gatlin Lake. It is possible that the
species is distributed throughout Cen-
1G. Bretschko: unpublished Progress Report 1972,
Volta Lake Research Project, Akosombo, Ghana.
M.L. Jones and K. Rutzler: Invertebrates of the Upper Gatun Locks 65
collections we found M. sallei to be abun?
dant in the middle chamber and present
in the upper chamber.
Several explanations might be offered
to explain the curious distribution of
Trochospongilla leidii at the lower end of
the upper chamber of the Gatlin Locks.
Since diminution occurs in an area adja?
cent to the gates between two consecu?
tive chambers, it might well be that the
turbulence attendent upon the opening
and closing of the gates might be impli?
cated. However, the wedge-shaped distri ?
butional pattern is not found near the
gates at the upper end of the chamber
where it connects with Gatlin Lake; fur?
ther, such a distribution is not found
at either end of the chamber of the
Pedro Miguel Locks, the Pacific counter?
part of the upper chamber of the Gatlin
Locks.
It is difficult to conceive the pat?
tern being a function of light, turbi?
dity, or temperature, for the distribu?
tion of Trochospongi lla leidii on the floors
and walls of the upper part of the cham?
ber shows no such restriction.
Jones and Dawson (1973, p. 90) have
noted that, although there is a general
vertical homogeneity of salinity and
temperature readings in the chambers of
all the locks, there may not be a hori?
zontal homogeneity within a given cham?
ber. While their examples of horizontal
heterogeneity were more striking, such
might also be the case in the upper cham?
ber of the Gatlin Locks (Fig. 1). Although
the differences in bottom salinities be?
tween the upper and lower ends of the
upper chamber are not great enough to
fall outside the limits of accuracy of
our equipment (0.3 and 0.8 .:t O. 3~ S) , the
suggestion that there is a real salinity
difference at the ends of the chamber is
strengthened by the distribution of
several of the invertebrates encountered
here. T. leidii and, perhaps, Mari onina sp.,
are excluded from the lower portion of
the chamber; Mytilopsis sallei, Gitanopsis
tortugae? and Munna sp., on the other
hand, are restricted to the lower por?
tion of the chamber. The most obvious
conclusion to be drawn would involve a
chemical difference between the lower
and upper ends of the chamber, most prob?
ably the subtle and apparently minor
salinity differences. Indeed, the dis?
tributional pattern of T. leidii at the
lower end of the upper chamber strongly
suggests the intrusion of some kind of
chemical "wedge". It is difficult to
accept this in light of the comments of
Jones and Dawson (1973, p. 89) concerning
vertical homogeneity and the turbulence
attendent upon the filling and draining
of lock chambers and the passing of
ships, but perhaps it is related, in
some way, to the density currents set
up by the opening of the gates between
successive chambers just before ships
move from one to the next (p. 90). Since
all filling and draining of the lock
chambers is by gravity-flow from Gatlin
Lake or a higher chamber, these density
currents are the only mechanism for mov?
ing more saline water from a lower cham?
ber to a higher one.
It is also of more than passing in?
terest that, while for the most part the
upper distributional boundary of Trocho?
spongilla leidii is the low operational
water level, upward extensions of the
sponge occur coincidentally with fresh?
water seeps. These apparently allow T.
leidii -to survive the inevitable drying
of the upper chamber wall during the
passage of each ship.
Acknowledgements. We thank the staff of the
Panama Canal Company, especially L. Barka, R.J.
Danielsen, and K.F. Millard, for their help and
for allowing us to make collections and observa?
tions in the GatUn Lock chambers; the Smithsonian
Tropical Research Institute for numerous cour?
tesies; the following taxonomic specialists for
identifications of the taxa indicated: J.L.
Barnard, National Museum of Natural History,
Smithsonian Institution (amphipods); C. Erseus,
University of Gothenburg (enchytraeid oligo?
chaete); L.F. Gardiner, Rutgers University,
Newark (tanaid); G.A. Schultz, Hampton, New
Jersey (isopod); W. Vervoort, Rijksmuseum van
Natuurlijke Historie, Leiden (hydroid); and T.S.
Wood, Wright State University, Dayton, Ohio
(bryozoan). We are grateful to C.E. Dawson, H.W.
Kaufman, and J. Rosewater for assistance in
gathering salinity and temperature data, and
extra thanks to J. Rosewater for molluscan
identifications and field observations.
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Dr. M.L. Jones
Department of Invertebrate Zoology
National Museum of Natural History
Smithsonian Institution
Washington, D. C. 20560
USA
Date of final manuscript acceptance: June 9, 1975. Communicated by M.R. Tripp, Newark