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Respiration in Nuttallina and Tonicella page 2
INTRODUCTION
Marine molluscs which occupy middle and upper
intertidal regions must be adapted to periodic exposure
to air. Whileadptations toward avoiding desiccation are
usually apparent, the means by which these animals cope
with respiratory problems during periods of emersion is
not so immediately evident. Studies of intertidal
molluscs have demonstrated an ability to breathe air
in several species, and some investigations (Sandison,
1966; Micallef and Bannister, 1967) have revealed
capacities for aerial respiration even greater than
those possible in the submerged state. Adaptations
to air breathing typically involve 1) increased
rigidity of the ctenidial lamellae (Steen, 1971) and
2) increased vascularization of the mantle skirt (Newell,
1970).
Several species of Polyplacophora occur in the
upper intertidal region and face the problems of exposure.
However, the only study known to me on aerial respiration
in chitons concerned the low intertidal species
Cryptochiton stelleri (Middendorff, 1846) (Petersen and
Johansen, 1973). The work described here involved
Nuttallina californica (Reeve, 1847), which is found at
+3 to +5 foot tidal levels and is frequently exposed to
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Respiration in Nuttallina and Tonicella
air, and Tonicella lineata (Wood, 1815), which occupies
subtidal to low intertidal zones and is very rarely
uncovered. These species were compared with respect
to respiratory anatomy, efficiency of aerial respiration,
and recovery from exposure upon resubmersion, in an,
attempt to reveal possible adaptive advantages of
Nuttallina over Tonicella under humid exposed conditions.
ANATOMICAL OBSERVATIONS
The general anatomy and function of the chiton
respiratory system has been described in detail by
Yonge (1939). Nuttallina and Tonicella were examined
for variations of this basic pattern in terms of
structure, number, and orientation of the ctenidia, as
well as shape and variability of the pallial groove.
Representative cross-sections of the two species are
shown in fig. 1, along with the typical ctenidial structure
of each.
The number of ctenidia per animal is known to be
variable, but preliminary counts gave.Tonicella 22-29
and Nuttallina 30-48 ctenidia along each side of the
foot. The greater number of gills in Nuttallina, which
reaches a larger adult size, agrees with the observations
of Johnson (1969). The general morphology of the ctenidia
appears very similar in the two species, although
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Respiration in Nuttallina and Tonicella
closer examination reveals that Nuttallina ctenidia are
somewhat stouter and contain 250% more lamellae. These
filaments do not appear to possess greater self-support
than do those of Tonicella.
Major differences between the two respiratory
systems include the shape of the pallial cavity and the
involvement of the girdle in its regulation. As seen in
fig. 1, Nuttallina has a relatively deep pallial groove
bounded by a highly thickened and bristly girdle. The
orientation of the girdle is variable and under voluntary
control; it may be partially or totally raised, promoting
exposure of the gills (fig. 2a), or clamped against the
foot, shielding the gills and undoubtedly decreasing
their efficiency (fig. 2b). The latter behavior is
generally avoided, except in response to a local disturbance
or to critical levels of desiccation. Normally, as observed
in the laboratory, the girdle is partially raised along
one side of the foot. In contrast, Tonicella maintains
a shallower pallial groove of constant shape, with the
smooth thin girdle pressed flat against the substrate
and lifted only locally as entrance and exit ports for
water circulation. There is no active response to shield
gills from desiccation.
Responses to exposure were studied as chitons
clinging to horizontal glass plates were taken out of
water. It was found that Nuttallina could hold water
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Respiration in Nuttallina and Tonicella
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in the pallial groove for several hours with its girdle
down, but in its normally raised position the gills were
freely exposed to air. Tonicella usually developed
air pockets in its pallial groove, though it too could
hold water beneath the gills under moist conditions.
In both animals, when the gills were exposed to air
they collapsed against the foot or the back of the pallial
groove. In this state they were kept moist with a mucus¬
like secretion.
RESPIRATION STUDIES
Materials and Methods
Chitons were collected weekly from the Mussel Point
area surrounding Hopkins Marine Station, Pacific Grove,
Calif. Nuttallina were obtained intertidally at the 13
to 15 foot tidal level on exposed rock faces, while
Tonicella were found subtidally in rocky areas at -15
to -25 feet. Animals were scrubbed to remove any
commensal algae, and a standard "wet weight" determined
after blotting for 30 seconds on a paper towel. Chitons
were then placed in "restraining chambers" (fig. 3)
designed to minimize activity and facilitate future
manipulation. Restraining chambers were constructed of
1.5 X 3 cm glass plates enclosed in perforated plastic
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Respiration in Nuttallina and Tonicella
page 6
tubing capped at both ends. Nuttallina and Tonicella
weighing between O.5 and 1.0 gram fit easily into these
chambers, where they remained throughout the experiment.
Chambers were placed in an aquarium supplied with circulating.
well-aerated sea water at 13.5°C where the chitons were
allowed to equilibrate in a horizontal position under
constant light for 3-5 days.
Oxygen consumption was measured on individual
animals under successive conditions of submersion.
exposure to humid air, and secondary submersion.
Respiration rates were determined by the direct method
of Warburg using standard manometric technique as
described by Umbreit et al. (1972). Restraining chambers
were adapted to fit available Warburg vessels so as to
maintain animals in the submerged condition and horizontal
position. All phases of the experiment were conducted
at 13.5°C with vessels agitated to maximize diffusion
of oxygen.
Submerged rates of oygen consumption were first
determined with chambers immersed in glass fiber-filtered
sea water, and readings taken half-hourly for a period
of 4-6 hours. Chambers were then removed, drained for
I minute on paper towels, and returned to vessels
containing 0.3 ml sea water to maintain humidity.
Aerial respiration rates were then recorded under these
conditions for the next 12 hours. Finally, filtered
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Respiration of Nuttallina and Tonicella
sea water was replaced to its original level in the
vessels and post-exposure respiration followed for 4-9
hours.
Each run consisted of a maximum of 3 Nuttallina
and 3 Tonicella. Final data were compiled from 10 of
each species, 7 of which were followed through re¬
submersion. Additionally, several of each species were
run as controls which were handled exactly as were the
experimental animals, but were maintained submerged
throughout the experimental period. Data were
expressed as oxygen consumption in ul 0/g wet weight/hr
for each of the three experimental phases, and as
ratios of aerial rate/ submerged rate and resubmerged
rate/ submerged rate for each individual.
Results
Both Nuttallina and Tonicella displayed remarkedly
constant oxygen consumption rates during the primary
submerged and aerial phases of the experiment, possibly
reflecting the standardization of activity imposed by
the restraining chambers. A greater variation of rate
with time was observed during the resubmersion period.
Table 1 summarizes the average primary submerged
rate, aerial/submerged rate ratios, and resubmerged/sub¬
merged rate ratios for experimental and control animals
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Respiration of Nuttallina and Tonicella
of each species. Results for experimental animals are
depicted in fig. 4. Individuals of both species exhibited
aerial respiration at a level significantly (p£.001:
paired points t-test) below that of the initial submerged
rate, while controls showed a slight elevation of rate
during this period. Furthermore, the average fractional
decrease in rate upon exposure was very similar («.73)
for both Nuttallina and Tonicella. The pattern of
post-exposure respiration, however, differed significantly
(pX.025; Student's t-test) between the two. While
Nuttallina's resubmerged rate returned to 94.5% of its
initial submerged rate and was depressed below.the
associated control, that of Tonicella rose 22% above
its original wef rate and was elevated above its control,
DISCUSSION
Anatomical studies present little evidence for any
adaptation to aerial respiration in Nuttallina. Excepting
the observed differences in numbers of ctenidia and
filaments, the basic orientation and structure of the
gills in Nuttallina and Tonicella appears very similar.
The expected adaptation towards increased rigidity is
apparently absent in the ctenidia of Nuttallina; the gills
collapse upon exposure as do those of Tonicella
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Respiration in Nuttallina and Tonicella
Manometric studies confirm that Nuttallina has no
significant advantage in aerial respiration over Tonicella,
Respiration rates were equally depressed in both species.
reflecting the lessened efficiency of the gills in
minimal retained water resevoirs and in the collapsed
state. Studies of Cryptochiton stelleri showed a
greater decline in rate upon exposure, with aerial rates
averaging 3 to 5 time less than the submerged value
(Petersen and Johansen, 1973).
Preliminary experiments (unpublished) show that
Nuttallina survives longer than Tonicella under humid
exposed conditions. The nature of Nuttallina's advantage
is suggested by the observed post-exposure respiratory
behavior. The elevated oxygen consumption of Tonicella
upon resubmersion probably indicates that an oxygen debt
was accrued during the period of decreased oxygen uptake
accompanying exposure. Nuttallina has no such increase
in respiratory rate and evidently avoids anaerobiosis
during emersion periods. In this manner, it is not
forced to cope with the problems of acid-product
accumulation and eventual re-oxidation of those
products. Intertidal Cryptochiton stelleri show a
pattern of post-exposure respiration similar to that
of Nuttallina; more significantly, no accumulation
of lactate in the body fluid has been observed during
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Respiration in Nuttallina and Tonicella
exposed conditions (Petersen and Johansen, 1973).
Similar chemical analysis for lactate or succinate in
body fluid of exposed Nuttallina and Tonicella would
help to confirm the presence or absence of oxygen debt.
This evidence suggests an internal regulatory
control rather than a structural alteration through
which Nuttallina has adapted to exposure. Newell (1970)
has reviewed in detail the recognized physiological responses
of intertidal animals to a decreased availability of
oxygen. Possible mechanisms of avoiding oxygen debt
include: 1) excretion of anaerobic products 2) maintenance
of stores of oxygen within respiratory pigments in the
body 3) reduction of activity and general body metabolism
during restrictive periods. The latter possibility is
supported by observations that Nuttallina becomes
extremely sluggish and remains sessile when emersed.
Similar behavior was noted in Cryptochiton stelleri
(Petersen and Johansen, 1973).
Finally, the possibility must be considered that
measurements in the laboratory may not accurately
reflect behavior during exposure in the field. Animals
were always maintained on flat glass surfaces during
manometric and observational studies of responses to
emersion. However, field studies show Nuttallina
to be found wedged in cracks or depressions in the
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Respiration in Nuttallina and Tonicella
substratum, with the girdle molded to the contour of
the local environment. A similar preference was
noted in the laboratory, where Nuttallina grouped
into corners of aquaria rather than remaining on flat
surfaces. This preferential "wedging" is possibly
an adaptive response which allows for a raised girdle
and maximum gill exposure while simultaneously
protecting the gills and maintaining a large water
reservoir in the pallial groove during aerial exposure
(fig. 5). In this manner, the collapse of the ctenidia
would be avoided and efficiency of respiration maximized
in the emersed state.
SUMMARY
The respiratory anatomy and function of inter-
tidal Nuttallina californica and subtidal Tonicella
lineata were compared in an attempt to elucidate
adaptations of the former to frequent aerial exposure.
Ctenidial structure was found to be very similar, with-
out additional self-support in Nuttailina. Most
significant is the thicker more actively flexible
girdle in Nuttallina which permits control of gill
exposure and possibly maintains a reservoir of water
beneath the gills in the naturally exposed state.
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Respiration in Nuttallina and Tonicella
Manometric studies have shown aerial respiration rates
to be equally depressed to 73% of the submerged rate
in both species. Though Nuttallina has not apparently
adapted for increased efficiency of air breathing,
studies of recovery from extended periods of exposure
have indicated a probable oxygen debt present in
Tonicella and absent in Nuttallina. Field and lab-
oratory observations suggest that Nuttallina decreases
its metabolic activity when uncovered. However, further
work is necessary to confirm these conclusions and to
elucidate the physiological basis of Nuttallina's
resistance to oxygen debt during periods of exposure.
ACKNOWLEDGMENTS
I would like to thank Dr. Fred Fuhrman for his
helpful assistance and advice, and Dr. Donald P. Abbott
for making this whole venture possible.
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LITERATURE CITED
Johnson, Kay M.
1969. Quantitative relationships between gill number.
respiratory surface, and cavity shape in chitons.
The Veliger 11 (3): 272-276; 6 figs.
Micallef, H. and W. H. Bannister
1967. Aerial and aquatic oxygen consumption of Monodonta
turbinata (Mollusca: Gastropoda). Journ. Zool. Lond.
151: 479-482 (8 November 1966).
Newell, Richard Charles
1970. Respiratory mechanisms. Pages 265-371.in R. C.
Newell, Biology of intertidal animals. New York, N.Y.
(American Elsevier Publishing Co., Inc.).
Petersen, Jorge A. and Kjell Johansen
1973. Gas exchange in the giant sea cradle Cryptochiton
stelleri (Middendorff). Journ. exp. mar. Biol. Ecol.
12: 27-43; 10 figs.
Sandison, Eyvor E.
1966. The oxygen consumption of some intertidal
gastopods in relation to zonation. Journ. Zool. Lond.
149: 163-173; 4 figs. (8 February 1966).
Steen, Johan Buch
1971. Transitional breathing. Pages 56-76 in J.B.
Steen, Comparative physiology of respiratory mechanisms.
New York, N.Y. (Academic Press, Inc.).
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Respiration in Nuttallina and Tonicella
Umbreit, W. W., R. H. Burris, and J. F. Stauffer
1972. Manometric and biochemical techniques.
5th ed. v + 387 pp.; 125 figs. Minneapolis, Minn.
(Burgess Publishing Co.).
Yonge, Charles Maurice
1939. On the mantle cavity and its contained organs
in the Loricata (Placophora). Quart. Journ. Micro.
Sci. 81 (3): 367-390. (June 1939).
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Respiration in Nuttallina and Tonicella
FIGURE EXPLANATIONS
Fig. 1
Top: Schematic cross-sections of Tonicella
and Nuttallina showing ctenidia in the pallial groove.
Center: Ctenidia lateral views. Bottom: Typical
cross-sections of the ctenidia.
Fig. 2
a. Schematic cross-section of Nuttallina
with girdle partially raised on the left side and
totally raised on the right.
b. Schematic cross-section of Nuttallina with girdle
in the clamped-down position. Note ctenidia pressed
against the foot.
Fig. 3
The "restraining chamber" in which animals
were maintained during experimentation. Actual size;
1.5 cm diameter X 3 cm length.
Fiq. 4
Mean relative respiratory rates of 10
Nuttallina and 10 Tonicella during successive periods
of submersion, exposure, and 7 animals of each species
after resubmersion. Lines through bars indicate standard
deviation.
Schematic cross-section of Nuttallina "wedged"
Fig. 5
in the corner of an aquarium. Note the raised girdle, the
exposed gills, and the large water-holding capacity of the
pallial groove.
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Respiration in Nuttallina and Tonicella
TABLE EXPLANATIONS
lable 1
Weight-specific oxygen consumption of Nuttallina
californica and Tonicella lineata under submerged
conditions; average fractional changes in respiratory
rate during 12 hours of aerial exposure, followed by
resubmersion. Controls were maintained submerged during
all phases of the experiment. Number in parentheses
is the number of individual animals studied. Variation
shown is + 1 S.D.
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Respiration in Nuttallina and Tonicella
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