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Shell Growth and Repair in the Gastropod Tegula funebralis
By
Margaret Caroline Peppard
Shells of Tegula funebralig (A. Adams, 1854) inhabiting
the intertidal areas of Mussel Point, Pacific Grove, California,
are rarely found to have more than four whorls, irrespective
of the size of the snail, due to heavy erosion of the upper
parts. Questioning the nature of repair of erosion damage led
to a consideration of the more general question of shell repair
in T. funebralis. Fretter and Graham (1962) discuss shell form-
ation in prosobranch molluscs, but little is understood of shell
repair mechanisms.
Eegla funebralis lives in what is essentially a tapered
tube, closed at the small end. This is clearly seen in Figs.
1 and 2. Macroscopically, there are three layers in the shell.
The thin, transparent periostracum on Mussel Point specimens is
present only on the body whorl near the shell aperture, if at all.
Underlying the periostracum is a black prismatic layer. These
two layers are secreted only by the mantle margin. Innermost
is a thick nacreous layer, white over most areas, but sometimes
yellow or greenish in the upper whorls. Slides of decalcified
shells embedded in paraffin show the laminar character of the
nacreous matrix.
Most of the specimens of Tagula funebralis from Mussel
Point have shells which are conspicuously eroded. Although
some of the erosion appears to be due to the radular action of
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predaceous snails, boring of bryozoans and polychaetes, or
mechanical wear, all save a minority of T. funebralis(indi-
viduals measuring 5 mm or less at the greatest basal diameter)
are pitted over most of the eroded surface. Under 30x magni-
fication, this damage closely resembles that caused by a
fungus described as infesting shells of marine animals by
Bonar (1936). Attempts were made to culture the fungus on
T. funebralis on a medium of 100 ml sea water, 1 gm Caco,,
1 ml 1 M NaNo,, 1 ml 1 M KH,PO., 1.5 gm agar, 1 gm humus,
5 gm Tegula funebralig shell, finely ground, O.1% glucose,
and 0.01% yeast extract. The fungus in culture shows chlamy-
dospores (see Fig. 3) which differ from those on the species
raised by Bonar, and also from those found by Johnson (1962)
in a fungus growing on smooth jingle shells (Anomia simplex
d'Orbigny) from Pivers Island beach, North Carolina.
Normal shell growth in Tegula funebralis was measured
for a fifteen-day period (May 14-29, 1963) on individuals
ranging 13.0-27.5 mm in greatest basal diameter. Measurements
were made with an ocular micrometer of growth increments on the
outer lip of the aperture, secreted on top of a baseline of
fingernail polish painted on the edge of the aperture at the
beginning of the study. Results are shown in Table 1. The
average overall addition to shell aperture was six microna per
day, but growth occurred in spurts, not evenly each day. Total
growth over the fifteen-day period did not measurable affect
the greatest basal diameter of the shells.
In order to assess the ability of Tegula funebralis to
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repair damage incurred to the shell, snails were operated on
in various ways, inflicting different types of shell damage,
as indicated in Table 2. Five individuals were operated in
each way. The holes (windows) made in the shell back of the
aperture were ground on an emery wheel, care being taken to
keep the shell wet and cool, and the internal tissues intact.
Table 2 gives the average change in each group on successive
days. The range of variation within each group was not so
great as to make the average irrelevant. In every case of
damage to the shell aperture, growth of the damaged part
proceeded faster than the growth at apertures of undamaged
controls. All such operations on the apertures were repaired
by the folds at the border of the mantle. The same was the
case with the windows over mantle cavities. New shell mater-
ial included a black prismatic layer.
When the mantle margin was slit in an otherwise undamaged
specimen, within two days a notch appeared in the shell aperture
at the point apposed to the incision. It is not clear whether
the notch was due only to lack of growth, or in part due to
active resorption of shell at the point, but wehin six days
the notch was repaired.
Snails with openings over the visceral hump first secreted
a soft, membranous layer across the inside of the hole; this
later became impregnated with calcium carbonate. Successive
layers, similar in appearance, were built up beneath the first
layer, which bulged through the opening. After thirty days
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(April 30-May 29, 1963), one specimen had plugged the shell
window with a hard patch of white material, apparently
calcium carbonate embedded in an organic matrix. The patch
protruded through the opening like a bubble, and was translucent
at the eperiphery, opaque in the center portion. Of three
females and two males, with windows cut over the visceral
hump, the females began repair sooner than the males. None
of the snails with windows over the visceral hump died, al-
though the gonad was frequently ruptured. On the other hand,
animals wih the shell damaged by grinding a hole over the
region of the heart and kidney died in all cases except one.
Death was due not to the operation, but to the later rupture
of the kidney or pericardial sac against the sharp edge of the
opening produced by the operation. One specimen which lived
an entire month with this operation failed to successfully
repair the damage, for each time the soft membranous layer
covering the hole became embedded with calcium carbonate, it
was sloughed off through the opening.
In the "windowed" animals, even where the holes penetrated
yellow and green layers, I observed no secretion of yellow or
green material by the mantle covering the body, nor is it
secreted by the mantle margin. Secretions by other than the
mantle margin were always either transparent membranous layers
or white inorganic material. However, natural repair does
sbow yellow or green material, particularly in eroded areas at
the shell apex. Perhaps the inner layers of nacreous material
are dyed by pigments secreted by the visceral hump, specifically
72
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either the digestive gland or the gonad (see MoGee, 1963a).
Cracked shells were bound firmly together in a solid unit
within five days by a calcium carbonate-embedded membrane on
the inner surface. Additional white nacreous material was laid
over the outer surface of the crack within sight days where the
break passed through the underside of the body whorl adjacent
to the shell aperture.
Summary
1. Shell erosion is caused by the activities of several animals
(bryozoans and polychaetes), by mechanical wear, and by a
fungus, which was cultured on agar plates.
Normal shell growth, recorded over a period of fifteen days
in twenty-eight animals, was intermittant, but averaged
six microns per day added to the outer lip of the aperture.
3. Repairs to shells damaged mechanically, by filing the
aperture, grinding holes in the body and upper whorls, and
by cracking in a vise are described.
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Table 1
Growth Studies on the Shell of Tegula funebralis
Legend:
no change since last observation
no observation
dead
Snail
Greatest
Addition to Shell at Aperture on Successive
Day
basal
mm)
no.
diam. (mm)
1
8911
13

7060 .09
13.0
- .090
13.5
- .120
.210
-
*
16.0
.030
.060
.090
.060
.030
17.0
.030
.060
17.0
.030
18.0
.030
.060
18.0
19.0
.090
20.0
.120
2.5
.090
.120
10
11
21.0
.165
12
.100
21.0
22.0
.165
100
13
.033
.132
23.0
.060
.090
.120 .150
3.C
090
3.0
.030
060.090
3.0
.030
060
18
23.0
.030
.060
.090
24.0
24.0
21
.090
24.0
.060
090.120
24.5
.060
25.0
.015
.030.120
25.0
.090.120
.030
.030
25.5
.132
.165
27.
.060
27.5
.060
.120
Average
growth
day
506
.014
.004
.006
.004
.002
.004
008
.008
.011
O11
006
.010
.006
.004
.006
006
008
008
008
002
O11
.007
.008
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Table 2
Repair of Shell Damage in Tegula funebralis
Legend:
no change since last observation
no observation
just visible trace of nacreous layers secreted
soft, membranous layer secreted over opening
calcium carbonate embedded in soft layer
Caco,
dead
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Table 2
Repair of Shell Damage in Tagula funebralis
Type of Operation
Repair on Successive Days After Operation
(mm Added to Shell at Aperture)
11
19
—
control(no opera¬
.165
.1
tion; normal growth
at aperture lip
recorded)
2mm notch
Mantle margin slit
notch filled
.O66mm.240mm
.3881m
2.6mm notch filed
.479mm.677mm
in edge of shell
of notch
of
of
of
aperture
filled notch
notch
notch
notch
filled
filled filled filled
window over
Caco,
2 Caco.
layers
visceral hum
window over heart
Gaco,
and kidney area
window over mantle
.18
.397
.198
.353
cavity
added
to hole
edge
shell cracked
white
Caco,
(with a vise)
nacreous
sevretion
over
crack(xx)
.354
shell broken at
.386
.187
.221
aperture (with
pliers)
shell aperture
.583
.333
.397
.353
ground off
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Figure Captions:
Fig. 1. Ventral view of Tegula funebralis, 8 mm in basal diameter; decaleified
cleared in cedarwood oil, and with black prismatic layer removed
Fig 2. Dorsal view of Tegula funebralis, same individual as shown in Fig. 1.
Fig. 3. Fungus found on Tegula funebralis shell; part of mycelium with
chlamydospores.
a, shell aperture; an, anus; em, columellar muscle; et, ctenidium; e, eye;
f, foot; g, gonad; h, heart; ht, head tentacle; k, kidney; me, mantle cavity;
8a, operculus; ad, esphradium x, rectum: z, chell; ge, spiral caecun;
2t. sterach.
C

a

D