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P. Todd Bulkle
PLEASE TYPE ABSTRACT DOUBLE SPACED BELOW
Buiklev, P. Todd. (Hopkins Marine Station of Stanford
University, Pacific Grove, Calif., USA.) Shell damage and repair
in five members of the genus Acmaea (Mollusca:Gastropoda: Prosobranchia).
The Veliger.
Examination of naturally damaged shells of Acmaea scabra, A. digitalis
A. pelta, A. scutum, and A. limatula showed that damage to the margin
due to chipping predominated, but shells were found with regions near
the apex crushed, with severe erosion by a fungus, with cracks extending
from the edge to near the apex, with the tops eroded apparently by
barnacles, and with small holes of unknown origin near the apex. A.
scutum was the species most commonly damaged. Repair was always acc¬
omplished by secretion of new nacreous layers below the damaged portion.
Rates of repair of artificially damaged shell margins exceed rates
of growth where the margin remains intaot - Author.
PLEASE DO NOT TYPE BELOW THIS LINE
Shell Damage and Repair in Five Members of the Genus Acmaea
(Mollusca: Gastropoda: Prosobranchia)
By
P. Todd Bulkley
Hopkins Marine Station of Stanford University
Pacific Grove, California
Shells of the common limpets of the genus Acmaea found in
the rocky intertidal area of the California coast often show
evidence of having sustained and repaired extensive damage.
Studies of shell damage and repair were made on Acmaea scabra
(Gould, 1846), Acmaea digitalis Eschscholtz, 1833, Acmaea pelta
Eschscholtz, 1833, Acmaea limatula Carpenter, 1864, and Acmaea
scutum Eschscholtz, 1833.
Peppard (1964) studied growth and repair in the shells of
Tegula funebralis and studies of normal growth over a 3 year
period were made of A. digitalis, A. pelta and Acmaea para
digitalis by Frank (1965). Seapy (1966) reports on the
growth of A. limatula over a period of one year. Fretter and
Graham (1962) discuss the general growth in prosobranch molluscs
and include a chapter on the shell. No previous work, however,
has been done on repair in the five species of Acmaea under
consideration here.
*Footnote 1.
75
P. Todd Bulkley
Specimens of each of the five species showing evidence of
shell damage were collected from Pescadero Point, Monterey
County, California and from Mussel Point, Pacific Grove,
California on May 24-26, 1966. All levels of the intertidal
area were covered, and both protected areas and regions exposed
to very heavy surf were included in the survey. One hundred
animals showing evidence of having repaired shell damage were
collected; all but two of these occurred where the wave action
was heavy and where debris such as rocks and large shells were
tossed and shifted about by the waves at high tide. Even in
the latter areas animals with damaged shells constituted a very
small minority of the total population.
Of several hundred A. digitalis examined only 3 were found
to be damaged significantly. A. scabra and A. pelta popula¬
tions showed the same small proportion of damaged shells,
although in the areas studied A. pelta was not as abundant as
the other species. Many more damaged shells of A. scutum and
A. limatula were found, but they comprised less than ten per-
cent of the observed populations of these species. Often
several A. scutum or A. limatula with damaged shells were
found in the same small area. Such groups of injured animals
were not found in the other species.
C
P. Todd Bulkley
The numbers of damaged animals collected and the types
of damage are shown in Table 1. As can be seen, injury to
the edge of the shell, with pieces of the margin chipped off,
is the most common (Plate 1, F). Repair of the margin can be
recognized readily not only by the newness of the repaired
portion but by the ridge which is produced where the new shell
material meets the old. In many cases pieces up to 20% of the
width of the shell have been broken off and replaced. Often
the entire margin of the shell has been broken off and a ring
of newer shell material can be seen clearly all around the
original shell. In A. limatula this new shell material has
the same characteristic file-like appearance as the original
shell.
In four of the animals collected a portion of the top
of the shell near the apex had been crushed (Plate 1. A).
Pieces of the shell had been pushed inward, pressing down the
viscera. Repair had been achieved by the laying down of new
nacreous layers below these pieces of shell, cementing to¬
gether the crushed fragments of the top of the old shell and
partially embedding them to form a solid unit. Any holes left
by missing pieces of shell had been covered over on the inside
by new shell material (Plate 1, B). Where the shell had been
P. Todd Bulkley
cracked severely from margin to apex, new nacreous material
had been laid down over much of the interior of the shell,
binding the pieces into a solid unit. In some cases quite
extensive damage of this sort has been repaired (Plate 1, C).
Four of the shells each contained a small hole from 1.5 to
3.5 mm in diameter. Although the cause of these holes is
unknown, they appear too irregular in outline to be the work
of predatory boring snails. The holes had been covered over
on the interior with new nacreous material.
Two of the shells of A. digitalis, one of A. limatula
and one of A. scabra had been very severely eroded by a fungus
growing in the shell (Plate 1, D). The erosion was heaviest
at the apex and extended down the sides to varying degrees.
The surface of these shells was much softer than that of
normal shells and had a spongy appearance when viewed under
the dissecting microscope. Bonar (1936) reported finding an
ascomycete, Didymella conchae, in the shells of Acmaea, and
noted that among A. digitalis uninfected shells are actually
rare. Examination of populations of A. digitalis bore this
out, but only in unusual cases was the damage particularly
noticeable. One A. digitalis shell was found with one large
and several small barnacles, Balanus glandula, living on it.
P. Todd Bulkley
There were deep pits in the shell, not caused by Didymella
conchae, which were clearly formerly inhabited by barnacles
and may actually have been eroded by them (Plate 1, E).
In conjunction with observations of natural damage,
laboratory studies of repair of artificially induced damage
were undertaken. Three animals of each species were chosen
with normal, undamaged shells. Slots were drilled in the shell
with a high speed dental drill. All the slots were of uni¬
form width, approximately 1.5 mm, and extended varying dis¬
tances from the margin towards the apex. In no case did the
slot extend up to the attachment of the shell muscle. Care
was taken not to damage the mantle. The animals were placed
on rocks in aquaria with constantly circulating seawater and
aeration and kept submerged throughout the experimental period
(May 16 - May 23). They were removed each day, placed on a
glass slide, and the extent to which they had repaired the
slots observed under a compound microscope and measured with
an ocular micrometer. The results are presented in Table 2.
In each case where repair took place, it began with the
laying down of a thin transparent layer at the interior end
of the slot. This layer was gradually extended towards the
margin of the shell and thickened from beneath. Often this
P. Todd Bulkely
new shell broke off. After about four days the new shell
became opaque as it continued to thicken. The mantle, whose
margin usually conforms exactly to the margin of the intact
shell, did not contract locally to a shape conforming to the
margin of the slot until after about two days. In no case
did repair begin until this had taken place. In those ani-
mals with deeper slots (see Table 2) the mantle margin was
unable to contract enough to conform to the margin of the
slot and no repair took place.
The high incidence of evidences of natural shell damage
and repair in A. limatula and A. scutum is probably the result
of three factors. First, both species inhabit the lower areas
of the intertidal (Ricketts and Calvin, 1952) where wave
action is heaviest, and where loose rocks and debris which
might be pounded against their shells are more prevalent.
Secondly their shells are both flatter and thinner than those
of the limpets of the higher intertidal areas, thus presenting
a larger and weaker surface. Finally, both species show a
much more rapid rate of repair than either A. scabra or A.
digitalis. Thus an A. scutum or A. limatula with a damaged
shell would be more likely to be able to repair any damage to
its shell before a predator could take advantage of the
weakened shell. Limited observations on shell growth in
P. Todd Bulkley
undamaged animals maintained in laboratory aquaria from April
27 to May 21, 1966, suggest that addition of new shell at the
margin occurs more rapidly in A. scabra than in the other
four species, and that new shell may be added at very uneven
rates at different regions of the shell margin in this species.
A. scabra is known to inhabit a particular "scar" on the sub¬
strate to which its shell conforms exactly and to which it
consistently returns (Hewatt, 1940; Test, 1945). The rapid
and uneven marginal growth it exhibits might enable animals
to achieve conformity of the shell to a new spot on the rock
more rapidly. However, shells of A. scabra with slots drilled
in the shell margin showed a slow rate of repair.
SUMMARY
1. Shells of Acmaea scabra, Acmaea digitalis, Acmaea pelta,
Acmaea scutum, and Acmaea limatula found in the field showed
a variety of types of natural damage. Damage through clipping
at the edge of the shell predominated, but shells were found
in which regions near the apex had been crushed; in which the
tops and sides were eroded due to an ascomycete, Didymella
conchae; in which cracks extending from the margin to near
the apex were present; in which the tops were eroded apparently
P. Todd Bulkley
due to barnacles, Balanus glandula, living on the shell, and
in which small holes of unknown origin occurred near the apex.
Repair of such damage in all cases had been accomplished
by the laying down of new nacreous material on the interior of
the shell below the damaged portion.
Rates of repair of artificially damaged areas on shell
margins were much higher than rates of growth on adjacent
undamaged shell margins.
ACKNOWLEDGMENTS
This work was made possible by Grant GY806 from the
Undergraduate Participation Program of the National Science
Foundation. Help in the project was given by Dr. Donald P.
Abbott, Hopkins Marine Station. The photographs in Plate 1
were taken by Mr. Samuel E. Johnson. Permission to collect
in the Pescadero Point area was kindly granted by the Monterey
Foundation.
P. Todd Bulkley
LITERATURE CITED
Bonar, Lee.
1936. An unusual ascomycete in the shells of marine
animals. Univ. Calif. Publ. Bot. 19:187-194.
Frank, Peter W.
1965. Growth of three species of Acmaea. The Veliger
7(3):201-202.
Fretter, Vera, and Alistair Graham.
1962. British Prosobranch Molluscs. Ray Soc., London.
xvi+755 pp, illus.
Hewatt, Willis G.
1940. Observations on the homing limpet, Acmaea scabra
Gould. Amer. Midland Nat. 24 (1):205-208.
Peppard, Margaret Caroline.
1964. Shell growth and repair in the gastropod Tegula
funebralis. The Veliger 6 (Supplement):59-63.
Ricketts, Edward F. and Jack Calvin.
1952. Between Pacific Tides. 3rd ed, revised by J. W.
Hedgpeth. Stanford Univ. Press, Stanford, Calif.
i-xiii, 1-502, illus.
0
10
P. Todd Bulkley
Seapy, Roger R.
1966. Reproduction and growth in the file limpet, Acmaea
limatula Carpenter, 1864 (Mollusca: Gastropoda). The
Veliger 8 (4):300-310.
Test, Avery Ransome (Grant)
1945. Ecology of California Acmaea. Ecology 26 (4):395-405.
82
P. Todd Bulkley
TABLE CAPTIONS
Table 1.
Frequency and types of natural shell damage in 100 limpets.
Table 2.
Repair of artificially damaged limpet shells.
12
P. Todd Bulkley
PLATE CAPTIONS
Plate 1.
A. Shells of A. scutum showing crushed tops.
B. Interior of shells shown in A. Note how new nacreous
material has been laid down.
C. Shell of A. scutum showing severe cracking.
D. Shell of A. digitalis showing erosion due to a fungus
in the shell.
E.
Shell of A. digitalis with barnacles growing on it.
Note deep pits.
F. Shell of A. limatula showing damage to edge and sub¬
sequent repair.
Footnote, page 1.
Permanent address:
P. Todd Bulkley
Species
Aea
scutum
Acmaea
pelta
Acmaea
digitalis
Acmaea
sdabra
Acmaea
limatula
Number
Collected
32
Damage
Edge
53
26
Table 1
Type of Damage
Apex
Crüshed Cracks
Didymella
Erosion
2
ed bake
Balanus
Small
Hole
Damage
Species
Acmaea
scutum
Acmaea
pelta
Acmaea
digitalis
Acmaea
scabra
Acmaea
limatula
Shel
LengtHm
20
38
20
21
23
Table 2
Depth
In jurym
10
Average
Daily
Repairum)
.075
.047
000
.05
.07
000
.001
000
000
.001
000
000
.023
.025
000
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