Growth and Development of Scar Tissue in Lavigeria Gastropod Shells
of Lake Tanganyika, East Africa
Student: Karen Hinkley
Mentor: Ellinor Michel
Abstract
The shells of Lake Tanganyika gastropods exhibit high levels of scarring from failed predation attempts by
various crab species (Phifer 2000, Socci 2001). Many individuals exhibit multiple scars. This project
aims to look at the rate of gastropod self-healing/ re-growth and the nature of the shell scar tissue. In many
other organisms, vertebrates for example, scars are formed rapidly, exhibiting disorganized structure as
compared to the original tissue. Scar tissues also exhibit different strength and flexibility characteristics
and changes in microstructure. I aim to address questions of importance for both biomimetics (the
engineering of biological materials) and evolutionary ecology of this highly derived group of endemic
gastropods. Considering the high-energy cost involved with shell deposition and the exposed position a
broken shell leaves the organism in, I hypothesize that self-healing of the shell materials will be rapid, the
microstructure disorganized and thus may lack the strength of original shell. However, the alternative
scenario is also possible, that repaired shell is stronger as a reaction to realized predation risk and thus
could be deposited slowly, with greater strength features. While some of these parameters will be analyzed
in the laboratory at the University of Washington, the actual rates and characteristics of re-growth must be
determined in the field.
Introduction
In the predator prey arms race of Lake Tanganyika, the Lavigeria gastropod species seem to be winning.
Although the Lake Tanganyika crabs have there own set of tools to overcome the snail shell defenses strong chelae and molariform dentition - they are clearly not always successful. The snails have highly
sculptured, heavy shells that seem to provide reasonable defense against crab attacks. This is evidenced by
the high percentage of scarred individuals found in many populations of Lavigeria (rates to 70% in some
species, Phifer 2000, Socci 2001). Some individual gastropods have as many as five shell scars from failed
predation attempts. Aquarium experiments by Annie Socci and previous work by Kelly West (1996)
demonstrate the crabs do enthusiastically eat Lavigeria individuals. Furthermore, there is predation risk at
smaller life stages from fish. Preliminary aquarium studies by Nathan Zorich (unpublished 1999) and
detailed gut content analysis by Felicite Nduwarugira (1999) showed that cichlid fish successfully consume
hatchling and very young juveniles. We have noted that juveniles are commonly found in more cryptic
habitats than adults, presumably to avoid predation.
Adult Lavigeria have three layers of crossed lamellar aragonite crystals providing heavy armor against the
crabs (West & Cohen 1996). This laminate structure diverts cracking stresses so shell breakage in adults is
difficult except at the growing edge. The growing edge is more fragile as not all the layers are deposited
initially (Hinkley, unpublished 2001). I am also testing whether there are ontogenetic shifts in shell
deposition. Juvenile shells appear to be very thin, and lack the pearly inner layer of the adults. Is the
juvenile shell more readily preyed upon due to its thinner construction? From scar occurrence data
collected for three Lavigeria species, L.nassa, L.grandis and L.coronata, the crabs attack gastropods of all
sizes but even juvenile gastropods can resist predation attempts.
In order to survive a predation attempt, a gastropod must be able to repair any damage inflicted during the
attack and then maintain health during the repair event. Shell deposition is an energetic expense, and must
be completed quickly to avoid being exposed to another attack. Is the scar tissue laid down in an
organized manner, requiring high-energy expenditure and time, or is it laid down quickly with little order,
like plugging a hole with plaster? Breaking shells in a controlled manner and watching the re-growth
process mimics what takes place after a gastropod has been attacked and the shell injured by a crab.
Biomimetic studies on scar microstructure and physical characteristics will include scanning electron
imaging, atomic force microscopy, nano-indentation, three point bend tests and crushing strength tests.
Methods
I used a mark, shell breakage and recapture process to follow re-growth in animals in the field. Three
species of Lavigeria were selected for this preliminary experiment: L. nassa, L. grandis and L. coronata.
Each of these three species exhibits a high natural frequency of scarring, is relatively easy to collect and is
abundant at Jakobsen’s Beach, Kigoma. This site was chosen for its relatively easy access both by road
and boat. I collected the samples by SCUBA and snorkeling.
While I initially selected a sample size of 300 snails, it soon became evident that a sample size of 200
would be more feasible to resample in this time period. I measured 7 morphometric variables with digital
calipers: overall length and width, aperture to top, spire to top, aperture length, lip thickness, and scar
presence on last two whorls only. I also took digital photos for vouchers and later morphometric work.
100 individuals per species served as a control group with a mark etched into their shells at the point where
their growth was when I collected them (as per Geller, 1990), but no further damage to the shell. These
were transported to the laboratory and treated in a similar manner to the experimental group. I cut the
shells of this group following a technique from Gehring (1900). The animal was tapped lightly with a
pencil and encouraged to withdraw back tightly into the shell. I cut the shell aperture back as far as the
animal could retract, approximately a half a whorl. A battery powered Dremmel tool cut cleanly through
the shell. (While the shells were broken, it is important to remember that shell tissue is not living and has
no nerves). The shells of all animals, experimental and control group, were marked with a individually
numbered, white tag. The tags were made by hole-punching plastic film, numbering with a permanent
marker and super-gluing the tag on the upper, visible surface of the shell. The collections were replaced in
the field within 24-48 hours, with the sites identified by surveyor’s tape underwater. Animals were
replaced aperture down, scattered broadly at replacement areas, to approximate normal life positions. After
a period varying between 10-42 days, each population was recollected using SCUBA. The re-growth on
injured shells was measured at the suture line. Control populations were re-remeasured using four
variables: overall length, overall width, aperture to top and lip thickness. To establish re-growth rates, the
overall growth average was divided by the total number of days after injury. I calculated the healing
rates/growth rates for adults and juveniles separately. Recapture rates for each species were also calculated
of the number of snails present and alive at recapture, divided by the total number collected originally.
Results
Recapture rates varied between species, with L. coronata having the highest recapture rate 67%, L. grandis
with 62% and finally L .nassa with a lower recapture rate of 42%.
Initial recaptures took place between 10 days and 42 days after initial injury.
Field observations demonstrated that re-growth could be surprisingly fast - some individuals had
measurable re-growth after only 5 days. The re-growth of experimental juveniles included color and
sculpture bands effectively immediately on the shell, while experimental adults seemed to have an area
lacking normal color and sculpture. Re-growth of juveniles was faster than adults across all species. In
Lavigeria grandis the re-growth average over a 21-day period was 0.133 mm per day while the juvenile
average was 0.177mm per day. The growth rates varied between species (Table 1).
Species
Sample Size
Recapture rate*
Number scarred
Growth Rate
individuals
14 days
recaptured
L.grandis
200
62%
34
0.133mm/day
L.coronata
200
67%
75
0.16 mm/day
L.nassa
200
42%
34
0.123 mm/day
Table 1: Please note recapture rates include control individuals so number scarred doesn’t match
recaptured number. Also dead animals figured into this number.
One population of L. coronata was recaptured three successive times with growth rates varying only
slightly between periods. The first 11 days averaged 0.14 mm per day. Over a 28-day period the growth
averaged 0.23 mm per day and subsequently after 41 days the growth average in this species, L. coronata
was 0.16 mm per day. This would indicate that the re-growth begins slowly and then significantly picks
up, then levels off as the re-growth approaches the original shell dimensions. From field observations I can
state that there is no visible re-growth until 5 days passes. My hypothesis is the collection, scarring and
release process put the animal into a state of shock. While they seem to survive in tanks for 2 days, these
are very fragile organisms and some die after 48 hours. This leads me to believe they aren’t really healthy
in the tank, just surviving. After a readjustment period they begin to heal themselves. The healing initially
looks like a very thin and transparent lip on the cut shell. It is paper-thin and could not be measured with
the calipers without injuring the tissue. During the next period of time, 28 days total, the growth rate nearly
doubles. Field observations include thickening of the new growth and the appearance of color bands and
ribs in the juveniles while adults show only the thickening and a darkening of the re-growth. At the 41-day
interval most juveniles were totally re-grown, the tissue closely resembling the old tissue. Only slight
discontinuities could be seen in the chords and coloration. In adults I saw re-growth that appeared to lack
chords and the coloration was distinctly different from the removed shell.
My subsequent SEM studies on the shell microstructure showed that juveniles have only two crossedlamellar layers (XLM), as opposed to three in adults, demonstrating ontogenetic variation in shell
deposition, (Hinkley and Michel in prep.). Future work will include crushing studies on the life stages to
verify the strength differences brought about by adding layers to your structure. Although juvenile shells
have fewer layers than adults, the presence of scars at juvenile stages implies that they are also capable of
resisting terminal breakage to some degree.
Discussion and comments for future work
I can give initial results for scar tissue re-growth based on less than a month’s growth. It will be very
interesting to see how long it takes for complete recovery of the half a whorl that was cut off. Further work
will include several population recaptures, although the most detailed growth information would come
from having an in-situ researcher making monthly recaptures.
Recapture rate differences tell us a lot about the movement of each of the three species. It would be very
interesting to study this aspect more deeply. Why do L.nassa wander so much while L. coronata tend to
stick to one location?
My technique for marking shells was less than optimal. While it had advantages such as being highly
visible, the disadvantages outweighed the good. The plastic tags separated from the glue, leaving behind
only a white spot and no number. Research into this problem had been done prior to beginning the
experiment, but no optimal method had been found. More recent delving into the underwater research
manual has lead to several different techniques to be tried next summer. These to include painting the
growing edge with marine paint, using an underwater epoxy for glue, and a version of bee tags for
numbering.
References
Phifer, M., 2000. Scarring and sculptured shells: Crab impacts on morphology of Lavigeria in Lake
Tanganyika. Nyanza Project Annual Report 2000.
Geller, J. B., 1990. Consequences of a morphological defense: growth, repair and reproduction by thinshelled morphs of Nucella emarginata (Deshays) (Gastropoda: Prosobranchia). J. Exp. Mar. Biol. Ecol,
144 (2-3): 173-184.
Socci, A. 2001. Interspecific differences in snail susceptibility to crab predation at Jakobsen’s Beach,
Lake Tanganyika. Nyanza Project Annual Report 2001.
West, K., A. Cohen, and M. Baron, 1991. Morphology and behavior of crabs and gastropods from Lake
Tanganyika, Africa: Implications for lacustrine predator-prey coevolution. Evolution 45:589-607.
West, K. and A. Cohen, 1994. Predator-prey coevolution as a model for the unusual morphologies of the
crabs and gastropods of Lake Tanganyika. Ergebnisse der Limnologie 0(44): 267-283.