Shell Thinning Research Articles

Organochlorine insecticide and polychlorinated biphenyl residues in eggs of coastal birds from the eastern Cape, South Africa

The concentrations of organochlorine insecticide and PCB residues were determined in eggs of jackass penguins Spheniscus demersus, Cape gannets Sula capensis, Cape cormorants Phalacrocorax capensis, white-breasted cormorants P. carbo, African black oystercatchers Haematopus moquini, roseate terns Sterna dougallii, kelp gulls Larus dominicanus, and grey-headed gulls Larus cirrocephalus. These species were selected as representatives of exclusively marine, mixed marine-estuarine-freshwater, intertidal-marine, scavenging-marine-estuarine-intertidal and scavenging-estuarine-freshwater habitats. All the eggs were positive for DDE with residue levels highest in the mixed scavenging species and lowest in the marine species. Similar trends were noted for dieldrin, but the levels were low or absent in most marine species. The eggs of most species exhibited residue levels of PCBs and the trends in terms of feeding habitats were similar to those noted with DDE. Based on a comparison with studies elsewhere, none of the organochlorine pesticide or PCB residue levels appear high enough to cause reproductive impairment or egg shell thinning.

Pesticide Residues and Shell Thinning in Eggs of Peregrines in Norway

The shell thickness of recent and old eggs, and of shell fragments, from 32 clutches of Peregrines were measured to investigate a possible shellthinning effect during the period following the use of DDT in north-western Europe from 1947 on. The pesticide contents of three recent eggs were also determined. The eggshells of the post-1947 sample were on average 19-23% thinner than the older eggs, depending upon the measuring method used. Up to 1000 ppm of DDTs and PCBs were found in the recent eggs, and one egg also contained 280 ppm dieldrin (values on a lipid basis). These results accord in general with the previously-published data for other Fennoscandian populations of Peregrines, and, at least partly, explain the substantial reduction in numbers of the Norwegian breeding population observed during the past few decades. A significant positive correlation was found between shell thickness and brood size.

Effects of Alpha-Chloralose Drugging on Blood Constituents in the Eastern Wild Turkey

CROMARTIE, E., W. L. REICHEL, L. N. LOCKE, A. A. BELISLE, T. E. KAISER, T. G. LAMONT, B. M. MULHERN, R. M. PROUTY, AND D. M. SWINEFORD. 1975. Residues of organochlorine pesticides and polychlorinated biphenyls and autopsy data for bald eagles, 1971 and 1972. Pestic. Monit. J. 9:11-14. HILL, E. F., R. G. HEATH, J. W. SPANN, AND J. D. WILLIAMS. 1975. Lethal dietary toxicities of environmental pollutants to birds. U.S. Fish and Wildl. Serv. Spec. Sci. Rep.-Wildl. 191. 61pp. KAISER, T. E., W. L. REICHEL, L. N. LOCKE, E. CROMARTIE, A. J. KRYNITSKY, T. G. LAMONT, B. M. MULHERN, R. M. PROUTY, C. J. STAFFORD, AND D. M. SWINEFORD. 1980. Organochlorine pesticide, PCB, and PBB residues and necropsy data for bald eagles from 29 states1975-77. Pestic. Monit. J. 13:145-149. McLANE, M. A. R., AND D. L. HUGHES. 1980. Reproductive success of screech owls fed Arochlor 1284. Arch. Environ. Contam. Toxicol. 9:661-665. MUNGARI, R. J. 1979. A report on the monitoring activities associated with the emergency release of endrin for pine vole control. Div. Plant Industry, New York State Dep. Agric. Markets, Albany. 185pp. RATCLIFFE, D. A. 1967. Decrease in eggshell weight in certain birds of prey. Nature 215:208-210. STICKEL, W. H., W. L. REICHEL, AND D. L. HUGHES. 1979. Endrin in birds: lethal residues and secondary poisoning. Pages 397-406 in W. B. Deichmann, organizer. Toxicology and occupational medicine. Elsevier/North Holland, New York, N.Y. U.S. ENVIRONMENTAL PROTECTION AGENCY. 1976. Endrin. Rebuttable presumption against registration. Fed. Reg. 41:31316-31338. .1979. Suspended and cancelled pesticides. Off. Public Awareness. Washington, D.C. 20pp. WHITE, D. H., AND E. CROMARTIE. 1977. Residues of environmental pollutants and shell thinning in merganser eggs. Wilson Bull. 89:532-542.

Effect of p-p'-DDE administered in vivo and in vitro on Ca2+ binding and Ca2+-Mg2+-ATPase activity in egg shell gland mucose of ducks.

Thinning of the egg shell is produced by p-p'-DDT and DDE in several species of birds. A study was made of the effect of DDE administered in vitro and in vivo on the Ca2+ binding and Ca2+-Mg2+-activated ATPase of a homogenate of the egg shell gland of ducks (Anas platyrhynchos var.). The concentration of Ca2+ was 1 X 10(-4) M and that of MgATP 1 X 10(-3) M. In vitro, DDE in concentrations of 2-16 micrograms/ml of incubation medium inhibited the Ca2+-Mg2+-activated ATPase in a concentration-dependent manner, whereas Mg2+-activated ATPase was not affected by these concentrations. The Ca2+ binding by the homogenate was reduced by DDE in the same concentrations. The sodium azide sensitive Ca2+ binding was most sensitive. In vivo, DDE administered in a concentration of 40 mg/kg dry weight of the food for 45 days reduced the egg shell index by 18% in comparison to controls. After 45 days of treatment the DDE concentrations in the egg shell gland mucosa was 1.20 +/- 0.16 micrograms/g of wet weight, while no DDE was detected in the controls. The Ca2+-Mg2+-activated ATPase was reduced by 32%, whereas the Mg2+-ATPase was not changed. The Ca2+ binding by the homogenate was reduced by 29%, the sodium azide sensitive part being most vulnerable, DDE increased the total Ca content of the egg shell gland mucosa by 44%. Since Ca is transported against a concentration gradient between blood plasma, and the lumen of the shell gland, it is suggested that DDE, by inhibiting the Ca2+-Mg2+-activated ATPase, decreased the Ca translocation over the egg shell gland mucosa.

Organochlorine Residues and Shell Thinning in Oregon Seabird Eggs

A single egg was collected at 62 nests of 10 seabird species from Oregon in 1979. The eggs were analyzed for organochlorine contaminants; con- temporary shell thickness was compared with eggshells collected during earlier time periods. Concentrations of DDE and PCB's in 1979 were generally low with the most contaminated species being the Double-crested Cormorant (Pha- lacrocorax auritus), Leach's Storm Petrel (Oceanodroma leucorhoa), and Fork- tailed Storm Petrel (0. furcata). Limited eggshell data provide evidence that shell thinning was more severe along the Oregon coast in the 1950's than in 1979. With the possible exception of the Fork-tailed Storm Petrel, the residues in 1979 posed no known threat to the welfare of the species. Seabirds have received increased attention during the last decade, partially because of the increase in man's activities in the coastal zones, but also because of the exposure of sea- birds to environmental pollutants in many locations (Ohlendorf et al. 1978). Pollutants in Oregon seabirds have not been systematically studied; however, an evaluation of a Common Murre (Uria aalge) die-off in 1969 included an analysis of dead birds for organochlorine pesticides and polychlorinated biphenyls (PCBs) (Scott et al. 1975). DDE and PCB residues in the murres were considerably below reported lethal concen- trations in other species of birds. Oregon's breeding seabird population is relatively large and diverse. Varoujean (1979) listed 46 colony sites in Oregon and about 1.2 million breeding seabirds of 13 species. The present study was undertaken to determine organochlorine burdens in seabird eggs collected along the Oregon coast, to evaluate the importance of the resi- dues detected, and to compare the residues with those reported from the same species in other localities.

Breeding Performance, Pollutant Burden and Eggshell Thinning in Common Loons Gavia immer Nesting on a Boreal Forest Lake

A population of 200 Common Loons Gavia immer nesting on a little-disturbed oligotrophic lake in northeastern Saskatchewan was studied from 1973 to 1975. The reproductive success, causes of nesting failure, pollutant content and shell thickness of eggs were reported and compared. Forty-six percent of pairs attempting to breed successfully raised young in any one year; a mean of 0.73 young were fledged per nesting pair. Thirty-eight percent of all eggs hatched. Predators, unexplained disappearance and inundation were the major causes of egg loss. Eggs contained significant levels of DDE, Dieldrin, PCBs and mercury; however no statistical relationship to reproductive performance was found. Shell thickness was inversely correlated, shell density directly correlated with DDE content. The mean shell thickness was identical to the pre-1947 mean although the variance was greater and some shells were 23% thinner. Altered shell density was interpreted as evidence of structural abnormalities. About 47 ppm DDE are necessary to induce 20% shell thinning in this species, but its large pore area may make it more sensitive to pollutant-induced alterations.

White-Faced Ibis Populations and Pollutants in Texas, 1969-1976

Eggshell thickness, levels of pollutant residues, and population status of the white-faced ibis (Plegadis chihi) were monitored in Texas from 1969 through 1976. Texas ibis nesting populations declined by 42%. Reproductive success apparently was limited by DDE-induced shell thinning and by dieldrin-caused mortality. Eggshells averaged 4% to 10% thinner than the pre-1943 mean thickness. Shells of numerous crushed eggs exceeded 20% thinning. Mean DDE residues in randomly collected eggs decreased from 0.94 ppm in 1970 to 0.25 ppm in 1976. DDE was higher, averaging 2.5 ppm, in thin-shelled eggs. DDE was negatively correlated with shell thickness. Dieldrin and PCB residues increased slightly from 1970 to 1976, but neither was correlated significantly with shell thickness. Residues in brains of many dead and dying ibises collected during years of extreme reproductive failure, 1970 and 1973, contained between 5 and 25 ppm dieldrin. Ibises were exposed to aldrin and dieldrin by feeding in rice fields where aldrin-treated rice seed was planted. Improved nesting success in 1976 may have been related to declining residues of DDE and to the discontinued use of aldrin in rice fields. Eggshell thinning has been detected in a number of declining species of fish-eating birds in the United States (Hickey and Anderson 1968, Blus 1970, Anderson and Hickey 1972, Blus et al. 1979). Laboratory investigations have shown that DDT or its metabolites, primarily DDE, induced shell thinning in susceptible species at concentrations found in some wild birds and their eggs (Heath et al. 1969, Wiemeyer and Porter 1970, Haegele and Hudson 1973). The recent population decline in the brown pelican (Pelecanus occidentalis) and the apparent decline in the white-faced ibis (Plegadis chihi) on the Texas Gulf Coast (King et al. 1977, 1978) prompted us to study the extent of eggshell thinning and the impact of organochlorine and heavy metal contamination on these species. The present paper presents information on eggshell thickness changes and chemical resi-

Habitat, Female Age, Organo-Chlorine Compounds and Breeding of European Sparrowhawks

(1) This paper examines associations between habitat, female age, organochlorine levels in eggs and breeding performance of Accipiter nisus in Annandale, south Scotland. (2) Breeding was better on low mixed farmland (on productive ground) than on upland sheep range or plantation forest (on less productive ground). This was shown by the earlier laying dates on farmland, larger brood sizes, faster growth of young, and by the greater proportion of nests that were successful. (3) Although sparrowhawks were restricted to woods for nesting, some aspects of their breeding performance deteriorated with increase in the extent of woodland around the nesting territory. This trend held both in low farmland and in the area as a whole, and was probably due more to the increase in woodland as such, than to the associated increase in breeding density. Prey densities were greater in small woods in open country than in continuous forest. (4) Territories in larch and pine were used most often, followed in order by those in spruce and broadleaved trees. Breeding performance was generally better in the favoured trees, but only for laying dates were these differences significant. The proportion of nests that produced young also rose with increasing amount of tree cover on the territory; this was partly due to lessened predation in the thicker territories. (5) There were fewer adult-yearling, and more yearling-yearling and adult-adult matings than expected if mating were random with respect to age. Yearling females had smaller clutches and broods than older ones, and in farmland their young grew less quickly and their nests were less often successful. (6) The failure of certain nests throughout the population was associated with high levels of DDE and PCB in eggs. The amount of shell thinning and egg-breakage was highly correlated with DDE concentrations. (7) Different aspects of breeding performance were most closely related with different factors. The frequency with which territories were used for breeding attempts was more closely linked with tree-species than with any other factor; laying dates and nestling growth rates were associated most strongly with surrounding landuse and productivity; sizes of clutches and broods were related most closely to age of female; while success or failure of the nest was associated both with habitat factors (especially thickness of cover on territories) and with organo-chlorine levels in eggs and with the extent of shell-thinning.

Chlorinated Pesticide Residues in Chicken Egg

Two hundred and twenty-one representative samples of chicken eggs of native and commercial strains of Gallus domesticus were collected during January 1975 to August 1977 throughout the Tehran area and the Northern province of Mazandarane. The samples were analyzed for chlorinated pesticide residues by gas-liquid chromatography. The insecticides [1,1,1-Trichloro-2,2-bis (P-Chlorophenyl)ethane] (DDT), [1,1-Dichloro-2,2-bis (P-Chlorophenyl) ethane] (TDE), [1,1-Dichloro-2,2-bis (P-Chlorophenyl)ethylene] (DDE), isomers of benzene hexachloride (BHC), Aldrin/Dieldrin, Heptachlor/Heptachlor epoxide, Dieldrin, and endrin were detected in varying concentration in the eggs. The eggs of native bred chickens, mainly as a result of their food sources, showed greater concentration of all pesticides except BHC isomers than those of commercial types. Though concentration of DDT compounds in 22% of native bred and 5% of commercial eggs exceeded WHO tolerance limit, the mean concentrations of pesticides residues were not exceeding these limits. There was no correlation between concentration of pesticides and egg shell thinning.

Egg shell characteristics of gannets Sula bassana, shags Phalacrocorax aristotelis and great black-backed gulls Larus marinus exposed to DDE and other environmental pollutants

Structure, strength and chemical composition of egg shells of gannets Sula bassana, shags Phalacrocorax aristotelis and great black-backed gulls Larus marinus have been studied in relation to egg residues of DDE. For each species, eggs that were highly contaminated with DDE tended to have thin shells. For the gannet, shell thinning was mainly due to loss of the vaterite cover. For the shag, cover loss was again considerable, but the true shell was also significantly reduced in thickness. Gull shells do not have a cover, and for the great black-backed gull most of the shell thinning was due to a reduced palisade layer. The different types of structural defect observed in the shells of these and other species could result from a single biochemical lesion such as enzyme inhibition in shell gland tissue. As the thickness of the shell decreased, so shell strength for the gannet and shag did not decrease as markedly as for the gull and other species. Partial loss of cover had little effect on strength compared with loss of true shell. Had the cover been entirely missing, however, strength would have been seriously affected.

Egg Production and Body Weight Changes of Japanese Quail and Leghorn Hens Following Deprivation of Either Supplementary Calcium or Vitamin D3

Dietary deprivation of supplementary calcium or vitamin D3 did not affect the body weight of female or male coturnix or Leghorn hens, even though feed consumption was significantly reduced. The egg production was reduced from about 74% to about 10% and 20% in the calcium deprived and the vitamin D3 deprived coturnix hens, respectively. Leghorn hens reduced their egg production from about 60% to about 25% on vitamin D3 deprivation, but virtually ceased egg production on calcium deprivation. The dietary deficiencies resulted in reduced egg weight, shell thickness, and female tibia ash of coturnix while not influencing ovarian and oviductal weights or testis weights and tibia ash of males. Each of Leghorn hens deprived of dietary calcium had thinner shells within 2 weeks, whereas in the vitamin D3 deprived hens shell thinning occurred within 3 weeks, but yolk weights were significantly heavier in this group. Plasma calcium, ovary weight, and oviduct weight were significantly reduced in Leghorn hens deprived of calcium, but not vitamin D3.

Changes in egg shell characteristics of the Sparrowhawk (Accipiter nisus) and Peregrine (Falco peregrinus) associated with exposure to environmental pollutants during recent decades

In Britain since the late 1940s, the Sparrowhawk (Accipiter nisus) and Peregrine (Falco peregrinus) have laid eggs with unusually thin shells. DDT and its metabolites have been blamed. This paper reports how various properties of modern shells differ from those of older shells of normal thickness.For the shells of both species, there were roughly proportional reductions in thickness of the two main component layers, the mammillary and palisade layers, suggesting that thin shells resulted from a decreased rate of deposition. In addition, in modern Sparrowhawk shells, the resistant surface layer was especially deficient perhaps due to a further reduction in deposition rate or to slight premature termination. Shell thinning was associated with a proportionately greater decrease in shell strength. For the Peregrine, porosity, measured as the rate of passage of water vapour, was lower for the thin shells than for normal shells. This difference could not be explained by a change in the area of the pore channels, since the modern shells did not have fewer pores and the pore channels were the same size and shape in the two samples of shells. However, in the thin shells of both species, mammillae height increased relative to mammillary layer thickness, and in the Peregrine shells the extent of this increase was related to the change in porosity.

Experimental reproduction of the egg drop syndrome 1976 with a haemagglutinating adenovirus

The experimental reproduction of an egg drop syndrome is described. Experimental oral infection of fowl with a haemagglutinating adenovirus resulted in the production of shell-less, soft-shelled and thin-shelled eggs in association with loss of egg shell colour and were similar to those changes seen in the egg drop syndrome 1976 (EDS 76). The first changes observed were 7 days post-inoculation when loss of egg shell pigmentation occurred. Colour patterns did not return to normal until 25 days post-inoculation. Thinning of egg shells was first seen 1 day after the loss of pigmentation and by 9 days post-inoculation soft-shelled and shell-less eggs were laid. From 13-16 days post-inoculation between 25 and 40% of eggs laid were either shell-less, soft-shelled or very thin-shelled. The majority of birds were laying normal eggs by 24 days post-inoculation. In spite of the marked egg shell changes the overall daily egg production remained at approximately 80% during the 17-week experimental period.

Shell structure of immature roman snails ( Helix pomatia) after exposure to p, p′-DDT

Shells were examined from immature snails ( Helix pomatia) that had been treated with DDT or deprived of access to calcium carbonate. Exposure to DDT resulted in shell thinning with the thickness of each of the three structural layers being inversely related to the body burden of DDT. The normal crossed lamellar structure was distorted in some of the thin shells. Withholding calcium carbonate led to similar changes, and DDT may cause thin shells by impairing calcium metabolism.

The Role of Different Organo-Chlorine Compounds in the Breeding of British Sparrowhawks

(1) In British sparrowhawks, organo-chlorine levels and shell thickness indices varied little between eggs in the same clutch; but they varied greatly among clutches from the same (known) female in different years, and among clutches from the same territory in different years (female mostly unknown). The average tendency was for OC levels in eggs to increase between first year and older females. Within populations, OC's showed a skewed distribution, with a small proportion of clutches containing very high concentrations. (2) In contrast, shell indices were distributed 'normally' in the statistical sense. Among 315 clutches, the overall ratio of DDE:PCB was 1:0-53, and of DDE:HEOD it was 1:0-19. Comparing clutches, the levels of all these compounds were significantly correlated with one another. DDE concentration was significantly related to the extent of shell thinning among clutches, the extent of egg breakage, the extent of egg addling and the extent of hatching failure. PCB concentration was significantly related to the extent of egg addling and hatching failure. (3) No significant relationship could be shown between the levels of any OC examined and the date of first egg, the initial clutch size, the incidence of egg desertion and the extent of nestling mortality. (4) No relationship could be shown between HEOD concentration and any aspect of breeding examined, but the concentrations of HEOD were relatively low. (5) In only one of nine areas examined was there evidence for a decline in the levels of all three OCs during 1971-4, but in the overall sample there was evidence for a decline in PCB levels. (6) The regression of shell index on log DDE content in the sparrowhawk was similar to those found by other workers for Falco peregrinus, F. mexicanus and Pelecanus occidentalis.

Prolonged effects of kepone on strength and thickness of eggshells from Japanese quail fed different calcium level diets

Japanese quail ( Coturnix coturnix japonica) were first fed low- and then high-calcium diets for 120 days each. A second group received the same diets contaminated with 200 ppm Kepone. Eggshell strength of vertical and horizontal axes as well as equatorial eggshell thickness were measured. Kepone administered in low calcium diet did not reduce equatorial shell thickness or strength; however, a temporary decrease of shell strength in vertical axis was recorded. Continued Kepone feeding produced significant shell thinning and weakening in both axes, evident especially during high-calcium diet. An increase in egg breakage and high mortality also became characteristic. Egg laying was a factor in survival. Kepone-fed quail that ceased egg laying exhibited neurological disorders and died. Post-mortem examination of the reproductive organs indicated that Kepone may have interfered with ovarian function.

Shell thinning and reproductive impairment in black ducks after cessation of DDE dosage.

Captive black ducks (anas rubripes) were fed dietary DDE [1,1-dichloro-2,2-bis (p-chlorophenyl)ethylene] at 10 ppm (dry weight; about 2 ppm on a natural diet basis) for 2 breeding seasons, then untreated feed for 2 succeeding years. Residues of DDE in the carcasses of adults declined 90% during the 2-year clean-up period. Following 2 years of dietary DDE, mean residues in eggs reached 64.9 ppm. Even after 2 years on clean feed, DDE residues in the eggs averaged 6.2 ppm or 9.5% of the mean DDE level reached after 2 years on treated feed. Shells of eggs from treated hens were about 20% thinner than shells of eggs from controls. Stoppage of DDE dosage resulted in progressively thicker shells, yet even after 2 years on untreated feed hens laid eggs with shells about 10% thinner than control hens. After DDE was removed from the diet, DDE residues in the eggs decreased, shell thickness increased, and reproductive success improved. Hens previously exposed to DDE, but then fed clean feed for 2 years, still produced significantly fewer surviving ducklings than did control hens.

Egg shell characteristics and incidence of shell breakage for grey herons Ardea cinerea exposed to environmental pollutants

Egg shell characteristics and incidences of shell breakage have been studied for the grey heron Ardea cinerea at several colonies in eastern England. Eggs collected in the late 1960s and in the 1970s had shells which were significantly thinner than shells collected before 1946. For a sample taken in 1973, shell thickness, shell strength, mammillary layer thickness and palisade layer thickness were all inversely related to the concentration of pp'-DDE(+ pp'-DDT + pp'-TDE) in the egg contents. Shell thinning was considered to be due to a reduced availability of shell components leading to a depressed rate of shell formation. A separate mechanism resulted in increased mammillae height relative to mammillary layer thickness. For comparison, the structure of other types of thin shells was also examined. Incidence of egg shell breakage was positively related to the residue levels of pp′-DDE or dieldrin in intact eggs. Eggs with shells thinner than 240 μm were unlikely to survive incubation. Since such eggs tend to contain higher-than-average residues, sampling intact eggs leads to a mean egg residue figure that is lower than the real mean for the colony. Measuring shell thickness is suggested as a rapid and economical bioassay of DDE levels. Shell thinning leads to a proportionally greater reduction in shell strength, and breakage of shells less than 240 μm in thickness was probably due to a combination of shell fragility and aberrant behaviour by the brooding adults. Breakage of thicker shells was believed to be mainly due to deliberate destruction. Despite more than half of the pairs breaking their shells during some years, heron populations have not declined overall perhaps because of the ability of this species to lay repeat clutches. There are indications that egg residues of DDE and other organochlorines are decreasing, and shell thickness and the incidence of shell breakage are returning to normal.

Reproductive Tissue Activity in Hypothyroid or Heat Stressed Hens

The ability of the reproductive tract of the White Leghorn hen under heat stress or methimazole thyroid blockage to assimilate 32P was examined. Both treatments increase the uptake of 32P by the shell gland and egg shell but only heat stress increases 32P uptake by the ovaries and reduces shell thickness.The greater shell thinness under heat stress is related to an increased deposition of phosphate in the shell. Gilbert (1967) has reported an increased serum calcium, due to calcification inhibitions by phosphate, leading to an increased gonadotropin secretion.Thyroid blockage appears to stimulate TSH and gonadotropin LH secretion but in the absence of phosphate inhibition of calcification shell thickness is not affected. It is also speculated that hypothyroidism does not stimulate FSH secretion and therefore ovarian tissue is not activated by this condition.

Patterns of chlorinated hydrocarbon contamination in New Zealand sub-Antarctic and coastal marine birds

Eggs and tissues of seabirds breeding on the coast of South Island, New Zealand, the Snares Islands and the Auckland Islands in the sub-Antarctic were examined for chlorinated hydrocarbon contamination. Higher levels of both industrial and agricultural compounds in the birds from the New Zealand coastal area reflected local contamination sources. In the sub-Antarctic, DDE residues were several times lower and PCB levels were lower by 1–2 orders of magnitude than in species occupying comparable ecological niches at Amchitka on a comparable latitude in the North Pacific. Concentrations of DDE in an unhatched egg of the New Zealand falcon from the sub-Antarctic were sufficiently high, on the basis of comparison with related species, to be associated with significant shell thinning.