Amphibia-Reptilia 28 (2007): 277-285

Do crocodiles co-opt their sense of “touch” to “taste”? A possiblenew type of vertebrate sensory organ

Kate Jackson∗, Daniel R. Brooks

Abstract. We recount here two experiments carried out which suggest the existence of the first described integumentaryosmoreceptor of its kind in a vertebrate. Domed pressure receptors, present on the cranial scales of alligators have previouslybeen demonstrated to convey the sensation of “touch” when flattened by pressure. Here we find that morphologically similardomed sensory organs present on the post-cranial scales of crocodylid but not alligatorid crocodilians flatten when exposedto increased osmotic pressure, such as that experienced when swimming in sea water hyper-osmotic to the body fluids. Whencontact between the integument and the surrounding sea water solution is blocked, crocodiles are found to lose their ability todiscriminate salinities. We propose that the flattening of the sensory organ in hyper-osmotic sea water is sensed by the animalas “touch”, but interpreted as chemical information about its surroundings.

Introduction

Sensory organs are essential for the survival ofanimals, providing information about the sur-rounding environment, both external and inter-nal. The sensation of “touch” (mechanorecep-tion), and “taste” (chemoreception) are two im-portant senses served by two different majortypes of sensory organ: mechanoreceptors, inwhich the physical pressure of touch causes achange in the shape of the sensory organ, stim-ulating a nerve impulse, and chemoreceptors,such as taste buds, in which cell surface mole-cules recognise a particular chemical, induc-ing a chemical change in the chemosensory cellwhich is interpreted as information about thatspecific chemical (Liem et al., 2001).

But between mechanoreception and chemore-ception there exists a grey area: osmorecep-tion. First described by Verney (1946, 1947),osmoreceptors sense the osmolality of a sur-rounding solution, thus providing chemical in-formation, but the mechanism by which the os-molality is sensed is mechanical: osmoreceptorsare cells which expand or shrink in response tochanges in the osmotic gradient between themand their surroundings. In vertebrates, osmore-

Department of Zoology, University of Toronto, Toronto,Ontario, M5S 3G5, Canada∗Corresponding author; e-mail: k.jackson@utoronto.ca

ceptors are groups of specialised nerve cells, lo-cated in various places in the circulatory sys-tem, where they sense changes in plasma os-molality, thus allowing hormonal regulation ofNaCl in the blood (Sibbald et al., 1988; Olietand Bourque, 1993; Bourque et al., 1994; Wells,1998; Voisin and Bourque, 2002). Here we de-scribe what we believe may be an entirely newtype of vertebrate sensory organ: an osmorecep-tor on the integument.

Animals inhabiting marine or estuarine envi-ronments must cope with the hyper-osmotic orfluctuating osmolality by either osmoconform-ing or osmoregulating. Like all tetrapod ver-tebrates, the Estuarine Crocodile, Crocodylusporosus (Meyer, 1795; Crocodylidae) osmoreg-ulates (Taplin, 1988). It maintains a plasmaosmolality of approximately 300 mOsm kg−1

(Mazzotti and Dunson, 1984; Taplin, 1984)while living in coastal waters in which thesalinity may range from occasionally availablefresh or hypo-osmotic sea water to full strength(35 ppt) sea water. C. porosus possesses a suiteof adaptations which allow it to reduce osmoticwater loss while immersed in hyper-osmotic seawater.

Interesting to the evolutionary biologist, theseadaptations appear to be phylogenetically con-servative traits, representing differences be-tween crocodylid and alligatorid crocodilians,

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278 K. Jackson, D.R. Brooks

as opposed to being specific attributes of C.porosus (reviewed below). Although some cau-tion must be exercised in making generalisa-tions from studies of individual species, com-parative physiological studies across crocodil-ian taxa have found that of the two mainfamilies of living crocodilians, the crocodylids(crocodiles) and the alligatorids (alligators andcaimans), major physiological differences ex-ist at the family level in ability to tolerate es-tuarine conditions. All crocodylids studied (in-cluding freshwater species) are morphologicallyadapted for hyper-osmotic conditions, while allalligatorids lack these adaptations. These adap-tations and the geographical distribution of liv-ing crocodilians have been interpreted as goodevidence in support of a marine phase in anancestor of living crocodylids (e.g. Taplin andGrigg, 1989). Estuarine adaptations possessedby Crocodylids, and absent in alligatorids, in-clude reduced integumental permeability (re-viewed by Taplin, 1988), heavy keratinisation ofthe buccal epithelium (Taplin and Grigg, 1989),and lingual salt-secreting glands (Taplin andGrigg, 1981). An additional mechanism usedis that of behavioural osmoregulation: the ani-mal drinks opportunistically when the surround-ing water is hypo-osmotic to its body fluidsand avoids drinking when it is hyper-osmotic.Crocodylids, even freshwater species with noexperience of sea water, will avoid drinkinghyper-osmotic sea water, while freshwater al-ligatorids will freely drink full sea water, evento the point of death (Bentley and Schmidt-Nielsen, 1965; Mazzotti and Dunson, 1984;Lauren, 1985; Jackson et al., 1996a) (but seeDiscussion).

Here we explore a further morphologicaldifference between crocodylid and alligatoridcrocodilians: the distribution of sense organs onthe integument. The study here presented in-vestigates the possibility that there may a con-nection between sensory organs present on theintegument of crocodylids but not alligatorids,and the capacity for sensation of sea water os-molality and thus behavioural osmoregulation.

The study of sensory organs on the integu-ment of crocodilians has been subject to greatconfusion as a result of inconsistent termi-nology. Two distinct types of integumentarysense organs exist: (1) facial mechanoreceptors,present in all crocodilians, and (2) sense or-gans present on the post-cranial integument incrocodylids and gavialids, but absent in alli-gatorids. The facial sense organs were deter-mined on the basis of detailed morphologicalstudy (von During, 1973, 1974; von During andMiller, 1979) and physiological study (Soares,2002) to be mechanosensory in function. Thesefacial mechanoreceptors appear to the naked eyeas small, black dots, present several per scale onall the cranial scales. Magnified they are seen tobe dome-shaped protuberances, innervated be-neath by branches of the trigeminal nerve. Itis presumed that deformation of the dome bytouch is perceived as mechanosensation.

The post-cranial sense organs were not al-ways known to be sensory organs. These havereceived much attention because they are ofphylogenetic interest, as they are absent in al-ligatorids. These were once thought to pos-sibly be secretory pores (King and Brazaitis,1971; Wermuth, 1978; Brazaitis, 1987; Griggand Gans, 1993) or alternatively sensory struc-tures. Brazaitis (1987) proposed the term “in-tegumentary sense organs (ISOs)”, a term thatstuck.

Jackson et al. (1996b) studied the mor-phology of the ISOs of C. porosus, in de-tail, and found that the ISOs very much re-semble von During’s facial mechanoreceptors(“touch papillae”) in structure, except that theyare approximately 300% the size of the for-mer, present only one per scale (rarely asmany as three), and presumably innervated byspinal nerves rather than trigeminally (fig. 1).They concluded that the ISOs were definitelysense organs, but refrained, in the absence ofphysiological data, to commit themselves tomechanosensation as their function.

Soares (2002) generated terminological con-fusion by lumping together the two types of

New type of vertebrate sensory organ 279

Figure 1. Ventral scales of (a) a Crocodylid (Crocodylus porosus), and (b) an alligatorid, (Caiman crocodilus), showing thepcDPRs on the crocodylid but not the alligatorid.

sense organ under the single term “domed pres-sure receptors” (DPRs). Using “DPRs” as ablanket term for both crocodilian facial touchpapillae and for post-cranial crocodylid/gavialidISOs makes it impossible to discuss the two aspossibly different types of sense organ. Sincethat is exactly what we propose to do here,we will modify Soares’ terminology further, di-viding it into “facial DPRs” and “post-cranialDPRs” (“pcDPRs”).

The present study consisted of two experi-ments. The first tested the hypothesis that inC. porosus, sensory information from the post-cranial sense organs is necessary for behav-ioural osmoregulation to occur. The second ex-periment tested the hypothesis that an individualpost-cranial sense organ could react mechani-cally to osmotic changes in its surroundings,providing a mechanism by which the pcDPRcould function as an integumentary osmorecep-tor (fig. 2).

Materials and methods

Experiment #1

This experiment was designed to test the hypothesis thatcrocodiles rely on sensory information from their skin todetermine the salinity of the water in which they are swim-ming. The experiment was carried out at the Long KuanHung Crocodile Farm in Singapore, in an outdoor shed ex-posed to outdoor temperatures (approx. 30◦C). Two groups

of six one year-old C. porosus were kept out of the wateruntil their body mass was reduced by 10% due to water loss(a quantifiable way of stimulating thirst). Mean mass of an-imals in the control group was 676 g ± 13.4 sem. Meanmass of the animals in the experimental group was 701 g± 32.9 sem. (An unpaired, two-tailed t-test shows no sig-nificant difference in mass between the two groups, p =0.5002). This drying out period took 2-3 days, and the an-imals were kept in the dark, in silence, in opaque plasticbins with lids in order to minimise stress. The animals in theexperimental group were then coated with petroleum jelly,weighed, and allowed to swim in 35 ppt seawater for 15min, after which they were weighed to determine whetherany water had been ingested. They were then moved to freshwater and weighed again after 15 min exposure. The sameprotocol was followed with the control group except withoutthe petroleum jelly. For the experimental group, the increasein mass following exposure to sea water was compared withthe increase in mass following exposure to fresh water, us-ing a paired t-test. The control group was tested in the sameway.

Experiment #2

This experiment was designed to test the hypothesis thatthe height of the domed pcDPRs increases in fresh water,and decreases in seawater or other hyperosmotic solutions.The animal used was a one yr old C. porosus (total length72 cm), housed under laboratory conditions at the Univer-sity of Toronto, with water and air temperatures of 27◦ anda 12:12 light:dark light cycle. The enclosure included swim-ming and basking areas, and the animal was maintained ona diet of neonate mice and live minnows.

Four tanks were prepared, deep enough for the crocodileto float in. Two were fresh water (dechlorinated reverse-osmosis tap water), while the other two held the experimen-tal solutions. The first of these was 1000 mOsm (mOsm =mosmol·kg water−1) (∼35 ppt) seawater, mixed from In-stant OceanTM powder. The second was a solution of su-crose, iso-osmotic to the seawater solution (1000 mOsm).

280 K. Jackson, D.R. Brooks

Figure 2. A diagrammatic explanation of the hypothetical mechanism of osmosensation proposed here. In (a), as has beendemonstrated experimentally, a cranial DPR responds to touch by flattening, thus transducing a nerve impulse. In (b) wepropose that a pcDPR flattens similarly but in response to exposure to a hyper-osmotic sea water solution (full-strength seawater: 35 ppt). In this scenario the flattening of the dome generates a nerve impulse as for the mechanosensory cranial DPR,but this signal is interpreted by the animal as chemical information.

Osmolality of both solutions was verified by freezing pointdepression (Advanced InstrumentsTM micro-osmometer,model 3MO).

An area approximately 3 cm2 of ventral scales on thecrocodile was outlined in permanent marker. The crocodilewas left in the fresh water tank for one hour. The animal wasthen removed from the water and held firmly ventral sur-face up, as quickly as possible. A layer of the light-bodied,low viscosity impression material, ReprosilTM hydrophilicvinyl polysiloxane impression material (light body) (Jack-son, 1997) was immediately applied to the area of scalesoutlined, allowed to cure to a rubbery consistency for 7min. The layer of PVS was then peeled off, having formeda high-resolution impression of each outlined scale and ofeach pcDPR. The crocodile was then left in the seawater

tank for 10 minutes, and the procedure was repeated, cov-ering exactly the same outlined area of scales. The animalwas moved back to fresh water for a period of 10 min, and acast was made of the outlined scales after their second timein fresh water. Finally the crocodile was made to swim in atank of sucrose solution for ten minutes and a cast was madeof the outlined scales.

The PVS impressions of the ventral skin were cut intoindividual scales. Each individual scale impression fromthe fresh water (1), sea water, fresh water (2), and sucrosesolutions, was matched up so that the pcDPR of an identicalindividual scale was compared after exposure to all foursolutions. The pcDPR was visible with light microscopyas a circular depression in the PVS cast. In order to makea “positive” copy from the “negative” created by the PVS

New type of vertebrate sensory organ 281

impression, each impression was filled using a hypodermicneedle, with a droplet of Spurr’s low viscosity embeddingmedium (firm mixture) (Bozzola and Russel, 1992), whichwas allowed to cure for 24 hrs at a temperature of 70◦C. Thisprocedure produced a rigid, transparent, high-resolutioncast of the scale and the pcDPR. Each cast was mountedwith double-sided tape to a scanning electron microscope(SEM) stub, grounded with carbon glue, sputter-coated withgold, and examined using a Hitachi H-7000 SEM, at anacceleration voltage of 20 kV.

Examined with the SEM, each cast was tilted until thepcDPR cast appeared as a domed structure in lateral view.It was impossible to be certain that the cast in the imagewas perfectly in side-view, making height measurements ofthe dome unreliable. To overcome this problem, an initialside view was photographed, and then the stub was rotatedand two more images captured at 120◦ angles from oneanother. The mean of these three measurements was used asthe height of dome measurement. Measurements of domeheight were made using Image JTM software.

Heights of ISO casts were compared using a series ofpaired t-tests, to determine whether the height of the domeschanged following exposure to the three solutions: (1) Thetwo fresh water groups, (2) the sea water and the sucrosegroups, (3) the first fresh water group and the sea watergroup, (4) the second fresh water group and the sucrose.

Results

Experiment #1

In the control group a paired t-test showed a sig-nificantly greater mass increase following expo-sure to fresh water as opposed to sea water (p =0.0469, t = 2.266, 10 degrees freedom), indicat-ing that the animals drank no sea water, but diddrink when moved to fresh water, replicating theresults of numerous other studies of crocodyliddrinking behaviour (reviewed in Introduction).In the experimental group, by contrast, an in-crease in mass was observed following expo-sure to sea water as well as to fresh water, anda paired t-test revealed no significant differencebetween the mass increase in the two solutions(p = 0.3572, t = 1.014, 5 degrees freedom).This result indicated that the crocodiles drankfresh water and full sea water indiscriminatelywhen the integument was coated with petroleumjelly (fig. 3). The (not significant) difference inthe amount of fresh water ingested by animals inthe two groups is thought to result from the ex-perimental animals having partially filled their

stomachs with sea water prior to exposure tofresh water.

Experiment #2

Measurements were made of casts from n = 10individual pcDPRs, exposed to four experimen-tal conditions: fresh water (first time), sea wa-ter, fresh water (second time), and sucrose solu-tion. The resulting four columns of values weresubjected to a series of paired t-tests. A com-parison of the pcDPR dome height in follow-ing exposure to the two fresh water solutionsfound no significant difference (P = 0.5682,t = 0.59, df = 9). Similarly, a comparison ofthe sea water with the sucrose solution foundno significant difference (P = 0.66, t = 0.46,df = 9). Comparisons of fresh water (first time)with sea water, and of fresh water (second time)with sucrose solution both revealed very signif-icant differences (P = 0.0028, t = 4.07, df= 9 and P = 0.0021, t = 4.26, df = 9, re-spectively) (fig. 4). This result indicated that thedomed pcDPR becomes flattened when exposedto hyper-osmotic solutions. There was no diffe-rence in the degree of flattening induced by ex-posure to hyper-osmotic solutions of two differ-ent solutes.

Discussion

Comparison of this osmoreceptor with others

Von Seckendorff Hoff and Hillyard (1993) andNagai et al. (1999) in their studies of behav-ioural osmoregulation in the toad, Bufo puncta-tus (Baird and Girard, 1852; Bufonidae), con-cluded, based on trials of solutions of NaCl,urea, and amiloride in various combinations,that although the toads were detecting changesin NaCl osmolality, the mechanism by whichthis was accomplished was partially osmosen-sory, but also with a chemosensory component.This scenario is the only vertebrate examplesimilar to the osmoreceptors described here thatwe are aware of. The ecological function (avoid-ing uptake of hyperosmotic NaCl solutions) is

282 K. Jackson, D.R. Brooks

Figure 3. Amount drunk, as inferred from mass increase, (mean ± sem) by crocodiles exposed to sea water (SW) and tofresh water (FW). Animals in the experimental group have the integument coated with petroleum jelly. Animals in the controlgroup do not. (n = 6 in each group, mean mass = 688 g).

Figure 4. Height of casts of pcDPRs (mean ± sem) following exposure to fresh water, hyper-osmotic sea water, fresh watera second time, and hyper-osmotic sucrose solution. No significant difference between the two fresh water solutions (p > 0.5);No significant difference between sucrose and sea water solutions (p > 0.6); Significant difference between fresh water (firsttime) and sea water (p < 0.01); Significant difference between fresh water (second time) and sucrose (p < 0.01).

comparable, although the physiological mecha-nism is different.

The pcDPRs investigated in the present studydiffer greatly from previously described os-

moreceptors. In location and function they areintegumentary and sense the external environ-ment. In structure they are enormous (approx300 µm in diameter in a 1-m long crocodile).

New type of vertebrate sensory organ 283

They are multicellular structures as opposed tosingle, specialised neurons. In mechanism, theyrespond equally to solutions of sea water andsucrose. Surveying the literature, we have failedto find any sensory organ like them. We are notaware of other amniotes that have chemo- orosmo-sensory organs on the integument (Liemet al., 2001). We propose here to dispense withthe cumbersome term “pcDPRs” in favour ofthe more descriptive “integumentary osmore-ceptors” or “IOs”. Thus the cranial mechanore-ceptors present in both crocodylids and alliga-torids need a new name to replace “DPRs” thatwill distinguish them from the IOs. We proposereverting to Von During’s term, “touch papil-lae”.

Implications for crocodilian phylogeny

To date, studies of sensory organs in crocodil-ians have been restricted to crocodylids and al-ligatorids. The physiology of gavialids, sister-group to the latter two lineages is poorly stud-ied. Several reasons account for this omission.The living Gavialidae are represented by onlyone (Brochu, 2003) or possibly two (Harsh-man et al., 2003) species worldwide: the gharial,Gavialis gangeticus (Gray, 1831; Gavialidae),and possibly also the “false” gharial, Tomis-toma schlegelii (Müller, 1838; Crocodylidae),traditionally thought to be a crocodylid. Bothspecies are restricted to South Asia in distri-bution and are endangered species. They arepoorly represented in live collections, and arenot captive-bred outside of India, all these fac-tors making them relatively inaccessible to com-parative physiologists. However, a further rea-son, which ought not to be overlooked, is thetemptation by crocodilian physiologists to as-sume that what is true of the single speciesthey have studied in detail, is applicable to allcrocodilian species. Given the significant dif-ferences in many physiological adaptations tohyper-osmotic environments found between al-ligatorids and crocodylids (or at least thosespecies from these two groups which have beenstudied!), it may be rash to make assumptions

about the physiology of gavialids, which arethe group the most distantly related to all othercrocodilians.

With that caution in mind, structures ap-pearing to the naked eye to be IOs likethose of crocodylids, are present on the post-cranial scales of gavialids (K. Jackson, pers.obs.). Looking at the phylogenetic relation-ships among the living crocodilian families, twoequally parsimonious, and equally puzzling hy-potheses account for the phylogenetic distri-bution of IOs: Either IOs have been derivedindependently in crocodylid and gavialid lin-eages, or, the presence of IOs represents theplesiomorphic condition for modern crocodil-ians, and these structures have been secondarilylost in alligatorids. Detailed morphological andphysiological study of the putative IOs of gavi-alids have the potential to resolve this question.

Sensing and interpreting

A further puzzle to be solved is the capac-ity for behavioural osmoregulation by estuar-ine populations of the alligatorid, A. mississip-piensis, observed by Jackson et al. (1996a). Al-though all alligatorids of freshwater origin stud-ied (including freshwater populations of A. mis-sissippiensis) have been found to drink freshwater and hyper-osmotic sea water indiscrimi-nately, as described in the Introduction, Jacksonet al. (1996a) found hatchling alligators from acoastal population in Georgia, USA, to discrim-inate salinities and to avoid drinking 35 ppt seawater. To discriminate salinities, these individu-als, which were morphologically indistinguish-able from individuals from freshwater popula-tions, must possess a mechanism to taste NaCl,and an instinct to avoid drinking hyperosmoticsolutions of it.

Very similar to the above scenario for A.missippiensis, differences in capacity for be-havioural osmoregulation have been demon-strated between coastal and freshwater popula-tions of snakes, Nerodia (Linnaeus, 1758; Col-ubridae) (Dunson, 1980) and turtles, Chelydra(Linnaeus, 1758; Chelydridae) (Dunson, 1986).

284 K. Jackson, D.R. Brooks

In both cases these are the same species withidentical sensory organs morphology and phys-iology. The behavioural differences observed indrinking appear to be instincts evolved recentlyin populations exposed to estuarine conditions.Juveniles captive-bred in fresh water from par-ents of estuarine origin retain the capacity toavoid drinking hyperosmotic sea water.

But how are estuarine alligators able to de-termine the salinity of their surroundings, lack-ing IOs? An obvious possibility is that theytaste NaCl with chemosensory organs on thetongue. Weldon and Ferguson (1993) demon-strated the capacity for lingual chemosensa-tion in A. mississippiensis. However, no othercrocodilian species were included in their study.If all crocodilians can sense NaCl lingually,what is the role of the IOs, and why do C. poro-sus lose their capacity to sense hyper-osmoticsalinities when the IOs (but not the tongue) arecovered?

Here we propose that a difference in capacityfor lingual chemosensation may exist betweencrocodylids and alligatorids. In crocodylids thesurface of the buccal epithelium is heavilycoated with keratin (Taplin and Grigg, 1989).This is thought to represent an adaptation to re-duce osmotic water loss across the large surfacearea of the buccal epithelium in a crocodile rest-ing with its jaws open. One possible adaptivescenario is that the keratin coating, though ef-fective at reducing osmotic water loss also hadthe effect of inhibiting lingual chemosensation.This may have set the stage for the adaptationof other sensory organs, the IOs, to take on thefunction of providing sensory information aboutsea water osmolality. Investigations of the ca-pacity for lingual chemosensation in a broaderrange of crocodilian species may shed light onthis question.

The result of our first experiment indicatesthat C. porosus is unable to behaviourally os-moregulate even with the lingual epithelium in-tact, when contact between the integument andthe surrounding solution is blocked. From thisresult we can only conclude that either, (a) C.

porosus cannot taste NaCl with its tongue, or(b) sensation of hyperosmotic NaCl from thetongue is not interpreted by the crocodile as asignal to avoid drinking. We believe the formerscenario most likely to account for the resultswe observed in C. porosus. We propose a sce-nario like the latter to account for the drinkingof hyper-osmotic sea water by freshwater alli-gatorids. That is, they sense but do not interpret.

Conclusions

From the results of the above experiments weconclude that C. porosus relies on sensory infor-mation from the integument to distinguish freshwater from hyper-osmotic sea water. Investiga-tion of the IO, the only sensory organ presenton the post-cranial integument, shows that thisdomed structure changes shape (flattens) on ex-posure to hyper-osmotic solutions. Much as flat-tening of the cranial touch papillae of A. missis-sippiensis has been shown by Soares (2002) tobe sensed by the animal and to be interpretedby the animal as “touch”, we propose that sim-ilar shape change in similar structures on thepost-cranial integument of C. porosus is sim-ilarly sensed, but is interpreted by the animalas chemical information about the osmolality ofits surroundings. We predict that the same re-sult will be found in other crocodylid (and pos-sibly gavialid) species. We propose that the IOrepresents an addition to the arsenal of morpho-logical adaptations to estuarine conditions thatcrocodylid crocodilians possess. To the best ofour knowledge this is the first report of an in-tegumentary osmoreceptor in any tetrapod ver-tebrate.

Acknowledgements. We thank Norm White, Lee BakKuan, Lee Peilin, Yap Boon Chark, Lam Toon Jin, KentVliet, John Brueggen, David Kledzik, Flavio Morrissiey,Eric Lin, Adam Britton, and Charlie Manolis for facilitatingthis work in many and various ways. This project wasfunded by a Natural Sciences and Engineering ResearchCouncil of Canada (NSERC) Postdoctoral Fellowship toKJ, and an NSERC Individual Discovery Grant to DRB.Experiments on crocodiles were authorised by Universityof Toronto Animal Use Committee protocol #20005148.

New type of vertebrate sensory organ 285

References

Bentley, P.J., Schmidt-Nielsen, K. (1965): Permeability towater and sodium of the crocodilian, Caiman sclerops.J. Cell. Compar. Physl. 66: 303-310.

Bourque, C.W., Oliet, S.H.R., Richard, D. (1994): Osmore-ceptors, Osmoreception, and Osmoregulation. Front.Neuroendocrin. 15: 231-274.

Bozzola, J.J., Russel, L.D. (1992): Electron Microscopy:Principles and Techniques for Biologists. Jones andBartlett Publishers, Sudbury.

Brochu, C.A. (2003): Phylogenetic approaches towardcrocodilian history. Annu. Rev. Earth Pl. Sc. 31: 357-397.

Brazaitis, P. (1987): Identification of crocodilian skins andproducts. In: Wildlife Management: Crocodiles and Al-ligators, pp. 373-386. Webb, G.J., Manolis, S.C., White-head, P.J., Eds, Surrey Beatty & Sons, Chipping Norton,NSW.

Dunson, W.A. (1980): The relation of sodium and water bal-ance to survival in sea water of estuarine and freshwaterraces of the snakes, Nerodia fasciata, N. sipedon, and N.valida. Copeia 1980: 268-280.

Dunson, W.A. (1986): Estuarine populations of the Snap-ping turtle (Chelydra) as a model for the evolution ofmarine adaptation in reptiles. Copeia 1986: 741-756.

Grigg, G.C., Gans, C. (1993): Morphology and physiol-ogy of the Crocodilia. In: Fauna of Australia, Vol. 2A:Amphibia and Reptilia, pp. 326-336. Glasby, C.J., Ross,G.J.B., Beesley, P.L., Eds, Australian Government Pub-lishing Service, Canberra.

Harshman, J., Huddleston, C.J., Bollback, J.P., Parsons, T.J.,Braun, M.J. (2003): True and false gharials: A nucleargene phylogeny of Crocodylia. Syst. Biol. 52: 386-402.

Hillyard, S.D. (1999): Behavioral, molecular and integrativemechanisms of amphibian osmoregulation. J. Exp. Biol.283: 662-674.

Jackson, K., Butler, D.G., Brooks, D.R. (1996a): Habi-tat and phylogeny influence salinity discrimination incrocodilians: implications for osmoregulatory physiol-ogy and historical biogeography. Biol. J. Linn. Soc. 58:371-383.

Jackson, K., Butler, D.G., Youson, J.H. (1996b): Morphol-ogy and ultrastructure of possible integumentary senseorgans in the Estuarine Crocodile (Crocodylus porosus).J. Morphol. 229: 315-324.

Jackson, K. (1997): Scanning electron microscopy of high-resolution casts for the study of tooth surface morphol-ogy in snakes. Herpetol. Rev. 28: 75-76.

King, F.W., Brazaitis, P. (1971): Species identification ofcommercial crocodilian skins. Zoologica-New York:New York Zoological Society.

Lauren, D.J. (1985): The effect of chronic saline exposureon the electrolyte balance, nitrogen metabolism, andcorticosterone titer of the American Alligator, Alligatormississipiensis. Comp. Bioch. Physiol. 81A: 217-223.

Liem, K.F., Bemis, W.E., Walker, W.F., Jr., Grande, L.(2001): Functional Anatomy of the Vertebrates: AnEvolutionary Perspective, 3rd Edition. Harcourt CollegePublishers, Philadelphia.

Mazzotti, F.J., Dunson, W.A. (1984): Adaptations ofCrocodylus acutus and Alligator for life in saline water.Comp. Bioch. Physiol. 79A: 641-646.

Nagai, T., Koyama, H., von Seckendorff Hoff, K., Hillyard,S.D. (1999): Desert toads discriminate salt taste withchemosensory function of the ventral skin. J. Comp.Neurol. 408: 125-136.

Oliet, S.H.R., Bourque, C.W. (1993): Mechanosensitivechannels transduce osmosensitivity in supraoptic neu-rons. Nature 364: 341-343.

Sibbald, J.R., Hubbard, J.I., Sirett, N.E. (1988): Responsesfrom osmosensitive neurons of the rat subfornical organin vitro. Brain Res. 461: 205-214.

Soares, D. (2002): An ancient sensory organ in crocodilians.Nature 417: 241-242.

Taplin, L.E. (1984): Drinking of fresh water but not seawater by the Estuarine Crocodile (Crocodylus porosus).Comp. Bioch. Physiol. 77A: 763-767.

Taplin, L.E. (1988): Osmoregulation in crocodilians. Biol.Rev. 63: 333-337.

Taplin, L.E., Grigg, G.C. (1981): Salt glands in the tongueof the Estuarine Crocodile, Crocodylus porosus. Science212: 1045-1046.

Taplin, L.E., Grigg, G.C. (1989): Historical zoogeographyof the eusuchian crocodilians: a physiological perspec-tive. Am. Zool. 29: 885-901.

Taplin, L.E., Grigg, G.C., Beard, L. (1993): Osmoregulationof the Australian freshwater crocodile, Crocodylus john-stoni, in fresh and saline waters. J. Comp. Physiol. B.163: 70-73.

Verney, E.B. (1946): Absorption and excretion of water: Theantidiuretic hormone. Lancet 6430: 739-783.

Verney, E.B. (1947): The antidiuretic hormone and factorswhich determine its release. P. Roy. Soc. B Biol. Sci.135: 25-106.

Voisin, D.L., Bourque, C.W. (2002): Integration of sodiumand osmosensory signals in vasopressin neurons. TrendsNeurosci. 25: 199-205.

von Seckendorff Hoff, K., Hillyard, S.D. (1993): Toads tastesodium with their skin: sensory function in a transport-ing epithelium. J. Exp. Biol. 183: 347-351.

von During, M. (1973): The ultrastructure of lamellatedmechanoreceptors in the skin of reptiles. Z. Anat. En-twicklungs. 143: 81-94.

von During, M. (1974): The ultrastructure of the cutaneousreceptors in the skin of Ca. crocodilus. AbhandlungenRhein.-Westfal. Akad. 53: 123-134.

von During, M., Miller, M.R. (1979): Sensory nerve endingsin the skin and deeper structures. In: Biology of the Rep-tilia, 9. Neurology A, pp. 407-442. Gans, C., Northcutt,R.G., Ulinski, P., Eds, Academic Press, New York.

Weldon, P.J., Ferguson, M.W.J. (1993): Chemoreception incrocodilians: Anatomy, natural history, and empiricalresults. Brain Behav. Evolut. 41: 239-245.

Wells, T. (1998): Vesicular osmometers, vasopressin secre-tion and aquaporin-4: A new mechanism for osmorecep-tion? Mol. Cell. Endocrinol. 136: 103-107.

Wermuth, H., Fuchs, K. (1978): Bestimmen von Krokodilenund ihrer Häute: Eine Anleitungzum Identifizieren derArt- und Rassen-Zugehörigkeit der Krokodile. GustavFischer Verlag, Stuttgart.

Received: July 19, 2006. Accepted: September 18, 2006.