Table Of ContentReference: Biol Bull. 181: 205-215. (October, 1991)
Ontogeny Versus Phylogeny in Determining Patterns of
Chemoreception: Initial Studies with Fiddler Crabs
MARC J. WEISSBURG'* AND RICHARD K. ZIMMER-FAUST12*
1Marine EnvironmentalSciences Consortium and~Department ofBiology. University ofAlabama,
P.O. Box 369-370. Dauphin Island. Alabama, 36528
Abstract. Fiddlercrab(L'ca longisignalis)first stagezoea ecological implications. Several broad scale patterns are
and adults were assayed for behavioral responses to 16 apparent, based on currentwork. In general,theparticular
amino acids and sugars. Larval chemosensitivity was ex- spectrum ofsubstances eliciting a response is correlated
amined using computer-video motion analysis ofswim- with diet. Flesh eaters tend to respond strongly to amino
ming behavior. Adult sensitivity was assayed by deter- acids, sometimes to organic acids and nucleotides, and
mining the substances that elicit feeding. The pattern of aregenerally unresponsiveto sugars(see reviewsofAche,
chemoreception expressed by V. longisignalis adults is 1982; Carr, 1988; Laverack, 1988; Zimmer-Faust, 1989).
strongly correlated with that measured previously in adult The particular substances generating large responses are
sand fiddler crabs, Uca pugilator. This concordance often dominant organic components in prey tissues. In
amongabilities probably reflectsshared trophic ecologies contrast to flesh eaters, herbivorous organisms are highly
of the two species. In contrast, a quantitative analysis responsive to carbohydrates (Zimmer et al.. 1979; Rob-
shows no significant correlation between the sets ofcom- ertson el al, 1981; Trott and Robertson, 1984; Rittschof
pounds inducing chemoreceptive behavior by larval and and Buswell, 1989), which occurasalgal storage products
adult U. longisignalis. The strongest responses (by both and components ofextracellular mucous sheaths (Fogg,
stages) are elicited by substances found in potential prey, 1966;Fogg#a/.. 1973;Craigie, 1974;Darley, 1977; Paul-
and differences in prey types among larvae and adults sen et al., 1978) and sometimes to amino acids that are
appear responsible forthe lack ofcorrelation. Larvae do, characteristic ofalgal material.
however, respond to substances abundant in prey con- In spite ofbroad scale associations between diet and
sumed by adults, even though these substancesareabsent, theresponsespectrumoforganisms, relatedorecologically
oroccurat low levels, in larval prey. Adults, on the other similar species can display quite dissimilar fine scale pat-
hand, appear insensitive to compounds that cue only lar- terns of chemosensitivity. The spiny lobsters Panulirus
val food, but which are maximally stimulatory to larvae. argus and P. interruptus each respond to a wide array of
Consequently, ourresultsindicatethattheabilitiesofone amino and organic acids, and exhibit a broad overlap in
life-history stage may be constrained, through develop- the substances eliciting a response. However, within the
ment, by the requirements oflaterstages. The patternsof overlapping subset of stimulatory substances, these two
correlation among adults ofdifferent species, and among species are differentially responsive to the same com-
life-history stageswithin aspecies, indicate that both eco- pounds. Forexample, P. argusismost responsivetocitric
logical context and developmental factors influence pat- acid, ascorbic acid, succinic acid, taurine, and glycine
terns ofchemosensitivity. (Acheetal., 1978), whereasP. interruptusrespondsmostly
Introduction to glycine, alanine, serine, succinic acid, oxalic acid, and
Studieson marineinvertebrateshaveengenderedalarge adenosinetriphosphate (Zimmer-Faust etal., 1984; Zim-
body ofliterature on chemosensory properties and their mer-Faust, 1987). Similarly, the coexisting copepods
Acartia hudsonica and Eurytenwra herdmani both re-
Received 26 April 1991;accepted 23July 1991. spond to amino acids, although Acartia is stimulated
* Presentaddress: Department ofBiology and Marine Sciences Pro- mostly by aliphatic amino acids (e.g., leucine and valine)
gram. UniversityofSouthCarolina.Columbia. SouthCarolina, 29208. whereasEurytemoraismostresponsivetothedicarboxylic
205
206 M. J. WEISSBURG ET AL
amino acids (i.e., glutamic and aspartic acid). When re- ming behavior in response to substances released by a
sponsestothesamecompoundsareobserved, eachspecies documented prey species. Thesolutionwasprepared from
reactstothem indifferentways(PouletandOuellet, 1982). acultureofyoungbrineshrimp(Artemiasalina) astan-
Acartia, for instance, is repelled by aspartic acid, while dard food source used in rearing carnivorous larvae of
Eurytemora is attracted to it. marine invertebrates, including fiddler crabs (e.g.,
To date, explanations ofchemosensory abilities have Herrnkind, 1968; Rabalais and Cameron, 1983; Christy,
focussed largely on adaptive hypotheses. It is implicitly 1989). Anemia nauplii were hatched from cultures in-
assumed that the response magnitude variesdirectly with cubated in 20 ppt artificial seawater media (ASW) (Forty
the relative concentration ofsubstances in prey, thus al- Fathoms Marinemix) prepared with HPLC grade deion-
lowing animals to locate potential food items more effi- ized water. When nauplii were 1-3 h old, 100 ml ofthe
ciently. Other explanations exist, especially where organ- batch culture were spun down in a Beckman (Model J2-
isms progress through a series of life-history stages re- 21) centrifuge at 10,400 X g and 2C for 30 min. The
quiringdifferent chemosensory abilities. At any one stage, resultingsupernatantwasdecanted, filteredthrougha0.45
abilities (1) may be functionally important, (2) may rep- ^m membrane filter,dilutedtoaconcentration of15 ppt,
resent incipient abilities required at a later stage, or (3) then stored at -87C.
may be relictual abilities that were required by previous Amino acids and sugars. Chemicals were prepared in
life-history stages. Developmental explanations for pat- ASW media at a concentration of 15.0 ppt, using HPLC
ternsofchemosensitivity maybeespecially important for grade deionized water. The following amino acids were
marine organisms where differences in the habitats oc- usedastestcompounds: /i-alanine(ALA),arginine(ARG),
cupied by larval and adult stages can be large. cysteine (CYS), glutamic acid (GAD), glycine (GLY),
Fiddler crabs (genus Uca) provide an excellent system proline (PRO), taurine (TAU), and valine (VAL). The
with which to quantitatively investigate correlations of following sugars were used: galactose (GAL), glucose
abilities among species, and life-history stages. All adult (GLU), maltose (MAL), sorbose (SOR), sucrose (SUC),
fiddler crabs are deposit feeders, foraging on intertidal and trehalose (TRE). Thesecompoundswereselected be-
sediments(Crane, 1975). Trophic habitsarequiteuniform cause they are important chemical constituents in either
(Teal, 1958; Miller, 1961; Crane, 1975; Robertson et al, plant or animal components ofthe diet of marine zoo-
1980; Weissburg, 1990), and several species often coexist plankters (e.g., Jeffries, 1969), or have been found stim-
within a limited geographic area (Crane, 1975). Further, ulatory to other deposit feeding crustaceans (e.g., Rob-
adult and larval habitats are markedly different in terms ertson et al., 1981; Rittschofand Buswell, 1989). Except
of abundance levels, and specific components of the where noted, all aminoacidswere L-isomers, and all sug-
chemicalenvironment(e.g., Degensetal., 1964;Andrews ars D-isomers. Chemicals were all reagent grade, obtained
etui., 1971; Clark f/fl/.. 1972; Mopperet al., 1980; Mop- from the Sigma Chemical Company. Test solutionswerMe
perand Lindroth, 1982). prepared in single batches at a concentration of 10M~4
The investigation reported here wasdesigned toexplore for experiments performed using larvae, or 10"' for
the potential influence of both ecological and develop- experiments performed using adults, divided into 20 ml
mental processes on adult and larval chemosensitivity. aliquots, and stored at 87C until needed. Where nec-
The logical starting point for detecting developmental essary, solutionswereadjusted topH 8.0using 1 A/NaOH
constraints on chemoreception is to determine patterns or HC1.
ofchemosensitivity at the endpoints ofthe ontogenetic
series. Similarly, the concordance ofchemosensory abil- Experiments testing larval chemosensory abilities
ities in at least one pair of species with similar trophic
habits should be established before a detailed survey of Gravid Uca longisignalis females, collected from Air-
ecologically and taxonomically related species is at- port Marsh, Dauphin Island, Alabama, were held until
tempted. Therefore, we have examined, quantitatively, theirbroodshatched. Femaleswereplaced in fingerbowls
the chemosensory abilities of first stage zoea and adult containing a small amount of filtered 15 ppt seawater,
stages ofthe crab, Uca longisignalis, and have compared and kept in incubators at 26 C under a 14:10 light-dark
the properties ofdifferent life-history stages in the same cycle. Females were checked for brood release twice per
species, andamongadultsofU. longisignalisand U. pug- day, and waterwaschanged daily. Usually several females
ilator. simultaneously released larvae, and these broods were
combined, and held in filtered seawater at densities of
Materials and Methods
about 10 larvae/ml prior to assaying chemosensory be-
Preparation oftest solutions havior.
Metabolites released by Artemia salina in culture. An Experimentson larvalchemosensoryabilitiesbegan24-
initial experiment was performed toexplore larval swim- 36 h after hatching and ran for 24 h. We specifically re-
CHEMORECEPTION IN FIDDLER CRABS 207
stricted ourexperiments to these early stages because lar- asthetranslational movementthroughspaceofanobject's
vae are only competent to settle on substrates and me- centroid (see Buskey and Stoecker, 1989; Zimmer-Faust
tamorphose to the adult form later in the developmental and Tamburri, in prep.). The video sampling rate ofthe
series, after they have reached the megalopa stage (e.g., motion analysis hardware was set at 10 frames/s, and,
Epifanio el a/., 1988). Therefore, behavior in response to based on preliminary results, only the first 5 min ofeach
test compounds cannot be ascribed to substrate selection trial were used in the analysis.
and settlement processes. Twenty larvae were transferred Analysis of larval behavior consisted of determining
in 5 ml ofASW into plexiglass microcosms (3 cm long swimming speed and net-to-gross displacement ratio
X 3 cm wide X 5 cm high), and 20 ml ofthe 10 4 A/test (NGDR) for each larval swimming path. The NGDR is
solution wasgently added. Afterabout 30 s ofgentle stir- the ratio ofthe linear distance between the starting and
ring to disperse the test compound, the microcosm was ending points (net distance) and the total distance tra-
placed into a darkened chamber, and larval swimming versed bythe path (grossdistance). The NGDR measures
behaviorwas recorded. Each trial was video-recorded for thetendencyofpathstobecircularortwisted, andreaches
a 10-min period. Order of the test solutions was deter- a maximum value of 1.0 for paths that are completely
mined by a random numbers table, subject to the con- straight. An NGDR ofzero implies a looped or circular
straintsthat: aspecificcompound was not presented twice path, with the origin and endpoint occurring at the same
in succession; and that seawater treatments were per- spatial coordinates. For each test substance, data for all
formed after every block of five test solutions. Five to paths were pooled across trials, and means compared to
eight replicate trials of each test compound were per- swimmingbehaviorobservedinseawaterusingaStudent's
formed. Individual larvae were tested in only one trial /-test. Experiments exposing larvae to Artemia culture
and then discarded. water and test compounds indicated mean speeds, and
Larval swimmingbehaviorin the horizontal planewas NGDRs did not differ significantly among paths during
filmed using a Sony infrared-sensitive CCD camera the 5-min analysis period.
(model HVM-200)equippedwithaTamron 180mm lens, Based on initial findings, glucose, valine, and trehalose
mountedbeneath themicrocosm, andfocused 1.0cm(25 were selected for dose-response trials because they were
to 35 larval body lengths) above the bottom. Similarly among the compounds producing the largest changes in
sized chambers and techniques have been successfully larval swimmingbehavior. Usingtheabove protocol, lar-
used for video recording locomotory behavior of small val swimmingbehaviorwasdetermined forlarvaeexposed
zooplankters, including copepods (Buskey, 1984), tintin- to a series ofhalflog-step dilutions from 10~4 to 10~6 M.
nids (Buskey and Stoecker, 1989), and polychaete (But- Three replicatetrialswere runat eachconcentration, and
man et ai, 1988) and molluscan larvae (Zimmer-Faust within each dilution series the order ofpresentation was
and Tamburri, in prep.). The chamber was illuminated randomly determined. Seawater controls were run every
with an infrared light source (>830 nm) oriented 90 to fifth trial, as before. Solutions were prepared by serial di-
the axis ofthe video camera. The field of view was 6.9 lutions of the primary stock used in the initial assays.
X 6.8 mm, about 6.4% ofthe total cross-sectional area of Individual larvaeweretestedonce, thendiscarded. Swim-
NGDR
the microcosm. Depthoffieldwasdetermined by mount- ming speed and were regressed on concentration
ing a pipette (tip diameter = 300 ^m) on a micromanip- using SAS regression procedures (SAS Institute, Carey,
ulator calibrated in 10 ^m increments. Using the com- North Carolina).
puter-video motion analyzer (see below) with gray scale
threshold set fordiscriminating larvae, the pipette tipwas Experiments testingadult chemosensory abilities
moved vertically to determine the limits at which thean-
alyzer would 'recognize' the tip as an object. The recog- Adult Uca longisignalis, also collected from Airport
nition distance was 1.5 mm in the vertical dimension. Marsh, were held in a 1 X 2 m sea table equipped with a
Quantification ofswimming speed and turning behav- continuous flow offiltered, UV sterilized seawater, and a
iorwasaccomplishedbyreplayingeachvideotapethrough sandy substrate allowing animals to construct burrows.
a computer video motion analysis system (Motion Animals had continuous access to both shallow water (5
AnalysisCorp. Model VP 1 10and ExpertVision software cm depth) and exposed sandy regions. Air temperature
package) interfaced with an Amdek 386 microcomputer. varied between 27-32C, watertemperaturewas24-27C
The system constructsa digitized record ofthe raw video at a salinity of 18-20 ppt. Animals were fed a diet of
data, using a gray scale detector to enhance the contrast commercial fish food pellets, and the benthic diatom Cv-
between objects in the video field and background. The lindrotheca closteriiim. Forty-eight hours prior to exper-
outlinesoftheobjectsaredefined,andthex,ycoordinates imentation, animals were removed from sea tables, and
ofthecentroid (geometriccenter)arecalculated. The path in small groups, isolated in bowls containing clean azoic
ofan object is reconstructed, on a frame-by-frame basis. substrate and 15 ppt ASW held at 26C.
208 M. J. WEISSBURG ET AL
Chemosensory assays were conducted on individual Approximately 75% ofthe animals met the criterion of
mm
adult crabs (mean carapace width: 18.4 1.8 SD) exhibitingsamplingbehavior,andadditionalanimalswere
exposed to one ofthe 16 amino acids or sugars used pre- tested as needed to produce sample sizes of 15 animals
viously in tests with larvae. A 40-1 aquarium was subdi- ofeach sex per compound. Thus, results cannot be con-
vided intothree sections usingopaque partitions, andthe sidered artifacts of differences in sampling frequency
bottom lined with a layer ofclean beach sand. The sand amongcompounds. Thetotal timespent feedingwasalso
was sloped so that when approximately 200 ml ofASW measured to indicate the intensity ofthe response, and
wasadded, thebackthirdofeach section containedabout was recorded to the nearest 1 s with the time counter on
1 cm of standing water. This arrangement allows crabs the video tape recorder.
access to water required in feeding (Miller, 1961), and Preliminary results indicated that males and females
crabsforagereadilyinthistypeofarena(Weissburg, 1990). were similar in their responses, so the data from both
A glass cover maintained humidity. sexes were pooled. A William's corrected G-test was used
Each crab was presented with a food patch embedded todetermine whetherthe percentage ofanimals respond-
in the sand at the front ofthe test section. The patch was ing to a substance wasgreater than that ofseawater con-
a2.3 cm diameterculturedish, containing4.0gsubstrate trols. Time spent feeding was analyzed using a Student's
mixed with 3 ml ofthe test solution. Based on the work /-test. Animals did not feed on theseawatercontrols, giv-
of Robertson el al. (1981), test solutions were prepared ing feeding times of0.00 (see Results). Because feeding
at 10"' A/, because this concentration proved maximally times on other compounds cannot be less than zero, this
stimulatory to adult U. pugilator. The parallel method- wasaone-tailed /-testtodeterminewhetherfeedingtimes
ology employed by Robertson el al. (1981) and by us fa- were significantly greater than zero.
cilitatesaquantitative comparison between the two stud-
ies. The substrate was mud, collected from a location at Comparing lan'alandadult chemosensory responses
Airport Marsh where crabs were observed feeding. Sedi-
mm
ment waswashed through aM1 sieve, ashed overnight Responses of larval and adult Uca longisignalis were
at 600C, treated in hot 10 HC1 for 8 h, then washed quantitatively compared using a non-parametric corre-
repeatedly with HPLC-grade distilled waterto removeall lation analysis. Kendal'sTauwascomputed based onthe
traces ofacid. Feeding experiments took place at 26C. rankingsofthesignificance testsoflarvaeand adults. Each
Crabswereallowed to feed for2 h while theirbehavior stimulatory compound (for either life history stage) was
wasrecorded on videotape, usinga NECblack and white ranked by the /-value (larvae) or G-value (adults). For
CCD video camera (Model Ti 22AII) equipped with a larvae, the highest /-value from measurement of either
Tokina 35-70 mm macrofocal lens and 2X multiplier, NGDR or swimming speed was used (see Results). We
connectedtoaJVC(Model BR 3200U)taperecorderand ranked the G-scores for the adults, because, unlike the /-
aPanasonicTR-124-MA videomonitor. Allthreeanimals test, they are lessaffected by variation in thetimeat which
in the aquarium were visible, and at the front ofthe tank an animal commenced feeding. If, forinstance, an animal
(where the patches were embedded), the image ofthe an- began feeding nearthe end ofa trial, feeding times could
imals was magnified by approximately 25%, facilitating betruncated asan artifact oftheexperimental trial length,
accurate analysis offeeding behavior. Each chemical was whereas estimates of the percentage of animals feeding
tested on 15 individuals ofeach sex. are immune to the timing ofthe response. In any case,
The tapes were reviewed and the frequency offeeding results obtained using /-test scores differ little from an
induction was determined, where feeding was defined as analysisdone usingthe G-test scores(see Results). Because
morethanthreesuccessiveliftsofthechelaetothemouth we are interested in comparing the subsets ofchemicals
in any 5-s interval. We chose this criterion to be conser- stimulatory to each life-history stage, compounds pro-
vative in distinguishing a feeding response from patch ducing no response in both larvae and adults were ex-
sampling behavior. We observed that 75-80% ofthe an- cluded from the correlation analysis.
imals, including those tested on seawater controls, lifted Responses of adult Uca longisignalis and adult Uca
a chelae to their mouth at least once during a trial, al- pugilator were also quantitatively compared using Ken-
though none of the animals lifted its chelae more than dal's Tau. Rankings for Uca pugilator were taken from
threeconsecutive timesin sedimentscontainingonlysea- Robertson et al. (1981), giving the number offoodballs/
water. Fiddlercrabspossesschemosensitive organsonthe animal produced by crabs foraging on a particular sub-
minor chelae (Robertson el al., 1981). and probing is a strate and compound. The rankings for the stimulus in-
mechanism to allow animals to determine the contents tensityofeachcompound on U. pugilatorwerecomputed
ofeach patch. Three consecutive lifts seems to discrimi- by averaging results from all trials performed presenting
nate feeding from sampling. Any animal that was not a particular compound. Several compounds were tested
scored as samplingthe patch wasexcluded from analysis. in only one experiment, and theconfidence limits forthe
CHEMORECEPTION IN FIDDLER CRABS 209
Table I
Samplesizesandt-va!liesforUca longisignalislarvalswimming
responsestochemicaltestsolutions, relativetoseawatercontrols
Compound
210 M. J. WEISSBURG ET AL.
monosaccharide glucose, and the glucose-containing di-
saccharides, maltose and sucrose, induced the greatest
percentage ofanimals to feed. Feeding was also elicited
bythesugars,trehaloseandsorbose,buttoamuch smaller
degree, and the amino acids serine, glutamic acid, and /3-
alanine also provoked minor responses. Except for valine,
which generated a non-significant increase in feeding,
none ofthe other compounds elicited any behavioral re-
240
sponse at all G-values were 0.00 (Table II).
The feeding intensity elicited by each ofthe test com-
D
pounds is also given in Table II, which lists foragingtime
for each treatment. All of the compounds that caused
crabsto feed generated feedingtimes significantly greater
than zero(one-tailed Mest)and, in general, the magnitude
of response initiation and intensity was correlated. A
Kendal's Tau, based on the rankings of feeding percent
240
256 256
x coordinate (pixels) < coordinate (pixels)
Figure3. Typicalswimmingpathsinthehorizontalplanebylarvae CO
exposed toseawaterand totest substancescausing maximal responses.
The x,y coordinates are in units ofpicture elements (pixels), where 1
pixel = 0.028 mm. All substances were tested at 10 4 M: A. seawater,
B.glucose,C.valine.andD.trehalose.Relativetoseawater,larvaeswim
fasteringlucose,fasterandstraighterinvaline,andturnmorefrequently
in trehalose.
seawater, and /-tests yielded values of 0.0022-1.183 (P
> 0.20,Table I). Mean NGDR valuesfornoNnG-sDtiRmulatory
compounds were all within 0.02 ofthe in sea-
water, and /-values ranged from 0.020 to 1.77 (P> 0.05,
Table
I).
The dose-response curves for glucose, valine, and tre-
halosegenerally displayalog-linearform (Fig. 4), allowing
analysis by linear regression. There is a significant rela-
tionship between solution strength and response magni-
tude for all compounds. Responses to valine display the
most sensitivity to dosage (slope = 0.097), followed by
glucose (slope = 0.062), with responsestotrehalose being
leastsensitivetodosage(slope = -0.048). They-intercept
ofeach curve is significantly different from seawatercon-
> > P
trols, according to a /-test (Fig. 4, / 8.94, d.f. 229,
<0.01, allcomparisons). Larvaethereforerespondtomi-
cromolar concentrations, and can probably respond to
sub-micromolarstimulusconcentrationsaswell. Noneof
the regressions explain a great deal ofthe observed vari-
ance(r <0.05 inallcases), eitherduetothesmallslopes,
or because additional factors may be important in deter-
mining larval swimming responses.
Experiments testing adult chemnsensory abilities
Adult Uca longisignaliswere induced to feed by a lim-
ited subset ofthe amino acids and sugars (Table II). The
CHEMORECEPTION IN FIDDLER CRABS 211
Table II
Feedingresponsesbyadult Ucalongisignalistochemicaltestsolutions
andseawatercontrols
Proportion feeding Feedingtime
Compound Proportion G-value Time(s) /-value
Atninoacids
212 M. J. WEISSBURG ET AL.
computed with trehalose ranked last, is -0.3423, and is of potential prey, or enabling larval settlers to conduct
-0.2319withtrehalose removed(P> 0.25 in bothcases). site-restrictedsearchesformicrohabitats favorabletosub-
The difference in line versus coarse scale patterns re- stratecolonization. Clearly,theincreasedturningbehavior
flects changes in the stimulatory capacity of substances displayed by fiddler crab larvae in trehalose solutions
excitatorytoboth lifehistorystages. Onlyfourcompounds might facilitate location ofprey within patches, because
were significantly stimulatory exclusively to larvae or to the turning causes animals to loop back, focusing loco-
adults (valine and galactose were highly excitatory solely motoryactivitywithin asmallarea(e.g.. Koopman, 1980).
NGDR
to larvae, while glutamic acid and serine induced only Paradoxically, increasesinswimmingspeedand
adultsto feed); the majorityoftheremainingcompounds also seem to be associated with prey detection. In this
tended to be more highly excitatory to one stage or the study, Uca zoea responded to metabolites from Anemia.
other. Maltose and sucrose, for instance, were highly a known zooplankton prey species, with an increase in
stimulatorytoadults, butlessstimulatory tolarvae, while both speed and NGDR. Buskey (1984) determined that,
trehalose was more stimulatory to larvae than to adults. when copepodsaregiven diatom suspensions, swimming
NGDR
In contrast to the within species comparison, the re- speed and decrease, whereas copepods exposed
sponses of adult U. pugilator and U. longisignalis are todiatom exudatesalone, increasebothspeedandNGDR.
highlycorrelatedatbothcoarseand finescales. Abinomial In the absence of mechanical stimuli, these behaviors
test indicates that adults from both species mutually re- might allowanimalstocoverlargeareasin search ofprey
spond either positively or negatively to the same com- patches, whereupon encounteringprey, paired mechanical
pounds more frequently than expected by chance (bino- and chemical stimuli would result in site-restricted be-
mialtest: n= 14..v= 13, P<0.001). Similarly,a Kendall's havior. Additional experimentsare necessarytodetermine
Tau, computed on the rankings in Table III, givesa value the functional roles ofchemoreception and mechanore-
of0.78 (P < 0.03), indicating a quantitatively similar re- ception in mediating prey search by Uca zoea.
sponsepattern to individual compoundsamongadultsof To some degree, substances stimulatory to larvae are
each species. Although we present our tabulation using those that signal the presence of potential food items.
rankings ofthe proportion feeding for Uca longisignalis, Zooplankton are the primary prey species ofUca larvae,
theresultsusingfeedingtimearenotsubstantiallydifferent which have been successfully reared only using various
(Tau = 0.57, 0.03 < P< 0.06). The monosaccharide glu- motile zooplankton (e.g., Herrnkind, 1968; Christiansen
cose, its disaccharide isomers, and the amino acid serine and Yang, 1976: Rabalais and Cameron, 1983; Christy,
were the most stimulatory, while the remaining amino 1989). The larvae lack setae necessary for capturing phy-
acids were weakly stimulatory to both species. toplankton, instead pinning prey between the telson and
anteroventral spine (Herrnkind, 1968). Uca longisignalis
larvae respond to the amino acids valine and /^-alanine.
Discussion Although the response spectrum is narrower than that
observed in other(adult) crustaceans (see reviewsofAche,
This is the first examination of patterns of chemore- 1982;Carr, 1988; Laverack. 1988),thesetwoaminoacids
ception in zoea stage larvae. We have shown that brach- strongly signal the presence of fleshy prey. Beta-alanine
yuran crab larvae possess chemosensory abilities, and that constitutes as much as 12% of the free-amino acids re-
these abilities are present, although perhaps not fully de- leased into the water by zooplankton, with both valine
veloped, at a very early stage. Larvae respond to various and /3-alanine being consistently among the most abun-
amino acids and sugars with a change in swimming be- dant amino acids released into the surrounding environ-
havior. Such changes havepreviously been demonstrated mentby potential zooplankton prey(WebbandJohannes,
as reliable indicators ofchemosensitivity in a variety of 1967). Beta-alanineisgenerallyoneofthemostabundant
zooplankters, suchascopepods(Poulet and Ouellet, 1982; freeaminoacids present in flesh, and valine isquiteoften
Buskey, 1984). tintinnids (Buskey and Stoecker, 1989), abundant, averaging over 5% ofthe free-amino acid pool
oysterlarvae(Zimmer-Faust and Tamburri. in prep.), and (Cowey and Corner, 1964; Srinivasagam et at.. 1971).
ciliates (Levandowsky et a/.. 1984). Zooplankton assemblages have 2-3 times the free amino
Workershaveinterpreted thechangesinswimmingbe- acid concentration of valine and /3-alanine as phyto-
haviorasindicating '> >i waterbornesubstancesacttocue plankton-dominated communities (Jeffries, 1969).
resource acquisition behavior, since changes often involve Some of the stimulatory sugars may also signal the
decreases in swimming speed and in NGDR, and an in- presence ofprey. Glucose isthe majorblood sugarpresent
creased rate of turning (Buskey, 1984; Buskey and in animals, while trehalose is found in the hemolymph
Stoecker, 1989; Zimmer-Faust and Tamburri, in prep.). ofsome crustaceans (Telford, 1968). Both ofthese com-
Together, these changes have been viewed as 'adaptive' poundsevoke filter feeding responses in the planktivorous
responses, allowing predators to remain in aggregations porcelain crab Petrolisthes cinctipes(Hartman and Hart-
CHEMORECEPTION IN FIDDLER CRABS 213
man, 1977). Thereislittleworkdoneon thedistributions ing material (Kennish, 1986). High background levels of
ofothersugarsin seawaterorin tissuesofmarine animals, substancesinsediments, andthelarge numberofpotential
althoughgalactoseandgalactosederivatives found insea- non-prey sources of stimulatory molecules, may make
water may originate from flagellates (Sakugawa el a/.. intertidal mudflats particularly "noisy" environments.
1985). Undertheseconditions, onlyafewsubstancesmostclosely
Adult U. longisignalis respond toa ratherlimited suite associated with the presence offood (i.e.. serine and car-
of the compounds tested, primarily carbohydrates, and bohydrates) may be good signal carriers, forcing conver-
stimuli that are more effective at inducing feeding also genceamongthepropertiesoforganismsoccupyingthese
produce more intense feeding responses (Table II). Glu- habitats. Future studieson animalsin subtidal, intertidal,
cose and glucose dissacharide moieties are particularly and semi-terrestrial habitats are needed to fully discern
potent stimuli. Contrary to what has been found in other the influence of habitat type on patterns of correlation
studies,thereislittleevidencethatdisaccharidesare more among chemosensory abilities ofrelated crustacean spe-
stimulatory than constituent monosaccharides (e.g.. cies.
Zimmer el ai. 1979; Robertson el al. 1981; Trott and In contrast to the between-species comparison, larval
Robertson. 1984). Judging by both the induction as well and adult Uca longisignalis express different patterns of
asthe intensity ofthe response, crabsrespond more highly chemosensory ability at the fine scale. It appears that dif-
to glucose compounds than to other substances. Some ferences in the chemical environment ofthe pelagic and
additional sugars produce appreciable responses of one benthic habitats, or differences in diet, may require that
type or the other (i.e.. induction or intensity), but the varying life-history stages respond disparately to similar
joint function ofthe two components is not as great as suitesofchemicals. A numberofcompoundsstimulatory
for glucose and its dissacharides. Although a few amino toonelife-history stageofU. longisignalisareeithernon-
acids, particularly serine, provoke responses, they have stimulatoryoronlyslightlystimulatorytotheotherstage.
small effectsbothasinducers, and in termsoftheresponse For instance, valine and galactose are excitatory only to
intensity. larvae; serine and glutamic acid only to adults. Valine
The pattern ofchemosensitivity suggests adult U. lon- and serine arecuesthatarewell-defined markers forprey
gisignalis are well constructed to allow efficient foraging types favored by a particular form, so it is perhaps not
in theirnatural habitat. Diatoms, bacteria, and, occasion- surprising that each substance is strongly stimulatory to
1 ally, blue-green algae are dominant flora ofthe mud-flats only one life-history stage. The significance ofglutamic
preferred by crabs. The primary storage product in dia- acid and galactose tocertain stages islessclear. Glutamic
toms is a glucan, and carbohydrates are primary constit- acid is not readily released into seawater by zooplankton
uents ofthe extracellular mucose sheaths ofdiatoms and (Webb and Johannes, 1967), but is an abundant constit-
blue-green algae(e.g.. Fogg, 1966). Thesesugarsareoften uent ofthe amino acids found in diatoms (Cheucas and
released intotheenvironmentbyalgal species(Fogg, 1966; Riley, 1969). Glucose appears to be unique in its ability
Foggt'/a/.. 1973;Craigie, 1974; Darley, 1977; Paulsen el tofunctionasacueforbothlarvalandadultfoodsources,
al. 1978). Serine is the dominant amino acid ofthe free and is highly stimulatory to both life-history stages.
amino acid pool ofdiatoms (Parsons el al, 1961), and it Patternsoflarvalandadultabilities must becautiously
appears that adult fiddlercrabs are unique in responding interpreted, because it appears that some stimulants may
primarily tothisamino acid (i.e.. Robertson elal, 1981). produce responses that do not enhance the ability ofthe
Strong responses to carbohydrates and serine may be ex- organism to detect prey. There is no evidence that the
pected in light oftheir specificity as a cue to microalgal sugars sorbose, maltose, and sucrose are present in zoo-
food resources. plankton, but they are intimately associated with adult
The methodology ofthis study and that ofRobertson food sources. It maybethatdetection ofthese substances
el al (1981) on the sand fiddler crab, Ucapugilator, are by larvae has some function. However, in view of the
parallel enough to quantify the correlation among che- close association ofthese substances with adult food, we
mosensory abilities of the two species. The analysis in- tentatively hypothesize that the chemosensory responses
dicates that, in at least this species pair, similar trophic of larvae to these sugars are incipient abilities. That is,
ecologies have produced a broad concordance among sensitivitytothesecompoundsby larvae may bethe result
patterns ofadult chemosensitivity, even on a fine scale. ofdevelopmental processes required in providing sensi-
The high correlation among the abilities of Uca species tivity by adults.
may beafunction ofthe habitat occupied byadult fiddler While larval abilities appearpartially aconsequence of
crabs. For the semi-terrestrial Uca. molecules present in adult requirements, adult abilities do not seem to reflect
sediments where crabs forage are derived from a variety larval needs. Substances which act as potential cues to
of sources, including microbial breakdown processes larval, but not adult prey items(e.g.. valine), do not elicit
(Rittschof, 1980), and leakage from deadcellsand decay- adult feeding behaviors, while still evoking strong larval
214 M. J. WEISSBURG ET AL.
responses. Central neural processingisknown toattenuate Literature Cited
chemical signals, thereby reducing or eliminating behav-
eiolreacltrroepshpyosnisoelso,giecvaelnatchtoivuigthy sfurbosmtarnecceesptmoaryceellvsok(Deestthrieorn,g Achem3,9a8nB,.ineVdTVs.h.e1AB9ci8ao2d.leomgiyCchoefPmCroersruses,tcaeNcpeetwaioIYn'o<>r/a.kn3..dLt.hAetrmwooroedceapntidoDn..CP.p.Sa3n6d9e--
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We
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The conclusions reached as a result ofour study must some marinediatoms. J. Mar. Biol. Assoc. U. A' 49: 117-120.
be regarded as working hypotheses, primarily becausewe Christiansen, M. E., and VV.T. Yang. 1976. Feedingexperimentson
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The authors thank M. N. Tamburri for assistance in Cosmochem Ada 28: 45-66.
Derby,C.D.,B.VV.Ache,andE.VV.Kennel. 1985. Mixturesuppression
performing video recording and motion analysis and Dr. in olfaction: electrophysiological evaluation ofthe contribution of
S. G. Morgan forguidance in raisinglarvae. Research was peripheral and central neural components. Cliem. Senses 10: 301-
sponsored, in part, by grants from the National Science 316.
Foundation (Rl 1-8996152, D1R-8954231, and DIR- Dethier, V. G. 1980. Evolution of receptor sensitivity to secondary
9013187), by a faculty development award from the Ma- plant substanceswith special referencetodeterrents. Am. Nat. 115:
45-66.
rine Environmental Sciences Consortium, and by an Kpifanio, C. E., K. T. Little, and P. M. Rowe. 1988. Dispersal and
award from the Research Grants Committee, University recruitment of fiddler crab larvae in the Delaware River Estuary.
ofAlabama. Mar. Ecol Prog. Ser. 43: 181-188.
