Symmetry Variation in the Heart-Descending ArterySystem of the Parthenogenetic Marbled Crayfish

Gunter Vogt,1* Christian S. Wirkner,2 and Stefan Richter2

1Zoological Institute and Museum, University of Greifswald, 17487 Greifswald, Germany2Institute of Biosciences, General and Systematic Zoology, University of Rostock, 18055 Rostock, Germany

ABSTRACT The internal anatomy of freshwater cray-fish is strictly bilaterally symmetric, with the conspicu-ous exception of the vertically oriented descending ar-tery (sternal artery), which originates from the heartand terminates in the subneural artery. Serial sectioningof 133 juveniles of the parthenogenetic marbled crayfishrevealed that the descending artery was bilaterally sym-metric in 4.5% of the specimens, right asymmetric in45.1%, and left asymmetric in 50.4%. In the bilaterallysymmetric variant two branches arise from the left andright chambers of the bulbus of the heart, run laterallyaround the hindgut, and fuse underneath it. The asym-metric variants have only one dorsal branch, whichloops around the hindgut on either the left or the rightside. Other structures of the heart, such as the pairedantennary and hepatopancreatic arteries and the ostiaor the unpaired anterior and posterior aortae, showedno symmetry variation. Because of the genetic identityof the experimental animals and their culture underidentical environmental conditions, the variation in sym-metry of the descending artery observed is interpretedas the result of developmental variation. We recommendthat the marbled crayfish be considered for investigationof the epigenetic mechanisms responsible for the mainte-nance and breaking of bilateral symmetry in metazoans.J. Morphol. 270:221–226, 2009. � 2008 Wiley-Liss, Inc.

KEY WORDS: symmetry; sternal artery; heart; epigen-etics; variation; development; marbled crayfish

The internal anatomy of freshwater crayfish isgenerally bilaterally symmetric (Vogt, 2002).Organs are either positioned mirror symmetricallyin both body sides when paired or in the mid-planeof the body when unpaired. The unpaired organs,which include the heart and the stomach, ofteninclude bilaterally symmetric substructures. Theonly conspicuous exception to bilateral symmetryis the descending artery (sternal artery), which isusually unpaired and asymmetric (Baumann,1921; Vogt, 2002). The cause for the asymmetry ofthis structure is unknown.

The descending artery originates together withthe posterior aorta from the bulbus, which is anunpaired compartment at the posteroventral mar-gin of the heart. The heart proper is a rhombicmuscular organ, which lies dorsally in the poste-rior cephalothorax. It gives rise to seven arteries

that lead into different body regions, five originat-ing anteriorly and two posteriorly. The antennaryand hepatopancreatic arteries are generallypaired, the anterior aorta and the posterior aortaare generally unpaired, and the descending arteryis usually unpaired but occasionally paired (Bau-mann, 1921; Wilkens et al., 1997; Wilkens, 1999;Vogt, 2002; McGaw, 2005).

The descending artery links the heart to theventrally located subneural artery, which suppliesthe mouthparts, chelipeds, walking legs, nervecord, and ventral thoracic and pleonal musculaturewith hemolymph (Vogt, 2002). Of the sevenarteries that leave the heart the descending arteryhas been shown to receive the highest cardiac out-put by far (Reiber, 1994; McGaw, 2005), underlin-ing its functional significance. On its way from theheart to the subneural artery the descending ar-tery usually loops around the left or right side ofthe centrally located gut (hindgut in crayfish,midgut in other decapods).

A descending artery occurs not only in fresh-water crayfish but also in other Decapoda and inLophogastrida, Mysida, Euphausiacea, and Anas-pidacea (e.g. Siewing, 1956; Wilkens et al., 1997;Wilkens, 1999; McGaw, 2005; Mayrat et al., 2006;Wirkner and Richter, 2007). It is thought to haveevolved from a segmental pair of lateral cardiacarteries at the posterior end of the thorax. Infreshwater crayfish and some other decapods, bothright and left asymmetric descending arteries havebeen found in the same species, and in a few speci-mens there were even paired descending arteries(Fulinski, 1908; Baumann, 1921; Defretin, 1934;Davidson and Taylor, 1995; Wilkens et al., 1997;Wilkens, 1999; McGaw and Reiber, 2002; McGaw,2005). These observations were made in the courseof studies on the general anatomy of the circula-

*Correspondence to: Gunter Vogt, Department of Zoology, Univer-sity of Heidelberg, Im Neuenheimer Feld 230, D-69120 Heidelberg,Germany. E-mail: gunter.vogt@web.de

Published online 20 October 2008 inWiley InterScience (www.interscience.wiley.com)DOI: 10.1002/jmor.10676

JOURNAL OF MORPHOLOGY 270:221–226 (2009)

� 2008 WILEY-LISS, INC.

tory system of decapods. Specific and comprehen-sive investigations regarding the symmetry of thevasculature have not yet been carried out.

It is not known whether the intraspecific varia-tion of the descending artery in freshwater cray-fish and other decapods is genetically determined,environmentally induced, or randomly created dur-ing development. The parthenogenetic marbledcrayfish, which was discovered in the mid 1990s(Scholtz et al., 2003; Vogt et al., 2004; Vogt,2008b), seems to be a particularly suitable candi-date for a detailed investigation into this problemas it is the only known decapod crustacean to pro-duce genetically identical offspring (Martin et al.,2007; Vogt et al., 2008). Moreover, this crayfishdevelops directly without metamorphosis (Vogt andTolley, 2004; Alwes and Scholtz, 2006) and can bereared in very simple environments includingmicroplates (Vogt, 2007, 2008a). To examinewhether symmetry variation in the descending ar-tery of crayfish is the result of developmental vari-ation we serial-sectioned isogenic embryos andearly juveniles of marbled crayfish raised underidentical environmental conditions.

MATERIALS AND METHODS

The symmetry of the heart-descending artery system of themarbled crayfish was investigated by serial sectioning of 133late embryos and juvenile stages 1 to 8. Although it has no sci-entific name as yet, the marbled crayfish is a clearly definedand well established parthenogenetic laboratory animal (Vogt,2008b). It is genetically very similiar to Procambarus alleni(Faxon 1884) and Procambarus fallax (Hagen 1870) (KeithCrandall, personal communication), two similar looking andcoexisting species from subtropical sloughs of South Florida(Dorn and Trexler, 2007). The specimens used for this studywere taken from the laboratory lineage of G. Vogt. The experi-ments and histological analyses were performed at the Zoologi-cal Institute of Greifswald.The embryos and juvenile stages 1 to 4 were sampled from

the pleopods of seven brooding dams (batches 1–7), which wereindividually kept in 30 cm 3 20 cm aquaria filled at a waterlevel of 7 cm. These aquaria were equipped with a layer of finegravel and a stone to provide shelter and to enable the breath-ing of atmospheric air. Juvenile stages 5 to 8 were communallyreared in the same aquaria but without their mother. The juve-nile stages were determined according to criteria listed by Vogtand Tolley (2004) and Vogt et al. (2004). The feeding stages, i.e.from juvenile stage 3 upwards, were fed ad libitum with Tetra-Wafer Mix (Tetra, Melle, Germany) as the sole food source.Excess food was siphoned out daily and water was completelychanged once a week. Water temperature in the aquaria fluctu-ated between 19 and 218C over time but was the same in allaquaria at any given time. It has to be emphasized that all themembers of a given batch were exposed to the same environ-mental conditions at any given time in their life and that thevariation between the batches, i.e. between different aquaria,was kept as small as possible by using the same type of aquar-ium, the same water source and the same food and by placingthe aquaria close to each other on the same shelf.For histological examination, the specimens were fixed with

Bouin’s fluid (Duboscq-Brasil), dehydrated in ethanol, and em-bedded in paraplast, using tetrahydrofuran as intermedium. Se-rial sections were taken with a Leica RM2125RT microtome,stained with Azan, and examined with an Olympus BX60 lightmicroscope equipped with an Olympus DP16 digital camera.

RESULTS

The heart-descending artery system of the mar-bled crayfish follows in principal the scheme givenfor freshwater crayfish in general (Fig. 1A). Theheart is an unpaired organ that is located dorsallyin the posterior cephalothorax in the body mid-plane (Fig. 2A). It is suspended in the pericardialcavity by ligaments and receives hemolymphthrough three pairs of ostia (Fig. 1A,C). The peri-cardial cavity is linked with the gills via sinuses(Fig. 2A). The substructures of the heart are eitherunpaired and located in the organ mid-plane orpaired and positioned mirror symmetrically in theleft and right halves of the organ. Unpaired struc-tures are the cardiac ganglion, the anterior aorta(Figs. 1B and 2B), the posterior aorta (Fig. 2B),and in most cases also the descending artery (Fig.2A–E). Paired structures are the antennary andhepatopancreatic arteries (Fig. 1B) and the dorsal,lateral, and ventral ostia (Fig. 1C).

The descending artery originates together withthe posterior aorta from the bulbus, which is anunpaired dilated compartment on the posteroven-tral margin of the heart (Fig. 1A). This bulbus isseparated from the heart by a valve (Fig. 2B,C)

Fig. 1. Paired and unpaired structures of the heart of themarbled crayfish. A: Schematic illustration of crayfish heart (afterBaumann, 1921, and Vogt, 2002) showing unpaired anterior orophthalmic aorta (oa), posterior aorta (pa), descending artery(da), and bulbus (b), and left representatives of paired antenn-ary artery (aa), hepatopancreatic artery (ha), dorsal ostium(do), lateral ostium (lo), and ventral ostium (vo). li, ligament;mp, muscle bundle of pericardial floor; p, pericardial cavity. S1to S5 indicate planes of transversal sections. B: Oblique trans-versal section through anterior margin of heart of stage 7 juve-nile (S1 in A), showing the five anterior arteries, all hit in thevalve region. ep, epicard layer of heart. Bar: 50 lm. C: Obliquetransversal section through heart of stage 3 juvenile (S2 in A),showing all of the six ostia. Bar: 50 lm.

222 G. VOGT ET AL.

Journal of Morphology

composed of broad dorsal and ventral flaps (Fig.2D). Interestingly, on transversal sections the mor-phologically unpaired bulbus displays a clear bilat-eral symmetry, being subdivided into left and rightchambers (Fig. 2D). The descending artery arisesventrally from the bulbus and connects the heartwith the subneural artery, which is located under-neath the nerve cord in the body mid-plane (Figs.2A,E and 3F). After leaving the bulbus, the de-scending artery loops around the hindgut (Fig. 3E)and then traverses the central musculature (Fig.2A) and the nerve cord (Fig. 3F) along the bodymid-plane to finally terminate in the subneural ar-tery. That vessel runs from the mouthparts to theend of the pleon.

The descending artery of the marbled crayfishcan be either paired or unpaired. Accordingly, itcan arise from both chambers of the bulbus (Fig.3A) or from the left (Fig. 2D) or right chamber(Fig. 3E) only. When paired, the two branches

encircle the hindgut and unite underneath it (Fig.3A,B). Thereafter, the descending artery runs asan unpaired structure along the body mid-planethrough the extensive musculature of the posteriorcephalothorax (Fig. 2A) and through an opening ofthe ventral nerve cord that is anteriorly and poste-riorly flanked by the ganglia of the 6th and 8ththoracomeres (Fig. 2E) and laterally by the spa-tially separate right and left ganglia of the 7ththoracomere (Fig. 3F). When unpaired, a singlevessel loops around the hindgut, either on theright side (Fig. 3E) or the left side (Fig. 3F). In the133 specimens investigated the descending arterynever originated in the middle of the bulbus. Like-wise, it never left the bulbus on one side and thenlooped around the hindgut on the opposite side ofthe body.

Histological investigation of 133 late embryosand early juveniles from seven batches revealedpaired descending artery systems in 4.5% of the

Fig. 2. Location of heart and descending artery in the marbled crayfish. A: Transversal section through posterior cephalothoraxof stage 4 juvenile, showing positioning of heart (he), ventral part of descending artery (arrow), and subneural artery (arrowhead)in body mid-plane. g, gill; gc, gill chamber; h, hindgut; m, central thoracic musculature; n, nerve cord; p, pericardial cavity; si,hemal sinus. Bar: 200 lm. B: Median longitudinal section through heart of stage 7 juvenile, displaying outlets of anterior aorta(aa) and bulbus (b), both equipped with valves (arrowheads). The bulbus gives rise to the posterior aorta (pa) and the vertically ori-ented descending artery (da). Bar: 200 lm. C: Close-up of bulbus with valve (arrow). ov, ovary. Bar: 50 lm. D: Oblique transversalsection through bulbus (S3 in Fig. 1A), showing bilaterally symmetric substructure of bulbus and outlet of descending artery fromleft chamber. Arrows denote valve flaps. Bar: 30 lm. E: Medial longitudinal section through ventral part of posterior cephalothoraxof stage 1 juvenile, showing fusion of descending artery with subneural artery (sa). g6 and g8, ganglia of 6th and 8th thoracomeres.Bar: 50 lm.

SYMMETRY OF DESCENDING ARTERY IN CRAYFISH 223

Journal of Morphology

specimens, right asymmetric systems in 45.1%,and left asymmetric systems in 50.4% (Table 1A).Paired descending arteries were found in juvenilestages 1, 2, 3, and 6–8 (Table 1A) and in batches 1,2, 3, and 5 (Table 1B). Their absence in the otherstages and batches is probably related to the smallsample size, and to chance. Of the six paired de-scending artery systems, two were completely sym-metric, having right and left dorsal branches ofidentical widths (Fig. 3A,B), three had broaderright dorsal branches (Fig. 3C), and one had abroader left branch. Of the 127 completely asym-metric descending artery systems eight (6.3%) dis-played remnants of a second branch, which leftthe bulbus at the opposite side of the main branchand ended after a short distance (Fig. 3D).

The other substructures of the heart, the fiveanterior arteries, the posterior aorta, the six ostia,and the cardiac ganglion, showed no symmetryvariation.

DISCUSSION

In the present study we have analyzed the sym-metry of the heart-descending artery system offreshwater crayfish and tested the hypothesis thatsymmetry variation is the result of developmentalvariation. To exclude variation caused by geneticdifferences in the test animals or the macroenvir-onment, we analyzed isogenic batch-mates of theparthenogenetic marbled crayfish that were raisedunder the same environmental conditions. The

Fig. 3. Bilaterally symmetric and asymmetric descending artery systems in early juveniles of the marbled crayfish. Transversalsections in bulbus region (S4 in Fig. 1A). A: Bulbus (b) with bilaterally symmetric dorsal branches of descending artery (da). h,hindgut. Bar: 30 lm. B: Fusion of equal left and right dorsal branches under hindgut in body mid-plane. Bar: 50 lm. C: Fusion ofnarrow left (arrow) and broad right branch in body mid-plane. Bar: 30 lm. D: Bulbus with main outlet on left side and shortbranch on right side (arrow). Bar: 30 lm. E: Right asymmetric descending artery looping around hindgut. Bar: 30 lm. F: Leftasymmetric descending artery traversing thoracic musculature (m) and nerve cord (g7, paired ganglia of 7th thoracomere) in bodymid-plane and terminating in subneural artery (sa). Bar: 50 lm.

224 G. VOGT ET AL.

Journal of Morphology

genetic identity of our marbled crayfish laboratorypopulation was demonstrated in advance using thehighly sensitive microsatellite technique (Vogtet al., 2008).

The descending artery of the marbled crayfish,and probably of freshwater crayfish and decapodsin general, is an extraordinary structure that orig-inates dorsally from either side of a bilaterallysymmetric organ, the bulbus of the heart, loopsthrough either side of the body, traverses the ven-tral part of the body along the body mid-plane,and terminates in an unpaired ventral vessel,which is also located in the body mid-plane. Inter-estingly, we found a variety of symmetry statesamong batch-mates of the marbled crayfish rearedin the same environment: 1) bilaterally symmetricsystems with two equal branches, 2) slightly asym-metric systems with unequal branches, 3) heavilyasymmetric systems with one functional branchand a second blindly ending branch, and, 4) com-pletely asymmetric systems with only one branch,either right or left. The asymmetry of the entirepopulation was slightly biased to the left. In con-trast to the descending artery, the other substruc-tures of the heart showed no symmetry variation.The occurrence of bilaterally symmetric and asym-metric descending arteries in all batches with big-ger sample sizes indicates that symmetry variationis not dependent on the mother.

There are not many studies to date, whichreport on the symmetry of the descending arteryin decapods, and in those which do, only smallnumbers of specimens were investigated. Fulinski(1908) and Baumann (1921) observed right andleft asymmetric descending arteries in the noblecrayfish, Astacus astacus, with a clear bias to theright side. The same was reported for the bluecrab, Callinectes sapidus, by McGaw and Reiber(2002). Defretin (1934) observed a paired descend-ing artery in one specimen of Astacus astacuswhile 75% of his study animals displayed rightasymmetry in the descending artery system. Inthe American lobster, Homarus americanus, thedescending artery is regularly accompanied by a

small bilateral artery that often has a separatevalve. This companion artery usually passes intothe musculature, but in one case it has beenobserved to fuse with the descending artery under-neath the intestine (Wilkens, 1999).

At present, nothing is known about the mecha-nisms that establish and maintain bilaterally sym-metric structures in the heart-descending arterysystem of crayfish (Baumann, 1921; Vogt, 2002) orthe mechanisms involved in symmetry breaking.The descending artery of the marbled crayfish issupposedly established in the mid-period of embry-onic development together with the heart (Alwesand Scholtz, 2006). On the basis of the detection ofa single bilaterally symmetric variant in anembryo of Astacus astacus, Fulinski (1908) specu-lated that asymmetric descending arteries mayarise from a symmetric anlage by removal of onebranch during later development. Our results nei-ther support nor reject this idea due to the lack ofdata from early embryos. It appears likely that asymmetric descending artery can be converted intoan asymmetric one by degeneration of either of thetwo dorsal branches during later development, butthe reverse process is highly improbable. Becausewe have also found bilateral symmetric systems inolder juveniles, we currently favor the idea thatthe different symmetry states are produced duringearly differentiation and are then retainedthroughout life. More detailed investigation of thisquestion is under way.

Phenotypic variation of a trait is the result ofgenetic variation plus environmental variationplus developmental variation. The latter is mainlycaused by the stochasticity of gene expression, pat-terning, and organogenesis (for literature see: Vogtet al., 2008). Since our experimental marbled cray-fish were genetically identical and reared underthe same environmental and nutritional condi-tions, the symmetry variation observed in the de-scending artery has to be regarded as the result ofdevelopmental variation. Symmetry variation inthe descending artery is the first example of devel-opmental variation in an internal organ in the

TABLE 1. Symmetry states of the descending artery in early life stages of the marbled crayfish

Descending arteryEmbryon 5 3

Juv 1n 5 12

Juv 2n 5 27

Juv 3n 5 31

Juv 4–5n 5 26

Juv 6–8n 5 34

Alln 5 133

A. Bilaterally symmetric and asymmetric variants in embryos and different juvenile stagesRight asymmetric 2 5 13 13 10 17 60 (45.1%)Left asymmetric 1 5 12 17 16 16 67 (50.4%)Symmetric 0 2 2 1 0 1 6 (4.5%)

Descending arteryBatch 1n 5 54

Batch 2n 5 31

Batch 3n 5 14

Batch 4n 5 14

Batch 5n 5 12

Batch 6n 5 4

Batch 7n 5 4

B. Bilaterally symmetric and asymmetric variants in different batchesRight asymmetric 25 14 5 7 5 2 2Left asymmetric 26 16 8 7 6 2 2Symmetric 3 1 1 0 1 0 0

SYMMETRY OF DESCENDING ARTERY IN CRAYFISH 225

Journal of Morphology

marbled crayfish, which is currently being intro-duced as a new model organism for development,epigenetics, and evolution (Vogt, 2008b). Develop-mental variation has been reported with regard tocoloration, life history traits, and the externalsense organs of the marbled crayfish (Vogt et al.,2008). An investigation into fluctuating asymme-try, the right-to-left difference of a trait, in a totalof 168 identically raised stage 2 to stage 6 juve-niles, for instance, revealed 14.9% perfect symme-try, 42.3% right asymmetry, and 42.8% left asym-metry in the gustatory corrugated setae (fringedsetae) on pereiopods 1 to 3, but 72.0% perfect sym-metry, 13.7% right asymmetry, and 14.3% leftasymmetry in the olfactory aesthetascs on the 1stantennae (Vogt et al., 2008). These data suggestthat developmentally related symmetry variationis not coupled between different traits.

The relevance of the present work lies in thedetection of an internal organ that can either bebilaterally symmetric, right asymmetric, or leftasymmetric even in a genetically uniform labora-tory population raised under identical environmen-tal conditions. It will be the challenge of futureinvestigations to identify the epigenetic and molec-ular mechanisms that cause this symmetry varia-tion. Because its embryos and early juveniles canbe cultured in simple facilities, including micro-plates (Vogt, 2007, 2008a,b), the marbled crayfishmay serve as an additional model to investigatethe establishment and maintenance of bilateralsymmetry and symmetry breaking in metazoans(Mercola and Levin, 2001; Palmer, 2004). Futurestudies using marbled crayfish raised under differ-ent experimental conditions could provide informa-tion on the modifiability of symmetry by the mac-roenvironment, and experiments with the sexuallyreproducing and closely related species, eitherProcambarus alleni or Procambarus fallax, couldprovide information on the genetic aspects ofsymmetry.

ACKNOWLEDGMENTS

We are grateful to Erika Becker and Ben Tuschy(Zoological Institute and Museum, University ofGreifswald) for preparing the serial sections, GerdAlberti for providing technical equipment, LucyCathrow for improving the English, and two anon-ymous referees for helpful suggestions on themanuscript.

LITERATURE CITED

Alwes F, Scholtz G. 2006. Stages and other aspects of the em-bryology of the parthenogenetic Marmorkrebs (Decapoda,Reptantia, Astacida). Dev Genes Evol 216:169–184.

Baumann H. 1921. Das Gefabsystem von Astacus fluviatilis(Potamobius astacus L.). Z wiss Zool 118:246–312.

Davidson GM, Taylor HH. 1995. Ventilatory and vascularroutes in a sand burying swimming crab, Ovalipes catharus(White, 1843) (Brachyura, Portunidae). J Crust Biol 15:605–624.

Defretin R. 1934. Sur la presence d’une artere sternale doublechez l’ecrevisse. Valeur morphologique de ce Vaisseau. B SocZool Fr 59:231–236.

Dorn NJ, Trexler JC. 2007. Crayfish assemblage shifts in alarge drought-prone wetland: The roles of hydrology and com-petition. Freshwat Biol 52:2399–2411.

Fulinski B. 1908. Beitrage zur embryonalen Entwicklung desFlubkrebses. Zool Anz 33:20–28.

Martin P, Kohlmann K, Scholtz G. 2007. The parthenogeneticMarmorkrebs (marbled crayfish) produces genetically uniformoffspring. Naturwissenschaften 94:843–846.

Mayrat A, McMahon BR, Tanaka K. 2006. The circulatory sys-tem. In: Forest J, von Vaupel Klein JC, Schram FR, editors.Treatise on Zoology—Anatomy, Taxonomy, Biology. The Crus-tacea revised and updated from the Traite de zoologie, 2. Lei-den: Brill. pp 3–84.

McGaw IJ. 2005. The decapod crustacean circulatory system: Acase that is neither open nor closed. Microsc Microanal11:18–36.

McGaw IJ, Reiber CL. 2002. Cardiovascular system of the bluecrab Callinectes sapidus. J Morphol 251:1–21.

Mercola M, Levin M. 2001. Left-right asymmetry determinationin vertebrates. Ann Rev Cell Dev Biol 17:779–805.

Palmer AR. 2004. Symmetry breaking and the evolution of de-velopment. Science 306:828–833.

Reiber CL. 1994. Hemodynamics of the crayfish Procambarusclarkii. Physiol Zool 67:449–467.

Scholtz G, Braband A, Tolley L, Reimann A, Mittmann B,Lukhaup C, Steuerwald F, Vogt G. 2003. Parthenogenesis inan outsider crayfish. Nature 421:806.

Siewing R. 1956. Untersuchungen zur Morphologie der Mala-costraca (Crustacea). Zool Jahrb Abt Anat Ontog Tiere 75:39–176.

Vogt G. 2002. Functional anatomy. In: Holdich DM, editor. Biol-ogy of Freshwater Crayfish. Oxford: Blackwell. pp 53–151.

Vogt G. 2007. Exposure of the eggs to 17a-methyl testosteronereduced hatching success and growth and elicited teratogeniceffects in postembryonic life stages of crayfish. Aquat Toxicol85:291–296.

Vogt G. 2008a. Investigation of hatching and early post-embry-onic life of freshwater crayfish by in vitro culture, behavioralanalysis, and light and electron microscopy. J Morphol269:790–811.

Vogt G. 2008b. The marbled crayfish: an new model organismfor research on development, epigenetics and evolutionarybiology. J Zool 276:1–13.

Vogt G, Tolley L. 2004. Brood care in freshwater crayfish andrelationship with the offspring’s sensory deficiencies. J Mor-phol 262:566–582.

Vogt G, Tolley L, Scholtz G. 2004. Life stages and reproductivecomponents of the Marmorkrebs (marbled crayfish), the firstparthenogenetic decapod crustacean. J Morphol 261:286–311.

Vogt G, Huber M, Thiemann M, van den Boogaart G, SchmitzOJ, Schubart CD. 2008. Production of different phenotypesfrom the same genotype in the same environment by develop-mental variation. J Exp Biol 211:510–523.

Wilkens JL. 1999. Evolution of the cardiovascular system inCrustacea. Am Zool 39:199–214.

Wilkens JL, Yazawa T, Cavey MJ. 1997. Evolutionary deriva-tion of the American lobster cardiovascular system: An hy-pothesis based on morphological and physiological evidence.Invertebr Biol 116:30–38.

Wirkner CS, Richter S. 2007. The circulatory system in Mysida-cea—Implications for the phylogenetic position of Lophogas-trida and Mysida (Malacostraca, Crustacea). J Morphol 268:311–328.

226 G. VOGT ET AL.

Journal of Morphology