THE MAIN FEATURES OF BRACHYURAN EVOLUTION

ZDRAVKO STEVCIC

AbstractStevcic, Zdravko. (Center for Marine Research, Rovinj, Yugoslavia). 1971. The main

features of brachyuran evolution. Syst. Zool., 20:331—340.—Past studies of the evolutionof the Brachyura (crabs, Crustacea: Decapoda) have, in general, provided conflictingand unsatisfactory results. This unfortunate situation stems in part from the objectivereason that their evolution has been very complex, but, it is also due to the basic inadequacyof the methodological approaches used. Brachyuran evolution has been considered pre-dominantly from only one point of view at a time (i.e., morphological, ontogenetic, etc.)without reference to a synthetic theory of evolution. The present paper attempts to initiatea synthetic approach to the study of crab evolution.

The identity of the ancestors of the Brachyura remains uncertain. Nevertheless, it seemsobvious that the Brachyura have reached the highest organizational level found amongthe decapod Crustacea. The genesis of the new organization—brachyurization—involveschanges in shape and structure accompanied by maximal diversification or organs and oftheir functions. This new level of organization has led to great biological improvementas manifested by taxonomic diversity, wide distribution and the ability to leave the primary(littoral) environment. The brachyuran organization is very plastic and has undergonemany secondary modifications in connection with further changes of habits and habitats.[Brachyura; Crustacea; evolution.]

INTRODUCTION

The abundance of very different phyloge-netic systems that have been proposed forthe crabs (Brachyura) reflects the manydiametrically opposed ideas about brachy-uran evolution that have been held bystudents of this group of animals. Themain reason for the disparity prevailingamong these systems is a methodologicalone. The evolution of crabs has usuallybeen studied from just one point of viewat a time (e.g., comparative morphology,ontogeny, paleontology) with concommitantoverreliance on some evidence and neglectof the remainder. Thus, in the course oftime, the various hypothesis have not ap-proached one another; but, on the con-trary, they have become more and moredivergent and the "gap" between them haswidened.

As we might expect, the majority of in-vestigations have involved morphologicalaspects of evolution, as these are generallythe most readily available. Investigationsof ontogeny have also clarified someimportant evolutionary problems; though,unfortunately, neither the relationship be-

tween ontogenetic and phylogenetic de-velopment of crabs, nor the laws ofdevelopment of larval structures and theirhabits are well known. Paleontologicalevidence has also frequently proven invalu-able to understanding the origins andevolutionary trends and tempo of brachy-uran groups. Phylogenetic interpretationsbased solely on paleontology have, how-ever, foundered because critical fossilforms, especially the oldest ones, are veryrare and often incomplete. Additionally,though the overall shape of a given fossilorganism and the form of some structures(orbits, dorsal furrows, appendages, etc.)can frequently be estimated, full reconstruc-tion and phylogenetic use of the fossils mustrely on knowledge of structuro-functionallaws (Gesetzmassigkeiten) as derived fromrecent forms. Other aspects of brachyuranevolution (e.g., those primarily involvingecology or ethology) have been, to date,rarely involved in studies of crab phy-logeny.

It stands to reason that the synthesis ofall available evidence is most likely to leadto a satisfactory conclusion to any phyloge-netic investigation. Unfortunately, very few

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carcinologists have availed themselves ofall of the accessible facts (Ortmann, 1896,1901; Bouvier, 1896, 1940; Balss, 1940-61),and their deductions are now outdated orsuffer (Balss) from uncritical use of theavailable evidence.

In order to meaningfully strengthen ourconcepts of brachyuran phylogeny it isnecessary that several basic and generalproblems be worked out in detail. Amongthe most urgent of these are the following:the origin of the group, the genesis of itsorganization, and the identification of theend products of brachyuran evolution.

THE ORIGIN OF THE BRACHYURA

A most difficult and still unsolved prob-lem concerns the origin of this group. Thetrue ancestors of the crabs are unknown;the oldest demonstrably applicable fossilforms (Eocarcinus, Prosopon) being truecrabs. All present hypotheses, no matterwhat evidence they rely on, cannot, with-out raising grave objections, explain theprincipal biological properties of the brachy-uran ancestors. Three major groups ofsuch hypotheses which attempt to attribute,respectively, macrouran, anomuran or pem-phicoid characters to the crab ancestors,now enjoy their own ardent partisans.

The first and the oldest hypothesis("macrouran") dates from Huxley (1878),who considered that the Brachyura hadorginated from the Astacidae. A similarinterpretation was that of Bouvier (1896,1940) who, in his comprehensive and splen-did essay, used not only comparative mor-phological evidence, but also all of thepaleontological and ontogenetic facts ofthat period. This author derived the Brachy-ura from the Nephropsidea (Homaridea).

A quite different hypothesis is that ofBoas (1880) who regarded the Brachyuraas having risen from forms related to thegenus Axius of the Thalassinidea. Ortmann(1892, 1896) derived the Brachyura fromthe Anomura, specifically from a form inter-mediate between the Paguridea and theGalatheidea and considered the Dromiaceaas either the most developed Anomura or

the most primitive Brachyura. Based onexamination of larval stages, Gurney (1942)and Burkenroad (1963) considered theThalassinidea to have been ancestral to theDromiacea. They placed the Dromiaceain the Anomura. Recently Pike and Wil-liamson (1960) derived the Dromiaceafrom a form intermediate between theNephropsidea and the Thalassinidea.

Finally, the third hypothesis is that ofvan Straelen (1928) who postulated, frompaleontological evidence, that the ancestorsof the Brachyura are to be found among theTriassic Pemphicoidea. This opininon hasbeen supported and supplemented by Beur-len (1930), Glaessner (1930, 1960) andForster (1967).

Which, if any, of these hypotheses iscorrect? For the reconstruction of the firsttransition stages ("missing links") betweenthe Brachyura and their ancestors it wouldbe necessary to know the laws of specificevolutionary development not only of thecrabs, but of all the decapod Crustacea.Unfortunately, such knowledge is, at pres-ent, largely lacking. Among these basiclaws it would be most important to knowthe relationship between morpho-physiolog-ical changes and changes in mode of lifeand habitat. The importance of such re-lationships is exemplified by two exampleselucidated by Schafer (1954). He estab-lished that typical (crab-like) forms (e.g.,Xantho or Eriphia) have ehelipeds foldedagainst the anterolateral margin of thecarapace. In this case, the chelae are com-monly heterochelic and heterodont. Thesecrabs have sternites arranged in paralleland move sideways. Such forms live pri-marily in places of strong current or waveaction and their condensed body is anadaptation to these environmental condi-tions. However the triangle-shaped ordeltoid-shaped forms (e.g., Spider Crabs)lack ehelipeds folded against the carapace,and the chelae are neither heterochelic norheterodont. Their sternites are radiallyarranged and therefore the animals canmove in all directions relative to the longi-tudinal axis of the body. These forms live

FEATURES OF BRACHYURAN EVOLUTION 333

in the places where motion of water isreduced because of roughness of the bot-tom. Citing another example, Stevcic (1967,1968) has "linked" environmental factors(bottom, food, etc.) to the chemical com-position of the body and to the behaviorof the Spinous Spider Crab (Maja squinadoHerbst). It is obvious that knowledge ofthese and similar relationships provides akey to the understanding of the process ofbrachyuran evolution.

With regards to the origin of crabs, afurther question intrudes: Are the Brachy-ura indeed a monophyletic group? As wehave seen before, there have been some at-tempts to consider Dromiacea as Anomura(Ortmann, 1892, 1896; Gumey, 1942).Bourne (1922) from his wide investigationson the Raninidae, argued that this grouporiginated from the Astacidae indepen-dently of other crabs. Lately Pichod Viale(1966) separated the Homolidea as beingmore primitive than the Brachyura. How-ever, today the predominate opinion holdsthat the Brachyura are a homogeneous,monophyletic group (Glaessner, 1930, 1960;Bouvier, 1940; Abrahamczik-Scanzoni, 1942;Balss, 1940-61) discrete from the otherdecapods. The following are considered asgeneral characteristics of the Brachyura:the abdomen is very much reduced andfolded under the cephalothorax; the lastthoracic segment is fused with the cephalo-thorax; the epistome is fused with thecarapace; the movable finger of the chelae(digitus mobilis, dactylus) is posed exter-nally; the females have a receptaculumseminis; the first two pleopods of malesare transformed into copulatory organs(gonopods) and the other pleopods are re-duced; the males have a penis; the inhalantopening lies primarily before the base ofthe cheliped. Other properties vary in con-nection with the grade of organization andwith adaptation to specific conditions ofexistence.

At present the question of the origin ofcrabs is still enigmatic and must be con-sidered open.

BRACHYURIZATION

One of the most important problems re-garding the evolution of the Brachyura isthe genesis of their organization. Borradaile(1916) was among the first to comprehendthe meaning of this process, calling it car-cinization. However, it would be betterto use the term brachyurization because itis more general and relevant to all crabs asa whole, without regard to shape and orga-nizational level. Since we do not know thetrue brachyuran ancestors, we can followthe genesis of their organization only par-tially, i.e., on the forms which are alreadycrabs. Further, the most primitive crabsare deep-sea inhabitants and are aberrant,so that it is very difficult to distinguishbetween adaptive peculiarities and ances-tral features in these forms. Nevertheless,by using all available evidence, we canmake many important deductions about thedevelopment of the new organization.

In order to make our understanding ofthe new organization and its successful ad-vances easier, we should firstly acquaintourselves with "typical" representatives,such as the crabs, Portunus and Xantho.Their shape is normally crab-like, i.e., thecarapace is usually transverse, depressed,wide in front with the anterolateral mar-gins regularly arched. Their head iscondensed, i.e., the occular segment is in-vaginated into the antennulary one (PichodViale, 1966). The antennulae are trans-versally folded. The antennae are fusedwith the epistome. The eyes can be pro-tected by complex orbits. The maxillipedsare depressed, and their third pair coversthe others as the operculum. The chelipedsare folded against anterolateral marginsand are, thus, heterochelic and heterodont.In females the first pleopod is absent. Thebranchiostegites close the gill chamber(branchial cavity). The gills are phylo-branches (usually 9 on each side). Theendoskeleton is very highly developed(Abrahamczik-Scanzoni, 1942), continuous(Drach, 1950) and the sternites are ar-ranged in parallel (Schafer, 1954). The

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zoea is typically brachyuran, i.e., withdorsal spine, and lacks both the naupliuseye and the frontal organ (Eloffson, 1963).

In fossil and recent crabs it is possibleto follow the gradual improvement oforganization from the very primitive tothe most advanced forms. Among the mostprimitive recent crabs are the Homolo-dromiidae, which are characterized by agreat number of ancestral features: thecephalothorax is cylindrical, the epistomeand the front are not quite fused, the headis not condensed, orbits are lacking, andthe antennae are not fused with the epi-stome. The antennal thorn and traces ofuropods remain. The gills are tricho-branches in great number on each side(21). On this organizational level are thefossil Eocarcinidae and Prosoponidae. Ahigher degree has been reached by theHomolidae and the Latreillidae, in whichthe number of ancestral properties is di-minished but they have not quite condensedthe head and the continuous endoskeleton.The next step of organization is evident inthe Dromiidae and Dynomenidae, whichhave still more advanced properties; thegeneral shape is crab-like, the chelipedsare folded against the carapace, the headis condensed and the endoskeleton is welldeveloped. This level of organization hasbeen attained by the Raninidae and Tymoli-dae. All the above mentioned groups havesome primitive and specific properties, how-ever, by which they differ from all theother Brachyura: the abdomen is not quitepressed against the sternum; the sternumis narrow; all of them have the primitivespermatheca (Gordon, 1950); and theypossess sternal furrows (excluding theHomolidae and Raninidae) as well as thecoxal sexual opening. Additionally, femalesretain the first pair of pleopods (excludingthe Tymolidae and Raninidae); the anten-nae are normally parallel and longitudinallyposed; the larvae (zoeas) lack the typicalbrachyuran shape and structures; and thelast one or two pairs of pereiopods areabnormal and posed dorsally (excludingthe Latreillidae and some Prosoponidae).

The majority of the crabs possess the moreor less typical brachyuran properties de-scribed above. The highest morphofunc-tional level has been reached by thesemiterrestrial and terrestrial forms such asthe Ocypodidae, Mictyridae, Grapsidae,Gecarcinidae and some species of Potamoni-dae.

The first step in the process of brachyuri-zation can be followed indirectly in some"pseudobrachyurans" i.e., in the crab-likeAnomura, where numerous brachyuran-likeshapes and structures, are attained withoutreaching the level of brachyuran organiza-tion. These properties can be found in thefamilies Lomisidae, Porcellanidae, Lithodi-dae, and partially in the family Hippidaeand in several genera of the Paguridae,namely: Ostraconotus, Porcellanopagurus,Tylaspis (Borradaile, 1916). These forms(excluding the Hippidae) have a depressedand broadened cephalothorax and the ab-domen folded partially under the cephalo-thorax (Porcellana can still swim by meansof the abdomen). The rostrum is oftenreduced with a broad front (Porcellana).Lithodes has the anterolateral margin welldeveloped. Some of these forms have re-duced the first pair of pleopods in thefemales and the last pleopods in the males(Lomisidae, Porcellanidae). The endo-skeleton may be continuous (Lithodidae,Porcellanidae) and the sternum broad(Porcellanidae) permitting sideways loco-motion. Gills are phyllobranches. Theuropods are often reduced (Lithodidae,Lomisidae). These are specialized formsfor specific environmental conditions, andare often aberrant (Lithodidae, Hippidae).These groups have reached the level oforganization at which Brachyura presum-ably began their evolutionary development.

It is very probable that folding of theabdomen was the first and perhaps de-cisive act making it possible for eithermacruran or anomuran ancestors to achievea higher grade of organization. Even inrecent Scyllaridae and Galatheidae the ab-domen may be flexed under the cephalo-thorax, protecting the tender ventral parts

FEATURES OF BRACHYURAN EVOLUTION 335

of the abdomen from damage. A similarphenomenon, where animals flex the ab-domen or fold the body in a ball, is wellknown in many animals such as the trilo-bites, some isopods, the hedgehogs, arma-dillos and others. The long abdomen inbenthonic Reptantia progressively lost itslocomotor function and grew weaker andmore folded under the cephalothorax, asnatural selection preferred forms with areduced abdomen and stronger ambulatorylegs. Simultaneously with the reduction andfolding of the abdomen occurred the proc-ess of gradual depression and broadeningof the cephalothorax. It is very probablethat this process commenced in littoralwaters, which agrees with Beurlen's (1931)point of view, who considered crabs pri-marily littoral inhabitants. As mentionedabove, the depressed form may be espe-cially advantageous in such habitat, wherewave action may be unusually strong. Con-sequent to the development of locomotionby means of pereiopods was the develop-ment of stronger muscles. These are at-tached to the endoskeleton, which itselfbecame stronger through fusion of thesternites into the rigid sternal plastron.The broadening of the cephalothorax andmodification of the sternites brought withthem the peculiar characteristic of thecrabs, their sideways locomotion. Thisprocess began in the Jurassic and by theCretaceous had resulted in the organizationof the true Brachyura.

In the process of brachyurization twodiametrically opposed processes can bedistinguished: differentiation and integra-tion. The segments, especially their ap-pendages, are strongly differentiated inrelation to those of the other decapodCrustacea. This differentiation is connectedwith the maximum division of labourreached among the decapods. Some seg-ments with reinforced functions such as thethoracic ones, are intensified, but segmentswith diminished function, such as the ab-dominal ones, are reduced. An especiallygreat amount of differentiation occurred inthe frontal region (antennae, antennulae,

orbits and maxillipeds). Concomitantlywith appendicular specialization there oc-curred modification and specialization ofthe inner organs and organ systems, (mus-cular and circulatory systems, endoskeleton,stomach and others).

Simultaneously with this differentiation,structural and functional integration (Franz,1924; Schafer, 1954) occurred. In part,integration was realized by concentrationof segments, resulting from the shorteningand compressing (Stauchung) of the tho-racic segments. Integration is also mani-fested through the fusion of the particularparts of the body as, for instance, the epi-stome with the carapace and all the thoracicsterna into the thoracic plastron. The proxi-mal segments of the antennae are fusedwith each other and with the epistome. Inall the pairs of pereiopods the J>asis andischium are fused. The concentration isespecially manifested in the central nervoussystem, where the ventral ganglia are fusedinto a great ventral ganglionic mass. Theintegration of the central nervous system haspresumably resulted in increased coordina-tion of movements and in complex be-haviour, especially in the amphibious andterrestrial forms.

These processes allowed considerablebiological progress (in the sense of Sewert-zoff, 1931) which is manifested not onlyby taxonomic diversity (the number ofspecies and individuals) but also in thediversity of habitats occupied by the Brachy-ura and the means by which they exploittheir environment. Thus 4450 of 8300species of the decapods (after Watermanand Chace, 1960) belong to the crabs; anunusually high number of species for ataxon which is of only infraordinal rank(Waterman and Chace, 1960; Glaessner,1960, 1969). In comparison, the orderIsopoda comprises 4000 species, while theAmphipoda contains 3600. Although themajority of crabs live in shallow water(especially in tropical seas) some exist onland and others live at depths as great asabout 5000 metres. Because of their greatquantity (biomass) and abundance they

336 SYSTEMATIC ZOOLOGY

represent a very conspicuous link in thechain of nourishment of inhabited biotopes.This evidence indirectly confirms the ef-ficiency and importance of the new brachy-uran organization.

THE END-PRODUCTS OF BRACHYURANEVOLUTION

From the aforementioned evidence it isobvious that the process of brachyurizationled to higher organization and to generalimprovement relative to ancestral forms.The process involved increasing the com-plexity of organization, which enabled bet-ter exploitation of environmental factorsand expansion into new habitats (deep-sea,fresh water, land as well as new niches inthe littoral zone). The Brachyura in com-parison with other decapod groups weremore plastic and adapted to life in variousenvironments. In the Brachyura, the chelae,for example, vary markedly in connectionwith their function, such as alimentation,so that the crabs as a group use a muchgreater variety of food than did their an-cestors. The increased mobility of varioustypes (involving, variously, running, swim-ming, burrowing or climbing) enabled thecrabs to achieve great hunting or foragingefficiency and speed; particularly quickare the terrestrial forms, and in water someportunids are even able to capture mack-erels. Brachyura vary in size to where someforms inhabit the phytal zone (algae), thecoral reefs and even the mantle cavitiesof molluscs and the tubes of worms.

Having attained this new level of orga-nization, the crabs began a large adaptiveradiation. The majority continued to livein the littoral zone (most members of thefamilies Xanthidae, Portunidae and Maji-dae). The remainder inhabited the inter-tidal zone and the land (Ocypodidae,Grapsidae and Gecarcinidae) and freshwater (Potamonidae), modifying their struc-ture according to the conditions of the en-vironment. However, these modificationswere not dramatic and the organizationremained on more or less the same level.

Some organs were further developedconcomitant with reduction of others;thus, in terrestrial forms the importance ofthe "lungs" is increased and that of the gillsdecreased; chemical receptors are weaker,and the role of vision (and the eyes) isincreased. Despite such modifications mem-bers of these groups are all of basically thesame crab-like form in which appear thegreatest number of subfamilies, genera,species and individuals.

Among the remainder of the crabs thereare a considerable number of aberrantforms, in which shape and structure havedeviated considerably from that of thetypical brachyuran, sometimes to the pointof their being difficult to identify as crabs.Such organisms have undergone especiallyfar-reaching modifications of the ectoso-matic organs (chelipeds, walking legs,eyes, antennae, and mouth parts) in re-sponse to specialization to a particularmode of life in more or less strongly limitedsurroundings. Such modifications have ap-peared in deep-sea, burrowing and com-mensal forms, and must certainly representan evolutionary two-edged sword, as, afterall, does every specialization. Better ex-ploitation of a limited range of environ-mental conditions has been attained but,presumably, at the expense of evolutionaryplasticity. Such conspicuous specializationhas occurred in the Homolodromiidae, Ho-molidae, Latreillidae, Cymopoliidae (deep-sea forms), Corystidae, Atelecyclidae,Calappidae, Leucosiidae, Raninidae (bur-rowing forms), Pinnotheridae and Trapezi-inae (commensals). Additionally, there area few species of other families (Majidae,Xanthidae, Portunidae, and others) whichhave become deep-sea inhabitants (Doflein,1904). Since the conditions under whichthese crabs live are simpler and more uni-form than those of typical crabs, there hasbeen a gradual decrease of their vital ac-tivity. In some forms there can be seensigns of regressive evolution (in the morpho-physiological sense) such as unrolling ofthe abdomen (Raninidae, Tymolidae, Do-rippidae, Corystidae), the reduction of the

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sensory organs (especially the eyes) andweakening of the locomotor system. Inconsidering the phylogenetic developmentof these aberrant forms it is necessary totake into account the fact that many ofthese properties are secondary and thatthey have originated from typical littoralforms.

Finally, far-reaching transformation hasoccurred in the Hapalocarcinidae, whichare so changed that such students of crabsystematics as Balss (1940-61) and othersconsider them a separate superfamily.Hapalocarcinids are obligate, life-longinhabitants of coral-galls; they are microph-agous (Potts, 1915) and have experi-enced striking regression of their locomotory(walking legs, muscles, endoskeleton) ali-mentary (chelae, mouth parts, gastric-mill)and sensory (eyes) organs.

These have been the main results ofbrachyuran evolution. It goes without say-ing that because of the enormous diversityof the crabs it is impossible always to es-tablish the pathway by which a givenbrachyuran group has evolved. The Geryon-idae, for example, being deep-sea forms,have some specific regressive characters,but on the other hand, are very active andraptorial crabs. Finally, it is worth reem-phasizing that our difficulty in determiningthe pathways of evolution within the Brachy-ura is a consequence not only of thediversity of crabs but also, and more criti-cally, of the dearth of knowledge about themode of life of the majority of crabs.

DISCUSSION

I have presented only the most generaloutline of the principal features and prob-lems of brachyuran evolution. From theforegoing pages, the extent of the difficultyand complexity of the problem of the originand subsequent development of the crabsmay be seen. I have focussed on these gen-eral problems first, so that it will be easierto subsequently solve specific cases of theevolutionary development of various smallergroups.

In work on brachyuran systematics up tothe present a common pitfall has been thedifferentiation of primary or ancestral char-acters from those which are secondary oradaptive. For example, most authors haveconsidered a dorsal position for the 4th and5th pairs of pereiopods (Homola, Dorippe),an elongated and cylindrical body (Ho-molodromia, Ranina, Corystes), elongatedmouth parts (Homolodromia, Calappa,Corystes), reduced orbits (Homola, Uca,Mictyris), and so on as primary (ancestral)features. Such an interpretation representsan oversimplification of the matter. It istrue that the primitive forms such as thelower Dromiacea have an elongated body,elongated mouth parts and incompleteorbits, but other crabs have secondarilyacquired these properties. The elongationof the body and mouth parts can be as-sociated with a burrowing mode of life.The reduction of the orbits is correlatedwith the movability of the eyestalks (PichodViale, 1966) and is often found in terrestrialanimals. The dorsal position of the legsis also a secondary feature and accordingto Dollo's law of the irreversibility of evo-lutionary process, it is impossible to returnthem into the 'normal' position of typicalcrabs. Additionally, previous workers havegenerally failed to recognize the wide-spread occurrence of convergence amongcrabs. Where several groups of variouslevels of organization have migrated intothe same habitat or acquired the same modeof life (deep-sea, burrowing, etc.) theyhave assumed similar shapes, structures andhabits (Tymolus-Dorippe, Matuta-Portunus,Ranina-Corystes). Thus, there exist assem-blages of convergent forms which, at firstsight, seem to be monophyletic as for in-stance the Oxystomata. The majority ofcarcinologists believe that this group is ahomogeneous and monophyletic one be-cause it possesses many linking propertiessuch as similar mouth parts, buccal cavernand respiratory system (course of watercurrents). Likewise the Oxystomata ismaintained as a primitive group becauseits members possess elongate mouth parts,

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and, in the Dorippidae, the last two pairsof pereiopods are positioned dorsally. Theseconclusions are without any real foundationbecause, as we have seen before, theseproperties may be adaptive (i.e., secondary)ones. Probably all these characters, ex-cluding the dorsal leg position, are reallyprimitive only in the Homolodromiidae,which possess the lowest overall organiza-tional level among the crabs. Only a fewcarcinologists have doubted that the Oxy-stomata are a monophyletic group (Boas,1880; Gurney, 1942; Williamson, 1965;Pichod Viale, 1966; Gordon, 1966). Thesame situation pertains with the Catomet-opa. It is very probable that the familiesin this group are uniform only superficially(Pichod Viale, 1966). Lately Guinot (1966)has cited several examples of genera beingquite erroneously classified on the basisof outward similarity.

The evolution of crabs has thus beenintimately associated with changes in modeof life and environment; brachyuran sub-groups have, in short, been formed throughadaptive radiation. The principal supporterof this point of view is Glaessner (1930,1957, 1960); others, such as Beurlen (1930,1933) and Russell (1962) have consideredthat adaptive radiation is not the only andindispensable mode of evolutionary change,believing that because new forms aroserapidly ("explosively") their evolution wasnot necessarily adaptive. These studentswere followers of the theory of ortho-genesis, and their theories are not suscep-tible to any form of objective proof. It istrue, that in the evolution of crabs thereare, as we have seen, many unsolved ques-tions, but our insufficient knowledge orcomplete ignorance of some evidence givesus no right to base explanations or hypothe-ses which are justified solely on the basisof random premisses. In addition, it isinteresting to note that Beurlen (1929,1931) unintentionally demonstrated factsthat were in opposition to his basic thesis.He stated that the evolution of the crabswas connected with changes occurring onthe earth's surface. He established that new

types arose in times of transgression of thesea, and that during regression severalforms invaded the land and fresh waterwhile others moved into the deep-sea. Hebelieved that this process was repeatedseveral times. Therefore it is not necessaryto postulate the existence of an autono-mous, undefined inner force for the explana-tion of the evolution of this group.

Undoubtedly the organization of theBrachyura represents an advance over thatof the other decapod Crustacea. However,this assertion is true for the Brachyura asa whole, but not for all of the sub-groups.Complete morpho-physiological and gen-eral biological progress has been attainedonly by several groups of higher crabs(Brachygnatha) in which there are manygenera and species (figures from Chace,1951): Majidae 145 genera with 673 species,Portunidae 38 genera with 297 species,Xanthidae 133 genera with 928 species, andGrapsidae 40 genera with 333 species.Several specialized groups have a lessernumber of genera and species: Homolo-dromiidae 4 genera with 6 species, Latreil-lidae 3 genera with 9 species, Cymopoliidae3 genera with 29 species, Dynomenidae 2genera with 13 species. Exceptionally,somewhat more plastic, though relativelyspecialized, groups have more genera andspecies: Leucosiidae 40 genera with 338species, Pinnotheridae 26 genera with 222species. Several groups show, in every way,a maximal progressive development, forexample, the terrestrial and semiterrestrialforms; by contrast some groups showed atotal regressive development to the pointof dying out: Eocarcinidae, Prosoponidae,Dacoticancridae (the most primitive groups)and Lobocarcinidae.

SUMMARY

1. In order to study the evolutionarydevelopment of any group it is necessaryto use all the available evidence withinthe framework of the general laws of evolu-tion. Most previous research on brachyuranevolution considered only a single suite

FEATURES OF BRACHYURAN EVOLUTION 339

(usually external morphology) of charac-ters and was frequently undertaken outsidethe context of a contemporary theory ofevolution.

2. Brachyuran ancestors and forms tran-sitional between the Brachyura and othergroups of decapods have not been identifiedwith certitude and the origin of the groupremains uncertain.

3. The genesis of the new organization(brachyurization) has involved a greatadvance over the other decapods. Thisprocess is connected with increasing dif-ferentiation of the segments and their ap-pendages and with maximal integrationcaused by the concentration of the seg-ments. The result of these changes is animprovement of organization to a higherlevel. The process started with the flexionof the abdomen under the cephalothorax,and proceeded with shortening, broadeningand depressing of the cephalothorax. Theassociated changes of the organs and theorgan systems caused far-reaching struc-turo-functdonal modifications and the entireorganization rose to a higher grade.

4. The attainment of this new organiza-tion enabled a large adaptive radiationconnected with the occupation of newhabitats and associated modifications ofshape, structure and habits. The majorityof the crabs evolved in typical brachyuranfashion, while a minority more or lessspecialized on limited environments, be-coming aberrant and, in some cases, under-went regressive evolution.

5. All the aforementioned modificationsare connected with changes in the biotopeand mode of life. The existence of ortho-genesis cannot be confirmed.

6. These investigations were necessaryto clarify the most elementary processes,establishing the limits reached by crabevolution. Consideration of the principalfeatures of brachyuran evolution is a neces-sary prerequisite to studying the phylogenyof single sub-groups.

ACKNOWLEDGMENTS

I would like to express my sincere grati-tude to Dr. Isabella Gordon (British Mu-seum Nat. Hist., London) for havingsuggested a number of improvements tothis text and for some critical remarksabout the manuscript. I am also gratefulto Dr. Gerd von Wahlert (Staatliches Mu-seum fur Naturkunde, Ludwigsburg) whomade some helpful suggestions on the man-uscript. I am indebted to DAAD (Deuts-cher Akademischer Austauschdienst, BadGodesberg) for a scholarship in WestGermany.

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Manuscript received Jan., 1969Revised Feb., 1971