THE MAIN FEATURES OF BRACHYURAN EVOLUTION

ZDRAVKO STEVCIC

Abstract Stevcic, Zdravko. (Center for Marine Research, Rovinj, Yugoslavia). 1971. The main features of brachyuran evolution. Syst. Zool., 20:331—340.—Past studies of the evolution of the Brachyura (, Crustacea: ) have, in general, provided conflicting and unsatisfactory results. This unfortunate situation stems in part from the objective reason that their evolution has been very complex, but, it is also due to the basic inadequacy of 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 initiate a synthetic approach to the study of evolution. The identity of the ancestors of the Brachyura remains uncertain. Nevertheless, it seems obvious that the Brachyura have reached the highest organizational level found among the decapod Crustacea. The genesis of the new organization—brachyurization—involves changes in shape and structure accompanied by maximal diversification or organs and of their functions. This new level of organization has led to great biological improvement as manifested by taxonomic diversity, wide distribution and the ability to leave the primary (littoral) environment. The brachyuran organization is very plastic and has undergone many secondary modifications in connection with further changes of habits and habitats. [Brachyura; Crustacea; evolution.]

INTRODUCTION tween ontogenetic and phylogenetic de- The abundance of very different phyloge- velopment of crabs, nor the laws of netic systems that have been proposed for development of larval structures and their the crabs (Brachyura) reflects the many habits are well known. Paleontological evidence has also frequently proven invalu- diametrically opposed ideas about brachy- able to understanding the origins and uran evolution that have been held by evolutionary trends and tempo of brachy- students of this group of . The uran groups. Phylogenetic interpretations main reason for the disparity prevailing based solely on paleontology have, how- among these systems is a methodological ever, foundered because critical one. The evolution of crabs has usually forms, especially the oldest ones, are very been studied from just one point of view rare and often incomplete. Additionally, at a time (e.g., comparative morphology, though the overall shape of a given fossil ontogeny, paleontology) with concommitant organism and the form of some structures overreliance on some evidence and neglect (orbits, dorsal furrows, appendages, etc.) of the remainder. Thus, in the course of can frequently be estimated, full reconstruc- time, the various hypothesis have not ap- tion and phylogenetic use of the must proached one another; but, on the con- rely on knowledge of structuro-functional trary, they have become more and more laws (Gesetzmassigkeiten) as derived from divergent and the "gap" between them has recent forms. Other aspects of brachyuran widened. evolution (e.g., those primarily involving As we might expect, the majority of in- ecology or ethology) have been, to date, vestigations have involved morphological rarely involved in studies of crab phy- aspects of evolution, as these are generally logeny. the most readily available. Investigations It stands to reason that the synthesis of of ontogeny have also clarified some all available evidence is most likely to lead important evolutionary problems; though, to a satisfactory conclusion to any phyloge- unfortunately, neither the relationship be- netic investigation. Unfortunately, very few 331 332 SYSTEMATIC ZOOLOGY carcinologists have availed themselves of the most primitive Brachyura. Based on all of the accessible facts (Ortmann, 1896, examination of larval stages, Gurney (1942) 1901; Bouvier, 1896, 1940; Balss, 1940-61), and Burkenroad (1963) considered the and their deductions are now outdated or Thalassinidea to have been ancestral to the suffer (Balss) from uncritical use of the . They placed the Dromiacea available evidence. in the Anomura. Recently Pike and Wil- In order to meaningfully strengthen our liamson (1960) derived the Dromiacea concepts of brachyuran phylogeny it is from a form intermediate between the necessary that several basic and general Nephropsidea and the Thalassinidea. problems be worked out in detail. Among Finally, the third hypothesis is that of the most urgent of these are the following: van Straelen (1928) who postulated, from the origin of the group, the genesis of its paleontological evidence, that the ancestors organization, and the identification of the of the Brachyura are to be found among the end products of brachyuran evolution. Pemphicoidea. This opininon has been supported and supplemented by Beur- THE ORIGIN OF THE BRACHYURA len (1930), Glaessner (1930, 1960) and A most difficult and still unsolved prob- Forster (1967). lem concerns the origin of this group. The Which, if any, of these hypotheses is true ancestors of the crabs are unknown; correct? For the reconstruction of the first the oldest demonstrably applicable fossil transition stages ("missing links") between forms (Eocarcinus, Prosopon) being true the Brachyura and their ancestors it would crabs. All present hypotheses, no matter be necessary to know the laws of specific what evidence they rely on, cannot, with- evolutionary development not only of the out raising grave objections, explain the crabs, but of all the decapod Crustacea. principal biological properties of the brachy- Unfortunately, such knowledge is, at pres- uran ancestors. Three major groups of ent, largely lacking. Among these basic such hypotheses which attempt to attribute, laws it would be most important to know respectively, macrouran, anomuran or pem- the relationship between morpho-physiolog- phicoid characters to the crab ancestors, ical changes and changes in mode of life now enjoy their own ardent partisans. and habitat. The importance of such re- The first and the oldest hypothesis lationships is exemplified by two examples ("macrouran") dates from Huxley (1878), elucidated by Schafer (1954). He estab- who considered that the Brachyura had lished that typical (crab-like) forms (e.g., orginated from the Astacidae. A similar Xantho or Eriphia) have ehelipeds folded interpretation was that of Bouvier (1896, against the anterolateral margin of the 1940) who, in his comprehensive and splen- carapace. In this case, the chelae are com- did essay, used not only comparative mor- monly heterochelic and heterodont. These phological evidence, but also all of the crabs have sternites arranged in parallel paleontological and ontogenetic facts of and move sideways. Such forms live pri- that period. This author derived the Brachy- marily in places of strong current or wave ura from the Nephropsidea (Homaridea). action and their condensed body is an A quite different hypothesis is that of adaptation to these environmental condi- Boas (1880) who regarded the Brachyura tions. However the triangle-shaped or as having risen from forms related to the deltoid-shaped forms (e.g., Spider Crabs) genus Axius of the Thalassinidea. Ortmann lack ehelipeds folded against the carapace, (1892, 1896) derived the Brachyura from and the chelae are neither heterochelic nor the Anomura, specifically from a form inter- heterodont. Their sternites are radially mediate between the Paguridea and the arranged and therefore the animals can Galatheidea and considered the Dromiacea move in all directions relative to the longi- as either the most developed Anomura or tudinal axis of the body. These forms live FEATURES OF BRACHYURAN EVOLUTION 333 in the places where motion of water is BRACHYURIZATION reduced because of roughness of the bot- One of the most important problems re- tom. Citing another example, Stevcic (1967, garding the evolution of the Brachyura is 1968) has "linked" environmental factors the genesis of their organization. Borradaile (bottom, food, etc.) to the chemical com- (1916) was among the first to comprehend position of the body and to the behavior the meaning of this process, calling it car- of the Spinous Spider Crab (Maja squinado cinization. However, it would be better Herbst). It is obvious that knowledge of to use the term brachyurization because it these and similar relationships provides a is more general and relevant to all crabs as key to the understanding of the process of a whole, without regard to shape and orga- brachyuran evolution. nizational level. Since we do not know the With regards to the origin of crabs, a true brachyuran ancestors, we can follow further question intrudes: Are the Brachy- the genesis of their organization only par- ura indeed a monophyletic group? As we tially, i.e., on the forms which are already have seen before, there have been some at- crabs. Further, the most primitive crabs tempts to consider Dromiacea as Anomura are deep-sea inhabitants and are aberrant, (Ortmann, 1892, 1896; Gumey, 1942). so that it is very difficult to distinguish Bourne (1922) from his wide investigations between adaptive peculiarities and ances- on the Raninidae, argued that this group tral features in these forms. Nevertheless, originated from the Astacidae indepen- by using all available evidence, we can dently of other crabs. Lately Pichod Viale make many important deductions about the (1966) separated the Homolidea as being development of the new organization. more primitive than the Brachyura. How- In order to make our understanding of ever, today the predominate opinion holds the new organization and its successful ad- that the Brachyura are a homogeneous, vances easier, we should firstly acquaint monophyletic group (Glaessner, 1930, 1960; ourselves with "typical" representatives, Bouvier, 1940; Abrahamczik-Scanzoni, 1942; such as the crabs, Portunus and Xantho. Balss, 1940-61) discrete from the other Their shape is normally crab-like, i.e., the decapods. The following are considered as carapace is usually transverse, depressed, general characteristics of the Brachyura: wide in front with the anterolateral mar- the abdomen is very much reduced and gins regularly arched. Their head is folded under the cephalothorax; the last condensed, i.e., the occular segment is in- thoracic segment is fused with the cephalo- vaginated into the antennulary one (Pichod thorax; the epistome is fused with the Viale, 1966). The antennulae are trans- carapace; the movable finger of the chelae versally folded. The antennae are fused (digitus mobilis, dactylus) is posed exter- with the epistome. The eyes can be pro- nally; the females have a receptaculum tected by complex orbits. The maxillipeds seminis; the first two pleopods of males are depressed, and their third pair covers are transformed into copulatory organs the others as the operculum. The chelipeds (gonopods) and the other pleopods are re- are folded against anterolateral margins duced; the males have a penis; the inhalant and are, thus, heterochelic and heterodont. opening lies primarily before the base of In females the first pleopod is absent. The the cheliped. Other properties vary in con- branchiostegites close the gill chamber nection with the grade of organization and (branchial cavity). The gills are phylo- with adaptation to specific conditions of branches (usually 9 on each side). The existence. endoskeleton is very highly developed At present the question of the origin of (Abrahamczik-Scanzoni, 1942), continuous crabs is still enigmatic and must be con- (Drach, 1950) and the sternites are ar- sidered open. ranged in parallel (Schafer, 1954). The 334 SYSTEMATIC ZOOLOGY zoea is typically brachyuran, i.e., with The majority of the crabs possess the more dorsal spine, and lacks both the nauplius or less typical brachyuran properties de- eye and the frontal organ (Eloffson, 1963). scribed above. The highest morphofunc- In fossil and recent crabs it is possible tional level has been reached by the to follow the gradual improvement of semiterrestrial and terrestrial forms such as organization from the very primitive to the Ocypodidae, Mictyridae, Grapsidae, the most advanced forms. Among the most Gecarcinidae and some species of Potamoni- primitive recent crabs are the Homolo- dae. dromiidae, which are characterized by a The first step in the process of brachyuri- great number of ancestral features: the zation can be followed indirectly in some cephalothorax is cylindrical, the epistome "pseudobrachyurans" i.e., in the crab-like and the front are not quite fused, the head Anomura, where numerous brachyuran-like is not condensed, orbits are lacking, and shapes and structures, are attained without the antennae are not fused with the epi- reaching the level of brachyuran organiza- stome. The antennal thorn and traces of tion. These properties can be found in the uropods remain. The gills are tricho- families Lomisidae, Porcellanidae, Lithodi- branches in great number on each side dae, and partially in the family Hippidae (21). On this organizational level are the and in several genera of the Paguridae, fossil Eocarcinidae and Prosoponidae. A namely: Ostraconotus, Porcellanopagurus, higher degree has been reached by the Tylaspis (Borradaile, 1916). These forms Homolidae and the Latreillidae, in which (excluding the Hippidae) have a depressed the number of ancestral properties is di- and broadened cephalothorax and the ab- minished but they have not quite condensed domen folded partially under the cephalo- the head and the continuous endoskeleton. thorax (Porcellana can still swim by means The next step of organization is evident in of the abdomen). The rostrum is often the Dromiidae and Dynomenidae, which reduced with a broad front (Porcellana). have still more advanced properties; the Lithodes has the anterolateral margin well general shape is crab-like, the chelipeds developed. Some of these forms have re- are folded against the carapace, the head duced the first pair of pleopods in the is condensed and the endoskeleton is well females and the last pleopods in the males developed. This level of organization has (Lomisidae, Porcellanidae). The endo- been attained by the Raninidae and Tymoli- skeleton may be continuous (Lithodidae, dae. All the above mentioned groups have Porcellanidae) and the sternum broad some primitive and specific properties, how- (Porcellanidae) permitting sideways loco- ever, by which they differ from all the motion. Gills are phyllobranches. The other Brachyura: the abdomen is not quite uropods are often reduced (Lithodidae, pressed against the sternum; the sternum Lomisidae). These are specialized forms is narrow; all of them have the primitive for specific environmental conditions, and spermatheca (Gordon, 1950); and they are often aberrant (Lithodidae, Hippidae). possess sternal furrows (excluding the These groups have reached the level of Homolidae and Raninidae) as well as the organization at which Brachyura presum- coxal sexual opening. Additionally, females ably began their evolutionary development. retain the first pair of pleopods (excluding It is very probable that folding of the the Tymolidae and Raninidae); the anten- abdomen was the first and perhaps de- nae are normally parallel and longitudinally cisive act making it possible for either posed; the larvae (zoeas) lack the typical macruran or anomuran ancestors to achieve brachyuran shape and structures; and the a higher grade of organization. Even in last one or two pairs of pereiopods are recent Scyllaridae and Galatheidae the ab- abnormal and posed dorsally (excluding domen may be flexed under the cephalo- the Latreillidae and some Prosoponidae). thorax, protecting the tender ventral parts FEATURES OF BRACHYURAN EVOLUTION 335 of the abdomen from damage. A similar orbits and maxillipeds). Concomitantly phenomenon, where animals flex the ab- with appendicular specialization there oc- domen or fold the body in a ball, is well curred modification and specialization of known in many animals such as the trilo- the inner organs and organ systems, (mus- bites, some isopods, the hedgehogs, arma- cular and circulatory systems, endoskeleton, dillos and others. The long abdomen in stomach and others). benthonic Reptantia progressively lost its Simultaneously with this differentiation, locomotor function and grew weaker and structural and functional integration (Franz, more folded under the cephalothorax, as 1924; Schafer, 1954) occurred. In part, natural selection preferred forms with a integration was realized by concentration reduced abdomen and stronger ambulatory of segments, resulting from the shortening legs. Simultaneously with the reduction and and compressing (Stauchung) of the tho- folding of the abdomen occurred the proc- racic segments. Integration is also mani- ess of gradual depression and broadening fested through the fusion of the particular of the cephalothorax. It is very probable parts of the body as, for instance, the epi- that this process commenced in littoral stome with the carapace and all the thoracic waters, which agrees with Beurlen's (1931) sterna into the thoracic plastron. The proxi- point of view, who considered crabs pri- mal segments of the antennae are fused marily littoral inhabitants. As mentioned with each other and with the epistome. In above, the depressed form may be espe- all the pairs of pereiopods the J>asis and cially advantageous in such habitat, where ischium are fused. The concentration is wave action may be unusually strong. Con- especially manifested in the central nervous sequent to the development of locomotion system, where the ventral ganglia are fused by means of pereiopods was the develop- into a great ventral ganglionic mass. The ment of stronger muscles. These are at- integration of the central nervous system has tached to the endoskeleton, which itself presumably resulted in increased coordina- became stronger through fusion of the tion of movements and in complex be- sternites into the rigid sternal plastron. haviour, especially in the amphibious and The broadening of the cephalothorax and terrestrial forms. modification of the sternites brought with These processes allowed considerable them the peculiar characteristic of the biological progress (in the sense of Sewert- crabs, their sideways locomotion. This zoff, 1931) which is manifested not only process began in the and by the by taxonomic diversity (the number of had resulted in the organization species and individuals) but also in the of the true Brachyura. diversity of habitats occupied by the Brachy- In the process of brachyurization two ura and the means by which they exploit diametrically opposed processes can be their environment. Thus 4450 of 8300 distinguished: differentiation and integra- species of the decapods (after Waterman tion. The segments, especially their ap- and Chace, 1960) belong to the crabs; an pendages, are strongly differentiated in unusually high number of species for a relation to those of the other decapod taxon which is of only infraordinal rank Crustacea. This differentiation is connected (Waterman and Chace, 1960; Glaessner, with the maximum division of labour 1960, 1969). In comparison, the order reached among the decapods. Some seg- Isopoda comprises 4000 species, while the ments with reinforced functions such as the Amphipoda contains 3600. Although the thoracic ones, are intensified, but segments majority of crabs live in shallow water with diminished function, such as the ab- (especially in tropical seas) some exist on dominal ones, are reduced. An especially land and others live at depths as great as great amount of differentiation occurred in about 5000 metres. Because of their great the frontal region (antennae, antennulae, quantity (biomass) and abundance they 336 SYSTEMATIC ZOOLOGY represent a very conspicuous link in the Some organs were further developed chain of nourishment of inhabited biotopes. concomitant with reduction of others; This evidence indirectly confirms the ef- thus, in terrestrial forms the importance of ficiency and importance of the new brachy- the "lungs" is increased and that of the gills uran organization. decreased; chemical receptors are weaker, and the role of vision (and the eyes) is THE END-PRODUCTS OF BRACHYURAN increased. Despite such modifications mem- EVOLUTION bers of these groups are all of basically the From the aforementioned evidence it is same crab-like form in which appear the obvious that the process of brachyurization greatest number of subfamilies, genera, led to higher organization and to general species and individuals. improvement relative to ancestral forms. Among the remainder of the crabs there The process involved increasing the com- are a considerable number of aberrant plexity of organization, which enabled bet- forms, in which shape and structure have ter exploitation of environmental factors deviated considerably from that of the and expansion into new habitats (deep-sea, typical brachyuran, sometimes to the point fresh water, land as well as new niches in of their being difficult to identify as crabs. the littoral zone). The Brachyura in com- Such organisms have undergone especially parison with other decapod groups were far-reaching modifications of the ectoso- more plastic and adapted to life in various matic organs (chelipeds, walking legs, environments. In the Brachyura, the chelae, eyes, antennae, and mouth parts) in re- for example, vary markedly in connection sponse to specialization to a particular with their function, such as alimentation, mode of life in more or less strongly limited so that the crabs as a group use a much surroundings. Such modifications have ap- greater variety of food than did their an- peared in deep-sea, burrowing and com- cestors. The increased mobility of various mensal forms, and must certainly represent types (involving, variously, running, swim- an evolutionary two-edged sword, as, after all, does every specialization. Better ex- ming, burrowing or climbing) enabled the ploitation of a limited range of environ- crabs to achieve great hunting or foraging mental conditions has been attained but, efficiency and speed; particularly quick presumably, at the expense of evolutionary are the terrestrial forms, and in water some plasticity. Such conspicuous specialization portunids are even able to capture mack- has occurred in the , Ho- erels. Brachyura vary in size to where some molidae, Latreillidae, Cymopoliidae (deep- forms inhabit the phytal zone (algae), the sea forms), , Atelecyclidae, coral reefs and even the mantle cavities Calappidae, , Raninidae (bur- of molluscs and the tubes of worms. rowing forms), and Trapezi- Having attained this new level of orga- inae (commensals). Additionally, there are nization, the crabs began a large adaptive a few species of other families (Majidae, radiation. The majority continued to live Xanthidae, Portunidae, and others) which in the littoral zone (most members of the have become deep-sea inhabitants (Doflein, families Xanthidae, Portunidae and Maji- 1904). Since the conditions under which dae). The remainder inhabited the inter- these crabs live are simpler and more uni- tidal zone and the land (Ocypodidae, form than those of typical crabs, there has Grapsidae and Gecarcinidae) and fresh been a gradual decrease of their vital ac- water (Potamonidae), modifying their struc- tivity. In some forms there can be seen ture according to the conditions of the en- signs of regressive evolution (in the morpho- vironment. However, these modifications physiological sense) such as unrolling of were not dramatic and the organization the abdomen (Raninidae, Tymolidae, Do- remained on more or less the same level. rippidae, Corystidae), the reduction of the FEATURES OF BRACHYURAN EVOLUTION 337 sensory organs (especially the eyes) and In work on brachyuran systematics up to weakening of the locomotor system. In the present a common pitfall has been the considering the phylogenetic development differentiation of primary or ancestral char- of these aberrant forms it is necessary to acters from those which are secondary or take into account the fact that many of adaptive. For example, most authors have these properties are secondary and that considered a dorsal position for the 4th and they have originated from typical littoral 5th pairs of pereiopods (Homola, Dorippe), forms. an elongated and cylindrical body (Ho- Finally, far-reaching transformation has molodromia, Ranina, ), elongated occurred in the Hapalocarcinidae, which mouth parts (Homolodromia, Calappa, are so changed that such students of crab Corystes), reduced orbits (Homola, Uca, systematics as Balss (1940-61) and others Mictyris), and so on as primary (ancestral) consider them a separate superfamily. features. Such an interpretation represents Hapalocarcinids are obligate, life-long an oversimplification of the matter. It is inhabitants of coral-galls; they are microph- true that the primitive forms such as the agous (Potts, 1915) and have experi- lower Dromiacea have an elongated body, enced striking regression of their locomotory elongated mouth parts and incomplete (walking legs, muscles, endoskeleton) ali- orbits, but other crabs have secondarily mentary (chelae, mouth parts, gastric-mill) acquired these properties. The elongation and sensory (eyes) organs. of the body and mouth parts can be as- These have been the main results of sociated with a burrowing mode of life. brachyuran evolution. It goes without say- The reduction of the orbits is correlated ing that because of the enormous diversity with the movability of the eyestalks (Pichod of the crabs it is impossible always to es- Viale, 1966) and is often found in terrestrial tablish the pathway by which a given animals. The dorsal position of the legs brachyuran group has evolved. The Geryon- is also a secondary feature and according idae, for example, being deep-sea forms, to Dollo's law of the irreversibility of evo- have some specific regressive characters, lutionary process, it is impossible to return but on the other hand, are very active and them into the 'normal' position of typical raptorial crabs. Finally, it is worth reem- crabs. Additionally, previous workers have phasizing that our difficulty in determining generally failed to recognize the wide- the pathways of evolution within the Brachy- spread occurrence of convergence among ura is a consequence not only of the crabs. Where several groups of various diversity of crabs but also, and more criti- levels of organization have migrated into cally, of the dearth of knowledge about the the same habitat or acquired the same mode mode of life of the majority of crabs. of life (deep-sea, burrowing, etc.) they have assumed similar shapes, structures and DISCUSSION habits (Tymolus-Dorippe, Matuta-Portunus, Ranina-Corystes). Thus, there exist assem- I have presented only the most general blages of convergent forms which, at first outline of the principal features and prob- sight, seem to be monophyletic as for in- lems of brachyuran evolution. From the stance the Oxystomata. The majority of foregoing pages, the extent of the difficulty carcinologists believe that this group is a and complexity of the problem of the origin homogeneous and monophyletic one be- and subsequent development of the crabs cause it possesses many linking properties may be seen. I have focussed on these gen- such as similar mouth parts, buccal cavern eral problems first, so that it will be easier and respiratory system (course of water to subsequently solve specific cases of the currents). Likewise the Oxystomata is evolutionary development of various smaller maintained as a primitive group because groups. its members possess elongate mouth parts, 338 SYSTEMATIC ZOOLOGY

and, in the Dorippidae, the last two pairs types arose in times of transgression of the of pereiopods are positioned dorsally. These sea, and that during regression several conclusions are without any real foundation forms invaded the land and fresh water because, as we have seen before, these while others moved into the deep-sea. He properties may be adaptive (i.e., secondary) believed that this process was repeated ones. Probably all these characters, ex- several times. Therefore it is not necessary cluding the dorsal leg position, are really to postulate the existence of an autono- primitive only in the Homolodromiidae, mous, undefined inner force for the explana- which possess the lowest overall organiza- tion of the evolution of this group. tional level among the crabs. Only a few Undoubtedly the organization of the carcinologists have doubted that the Oxy- Brachyura represents an advance over that stomata are a monophyletic group (Boas, of the other decapod Crustacea. However, 1880; Gurney, 1942; Williamson, 1965; this assertion is true for the Brachyura as Pichod Viale, 1966; Gordon, 1966). The a whole, but not for all of the sub-groups. same situation pertains with the Catomet- Complete morpho-physiological and gen- opa. It is very probable that the families eral biological progress has been attained in this group are uniform only superficially only by several groups of higher crabs (Pichod Viale, 1966). Lately Guinot (1966) (Brachygnatha) in which there are many has cited several examples of genera being genera and species (figures from Chace, quite erroneously classified on the basis 1951): Majidae 145 genera with 673 species, of outward similarity. Portunidae 38 genera with 297 species, The evolution of crabs has thus been Xanthidae 133 genera with 928 species, and intimately associated with changes in mode Grapsidae 40 genera with 333 species. of life and environment; brachyuran sub- Several specialized groups have a lesser groups have, in short, been formed through number of genera and species: Homolo- adaptive radiation. The principal supporter dromiidae 4 genera with 6 species, Latreil- of this point of view is Glaessner (1930, lidae 3 genera with 9 species, Cymopoliidae 1957, 1960); others, such as Beurlen (1930, 3 genera with 29 species, Dynomenidae 2 1933) and Russell (1962) have considered genera with 13 species. Exceptionally, that adaptive radiation is not the only and somewhat more plastic, though relatively indispensable mode of evolutionary change, specialized, groups have more genera and believing that because new forms arose species: Leucosiidae 40 genera with 338 rapidly ("explosively") their evolution was species, Pinnotheridae 26 genera with 222 not necessarily adaptive. These students species. Several groups show, in every way, were followers of the theory of ortho- a maximal progressive development, for genesis, and their theories are not suscep- example, the terrestrial and semiterrestrial tible to any form of objective proof. It is forms; by contrast some groups showed a true, that in the evolution of crabs there total regressive development to the point are, as we have seen, many unsolved ques- of dying out: Eocarcinidae, Prosoponidae, tions, but our insufficient knowledge or Dacoticancridae (the most primitive groups) complete ignorance of some evidence gives and Lobocarcinidae. us no right to base explanations or hypothe- ses which are justified solely on the basis SUMMARY of random premisses. In addition, it is interesting to note that Beurlen (1929, 1. In order to study the evolutionary 1931) unintentionally demonstrated facts development of any group it is necessary that were in opposition to his basic thesis. to use all the available evidence within He stated that the evolution of the crabs the framework of the general laws of evolu- was connected with changes occurring on tion. Most previous research on brachyuran the earth's surface. He established that new evolution considered only a single suite FEATURES OF BRACHYURAN EVOLUTION 339

(usually external morphology) of charac- ACKNOWLEDGMENTS ters and was frequently undertaken outside I would like to express my sincere grati- the context of a contemporary theory of tude to Dr. Isabella Gordon (British Mu- evolution. seum Nat. Hist., London) for having 2. Brachyuran ancestors and forms tran- suggested a number of improvements to sitional between the Brachyura and other this text and for some critical remarks groups of decapods have not been identified about the manuscript. I am also grateful with certitude and the origin of the group to Dr. Gerd von Wahlert (Staatliches Mu- remains uncertain. seum fur Naturkunde, Ludwigsburg) who 3. The genesis of the new organization made some helpful suggestions on the man- (brachyurization) has involved a great uscript. I am indebted to DAAD (Deuts- advance over the other decapods. This cher Akademischer Austauschdienst, Bad process is connected with increasing dif- Godesberg) for a scholarship in West ferentiation of the segments and their ap- Germany. pendages and with maximal integration caused by the concentration of the seg- REFERENCES ments. The result of these changes is an ABRAHAMCZIK-SCANZONI, H. 1942. Beitrage zur improvement of organization to a higher Kenntnis der Musculatur und des Innenskeletts der Krabben. Zool. Jahrb. Abt. Anat. Ontog. level. The process started with the flexion 67:293-380. of the abdomen under the cephalothorax, BALSS, H., W. BUDDENBROCK, H. E. GRUNEB, AND and proceeded with shortening, broadening E. KORSCHELT. 1940-1961. Decapoda. In: and depressing of the cephalothorax. The H. G. Bronn, Klassen und Ordnungen des Tier- associated changes of the organs and the reich. 5, pt. 1. Crustacea, 2nd edit. Bk 7, no 1-15, p. 1-2169. organ systems caused far-reaching struc- BEURLEN, K. 1929. Paralellenentwicklung und turo-functdonal modifications and the entire Iterationen bei Decapoden. Palaont. Z., Berlin. organization rose to a higher grade. 11:50-52. BEURLEN, K. 1930. Vergleichende Stammes- 4. The attainment of this new organiza- geschichte. Grundlagen, Methoden, Probleme tion enabled a large adaptive radiation unter besonderer Beriicksichtigung der hoheren connected with the occupation of new Krebse. Fortschr. Geol. Palaont. 8, (26):317- habitats and associated modifications of 586. BEURLEN, K. 1931. Die Besiedlung der Tiefsee. shape, structure and habits. The majority Natur und Museum. 61, (6): 269-279. of the crabs evolved in typical brachyuran BEURLEN, K. 1933. Zur Entfaltung der Brach- fashion, while a minority more or less yuren und zu der Frage der explosiven Formen- specialized on limited environments, be- bildung iiberhaupt. Cbl. Min. Geol. Palaont. Stuttgart. 1933B:473^79. coming aberrant and, in some cases, under- BOAS, J. E. V. 1880. Studier over Decapodernes went regressive evolution. Slaegtskabsforhold. Danske Selsk. Skr. Nat. og Mat. ser. 6. 1:25-210. (In Danish with French 5. All the aforementioned modifications summary). are connected with changes in the biotope BORRADAILE, L. A. 1916. Crustacea. I. De- and mode of life. The existence of ortho- capoda II. Porcellanopagurus; an instance of genesis cannot be confirmed. carcinization. Brit. Antarct. ("Terra Nova") Exp. 1910. Nat. Hist. Rep. Zool. 3:75-126. 6. These investigations were necessary BOURNE, G. C. 1922. The Raninidae: a study to clarify the most elementary processes, in carcinology. J. Linn. Soc. (Zool.). London. 35:25-78. establishing the limits reached by crab BOUVIER, E.-L. 1896. Sur l'origine homarienne evolution. Consideration of the principal des crabes. fitude comparative des Dromiaces features of brachyuran evolution is a neces- vivants et fossiles. Bull. Soc. Philom. Paris, ser. 8, 8:34-110. sary prerequisite to studying the phylogeny BOUVIER, E.-L. 1940. Decapodes marcheurs. of single sub-groups. Faune de France. 37:1-404. 340 SYSTEMATIC ZOOLOGY

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