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Tuatara: Volume 2, Issue 1, March 1949
A Guide to the Brachyrhynchous Crabs
A Guide to the Brachyrhynchous Crabs
The New Zealand crabs have had a stormy systematic history. Even today there is no complete convenient account available describing these common and interesting animals which are such an important element in our marine fauna. This guide is not a final statement of the systematics of these animals. It is only a step in bringing this part of this fauna within the reach of students and cannot as yet be carried to completion because much literature is unavailable. It has been developed from my personal needs and is presented here as an aid and stimulus to the researches which are yet to be undertaken. I will be very grateful for any corrections, alterations or additions which can be suggested. Many species of our crabs are poorly known, some only from a brief account of the one original specimen and the status of others, even of common species (e.g. Ozius truncatus) is still in doubt. Species are included in the guide which are doubtfully part of our fauna. These species are bracketed and noted here to enable their recognition if actually present.
This is a guide to our crabs of the families Portunidae, Cancridae, Xanthidae, Grapsidae, Ocypodidae and Pinnotheridae, which are Brachrhyncha—crabs in the ordinary sense, lacking a rostrum, having a round to squarish body, the mouth located in a square to oblong area (the mouth-field), covered by the mouth-parts of which the third maxillipeds are the external elements. These Brachyrhyncha contrast with the spider-crabs and masking-crabs which also have an oblong mouth-field, but in these the carapace is commonly triangular in outline with the apex anterior so that these crabs are grouped as the Oxyrhyncha. Because of the common oblong form of the mouth-field, crabs and spider-crabs are grouped together as Brachygnatha. In this respect they contrast with another common group of crabs, the Oxystomata, in which are included the ball-shaped crabs, the box-crabs, and the flat-back crabs—varying much in shape but having in common a narrow, elongate and rather triangular mouth-field.
Several portunid crabs not listed in the key have been recorded from New Zealand. In particular species having 9 antero-lateral spines should be watched for and reported if found. There appear to be only the two species of cancrid crab; the xanthid crab fauna is richer but very inadequately known and much further research is required. I have handled two additional distinct species, neither identifiable with the species listed in the key. The grapsid and ocypodid crabs are reasonably in order but the pinnotherid crabs are in need of extensive page 30 revision. P. pisum, a European species has been identified in New Zealand material by at least two specialists, one of whom considered our dwarf Portunus as the northern hemisphere Portunus corrugatus. Local students may find the opportunity for important work in the description and revision of Xanthidae and Pinnotheridae.
The key is drawn from many sources, but its content is largely determined by Chilton and Bennett's review of the Brachyura (Trans. N.Z. Inst., v. 59, 1928) which clears the fauna of some species, establishes the nomenclature for others, and gives a ready entry into the literature on this subject. Rathbun's monographs (U.S. Nat. Mus., Bulls. 97, 129, 152, 166) are invaluable as a reference and guide in many problems, and will be found most useful for further detail required in advanced studies.
The figures are restricted to an outline of the carapace and it must be noted that to enable presentation of all these species in two plates, the figures are drawn to much the same size. The figures are not drawn to scale which would have been well-nigh impossible for this guide since the species illustrated range in width from one half inch to four inches and a half. An indication of the width of each species is given in the key. For much the same reason, the ornamentation of the surface of the carapace is commonly omitted. The figures are useful only in conjunction with the key. Each illustration shows the fronto-orbital width (between the lateral orbital spines, marked by the outer pair of arrows), the contour of the front (between the orbits, between the medial pair of arrows), and the antero-lateral and postero-lateral margins of the right side. Excepting where noted in the key, the spines at the orbital angles are included in the count of the antero-lateral spines, and frontal spines respectively.
The keys are set out as far as possible on a natural systematic basis, so that confusion between the species included and others is unlikely. The features selected are generally readily observable, but difficulty will be found in the case of the Xanthidae. The important ridges which are mentioned in 1 (4) can only be seen by turning the mouth-parts well down. If present, the ridges may be low, rounded and not very conspicuous; if absent, the plate anterior to the mouth will be quite smooth, as for example in Ovalipes. The reference numbers, e.g., Key to Families, 1(6), are alternatives. Where there is not agreement with 1, then refer to 6. Where the agreement is with 6, then proceed to 7, etc. This type of key is advantageous in that the summary of the features fitting the specimen give the essential points in the generic and familial definitions. Also, the method permits a ready return through the key when errors have been made, or when the review of the features dealt with is desired.
page 31Key to the Families of the Brachyrhyncha
| 1(6). | Palp of external maxilliped articulating at or near the anterior medial corner of the merus (Fig. 4a). Body usually round, or oval, not squarish. Right chela often larger than left. | |
| 2(3). | Legs flattened, paddle-like, adapted for swimming. | F. Portunidae. |
| 3(2). | Legs not adapted for swimming. | |
| 4(5). | First antennae fold length-wise (i.e. vertically) | F. Cancridae. |
| 5(4). | First antennae fold obliquely or transversly (i.e., more or less horizontally) | F. Xanthidae |
| 6(1). | Palp of external maxilliped not articulating at or near the anterior medial angle of the merus (Fig. 18a). Body usually more or less squarish. Right chela never larger than left. | |
| 7(10). | Free-living crabs with a more or less straight-sided body and well-developed eyes. | |
| 8(9). | At least a narrow gap between the external maxillipeds (Fig. 18a). Front broad, eye-stalks relatively short, less than one-third width of carapace, dactyl of walking legs often compressed and spinous. | F. Grapsidae |
| 9(8). | External maxillipeds almost or quite closing across the mouth. Front narrow; eye-stalks relatively long, subequal or exceeds one-third width of carapace; dactyl of walking legs, styliform. | F. Ocypodidae. |
| 10(7). | Symbiotic crabs with reduced eyes and orbits. Body swollen. | F. Pinnotheridae. |
Key to the Species of Portunidae (Swimming Crabs)
| 1(10). | Antero-lateral teeth 3 to 5. | |
| 2(5). | Antero-lateral teeth, 4. | G. Nectocarcinus. |
| 3(4). | Front margin, narrow, prominent, divided into 4 teeth (not counting the inner orbital spine). N. antarcticus (Fig. 1) (Rough swimming crab, below tide level and offshore, sandy beaches, up to 1 ½ inches.) | |
| (4)3. | Front margin wide, arched, entire | (N. integrifrons). |
| 5(2). | Anterolateral teeth, 5. | |
| 6(9). | 3 or 4 sharp teeth on frontal margin. | G. Ovalipes. |
| 7(8). | Front with 4 small teeth; dorsal margin of orbit divided by 2 fissures, so forming 3 lobes of which the median is spinous. O. bipustulatus (Fig. 2). (Smooth swimming crab, below tide level, sandy beaches, up to 2 ½ inches.) | |
| 8. | Front with 3 acute teeth; only one fissure in the dorsal margin of the orbit. | (O. ocellatus). |
| 9(6). | Frontal margin with 3 finely crenulated lobes. (Portunus borradailei (Fig. 3) (Dwarf swimming crab; offshore; about ½ in.) | |
| 10(1). | Antero-lateral teeth, 9. (Several species of Neptunus reported, all questionable.) |
Key to Species of Cancridae
| 1(2). | Antero-lateral margin with many (ten) obtuse teeth, the number varying with age; 5 acute frontal teeth (all teeth relatively larger and more definite in young specimens). Cancer novae-zealandiae (Fig. 4). (Lower tidal of rocky and stony beaches; offshore in deeper waters; up to 4 inches.) |
| 2(1). | Antero-lateral margin with a few (three) broad flat lobes, frontal margin lacking teeth. Heterozius rotundifrons (Fig. 5) (Big-hand crab; tidal of stony and rocky beaches; up to 1 in.) |
Key to the Species of Xanthidae.
| 1(4). | Ridges on roof of mouth field define the efferent branchial channels and reach to the anterior boundary of the mouth field, ridges often strong. |
| 2(3). | Fronto-orbital border half or less than half the greatest width of the carapace; anterior tooth very broad, almost indistinct; carapace flat. Ozius truncatus (Fig. 6) (Black-finger crab, lower tidal, gravel, rocky and stony beaches, up to 1 ½ inches.) |
| 3(2). | Fronto-orbital border much more than half the greatest width of the carapace. G. Pilumnus. (The hairy crabs; lower tidal and offshore.) |
| Antero-lateral margin with three posterior obtuse and two anterior poorly defined spines. P. novae-zealandiae (Fig. 7). (Less than one inch.) | |
| Antero-lateral margin with 5 slender subequal spines. P. maori. (Fig. 8). (Half inch.) | |
| Antero-lateral margin with 4 spines. P. spinosus (Fig. 9). | |
| 4(1). | Ridges defining efferent branchial channels, if present, very weak and never reach to the anterior boundary of the mouth field. |
| 5. | Carapace more or less hexagonal or subquadrate; five antero-lateral teeth of which the first and second are partially fused. Panopeus otagoensis (Fig. 10). (Half inch.) |
Key to Species of Grapsidae
| 1(4). | Antennules visible in dorsal view, situated in deep clefts in carapace. |
| 2(3). | Interantennular region of carapace wider than long. Carapace broader than long. Plagusia chabrus (Fig. 11). (Red rock crab, lower tidal, in crevices, up to 3 ¼ inches.) |
| 3(2). | Interantennular region of carapace longer than wide; anterior margin of merus of legs acutely spined; carapace and legs extremely flat. Percnon planissimum (Fig. 12). (Spray crab; wave-washed rocks; 1 inch.) |
| 4(1). | Folded antennae not visible in dorsal view (concealed as is usual). |
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PLATE 1.—Nectocarcinus antarcticus; la, paddle. 2, Ovalipes bipustulatus; 2a, paddle; 3, Portunus borradailei; 3a, paddle; 4, Cancer novae-zealandiae; 4a, external maxilliped; 5, Herterozius rotundifrons, male; 6, Ozius truncatus; 7, Pilumnus novae-zealandiae; 8, P.maori; 9, P.spinosus; 10, Panopeus otagoensis; 11, Plagusia chabrus; 12, Percnon planissimum. Figures 7, 9 and 10, after Filhol; 8, after Borradaile.
| 5(12). | An oblique hairy ridge on the exposed surface of the external maxilliped. |
| 6(9). | Carapace quadrate or subquadrate; sides straight, incised or toothed. |
| 7(8). | Front inclined downward, slightly concave above, broadest at base where it is about one-third width of carapace; external maxillipeds with the third joint short, truncate above. Helice crassa (Fig. 13). (Tunnelling mud-crab; tidal mud flats; three-quarters of an inch.) |
| 8(7). | Front abruptly deflexed, more than half width of carapace, divided into lobes; third joint external maxillipeds longer than broad, rounded above. (Sesarma pentagona). |
| 9(6). | Anterior half of carapace with entire arched margin, posterior half rectangular. G. Cyclograpsus. |
| 10(11). | Edge of carapace finely granulated; outer corner of orbit raised to a point; hairs along length of tarsus. C. lavauxi. (Among stones, lower tidal, ½ to ¾ inch.) |
| 11(10). | Edge of carapace smooth; orbit lacking a raised point at outer corner; tarsus with hairs only at tip. C. whitei (Fig. 14). (Upper tidal, among rocks and stones, about one inch.) |
| 12(5). | No oblique hairy ridge on exposed surface of external maxilliped. |
| 13(16). | Lower border of orbit not running downward toward the buccal cavern but is supplemented by a rather distant suborbital crest in line with the anterior border of the epistome. G. Hemigrapsus. |
| 14(15). | Legs hairy; chela of male hairy on medial surface; suborbital ridge in male not uniformly granular being more strongly granular along medial half and less, if at all, on lateral half. H. crenulatus (Hairy-handed crab; green to white, mud-flats and sonv beaches; middle and lower tidal levels; about 1 in.) |
| 15(14). | Legs hairless; suborbital crest in male uniformly granular for full length. H. sexdentatus (Fig. 15). (Reddish-purple; beneath stones and boulders, sand-flats to rocky shores; upper and middle tidal levels; commonly up to 1 ½ inches.) |
| 16(13). | Lower border of orbit runs downward toward the buccal cavern. |
| 17(20). | Front, half or more than half greatest width of carapace. |
| 18(19). | Carapace convex, almost smooth. Planes minutus (Fig. 16). (Gulf-weed crab; in floating weed; ¾ inch.) |
| 19(18). | Carapace depressed, distinctly striated, toothed. (Pachygrapsus transversus) (Fig. 17). (Among stones, on mud or sand; about ½ inch.) |
| 20(17). | Front much less than half greatest width of carapace. |
| 21(22). | Merus of external maxilliped not longer than broad (Fig. 18a). Leptograpsus variegatus. (Fig. 18). Upper tidal of rocky beaches; growing to 2 ½ inches.) |
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PLATE 11.—Fig. 13, Helice crassa; 14, Cyclograpsus whitei; 15, Hemigrapsus sexdentatus; 16, Planes minutus; 17, Pachygrapsus transvorsus; 18 Leptograpsus variegatus; 18a, external maxillipeds; 19, Grapsus grapsus; 20, Ucahuttoni; 21, Ommatocarcinus macgillivrayi; 22, Hempilax hirtipes; 23, Pinnotheres pisum. Fig. 17, after Rathbun; 19, after Boone; 20, after Filhol.
| 22(21). | Merus of external maxilliped longer than broad (Grapsus grapsus) (Fig. 19). (Spray zone and upper tidal of rocky beaches; up to 3 ½ inches.) |
Key to Species of Ocypodidae
| 1(2). | An orifice or recess, margined thickly with hair, between the bases of the second and third pairs of walking legs; antennular flagella small, folding vertically or obliquely. (Uca huttoni). (Fig. 20.) (Fiddler crab; known from one specimen.) |
| 2(1). | No special recess between the bases of any of the legs; antennular flagella well-developed, folding transversely. |
| 3(4). | Antero-lateral angles extended as a long spine directed laterally; front very narrow. Ommatocarcinus macgillivrayi. (Fig. 21.) (Offshore, 10 fathoms and more; found in the stomach of dogfish and other fish; 1 inch including spines.) |
| 4(3). | Antero-lateral margin tridentate; front 1/3rd. the width of the carapace which has almost straight sides. Hemiplax hirtipes. (Fig. 22.) (Mud-flats and Zostera beds; 1 inch.) (Superficially resembles H. crassa but distinct in having eye-stalks equal to front.) |
Key to Species of Pinnotheridae
(NOTE: The free-living Hymenosomidae, for long included in this Family, are dealt with later as Oxyrhyncha. These are small flat-backed crabs of the genera Hymenicus, Halicarcinus, Hymenosoma and Elamena.)
Carapace swollen, obese, globular, second joint of external maxillipeds small or lacking G. Pinnotheres.
| 1(2). | Front ponderous, concealing eyes from dorsal view; margin of female abdomen, smoothly rounded. Pinnotheres pisum. (Fig. 23.) (Pea crab; symbiotic, inhabiting mantle chamber of mussels, etc.; ½ inch.) (This European species has been carefully checked by several workers.) |
| 2(1). | Front not concealing eyes in dorsal view; posterior end of female abdomen not smoothly rounded, but ending in a short acute point. (Pinnotheres novae-zealandiae.) |