Zool. J. Linn. Soc., 54: 241-245. With 3 plates and 2 figures
April 1974
The cuticle of the crabs Cancer pagurus L. and
Carcinus maenas (L.)
R. DENNELL, F.L.S.
Department of Zoology, The University, Manchester
Accepted f o r publication September 1973
Fine closely-packed parallel fibres pass obliquely, at an angle of about 10’ to the horizontal,
through the cuticles of Cancer and Carcinus. They lie on axes parallel to the four faces of an
obtuse pyramid, and have no counterpart in the model proposed b y Bouligand (1965, 1971,
1972).
Replication of vertically broken surfaces of cuticle on cellulose acetate film shows that the
laminae of the cuticle are discrete structural entities which preserve their identity around the
angle formed by two vertical faces meeting at right-angles. This does not conform to the
requirements of the Bouligand model.
CONTENTS
Introduction
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Material and methods
Observations
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Discussion
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References
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241
242
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INTRODUCTION
In 1965 Bouligand advanced the hypothesis that the arthropod cuticle is
composed of a helicoidal array of parallel fibres. According to this view the
laminae, and the parabolic arcs of fibres often seen between them, are no more
than artifacts caused by the sectioning of the fibres. Bouligand re-asserted his
hypothesis in 1971, and in 1972 adapted it to meet the requirements of
growing knowledge of morphological complexity. He used his revised model to
explain how apparent laminae may lie at an oblique angle to the plane of
horizontal sheets of curved fibres. Green & Neff (1972) have followed
Bouligand in their interpretation of the cuticle of the fiddler crab.
Bouligand’s elegant and attractive hypothesis owes much to his study of the
cuticle of Carcinus, but Dennell(1973) was unable to relate his observations on
this crab to the hypothesis. The laminae appeared to be distinct and separable
structures, and the presence of parabolae between them due to the presence of
continuous interlaminar fibres. Large sinuous fibres, perhaps tubular, with no
counterpart in Bouligand’s model, were also found to be present.
241
242
R. DENNELL
According to Bouligand’s model, all oblique sections of the cuticle, however
orientated, should present the appearance of parabolic fibres between the
apparent laminae. Moreover, if the cuticle is broken vertically in planes at right
angles, the laminae should not seem to be continuous in passing from one plane
to another, but t o be out of register to the extent of half the interlaminar
interval. The acquisition of very thick cuticle from the cheliped of Cancer
pagurus gave the opportunity of discovering whether these requirements of the
Bouligand model are met by the structure of the cuticle.
MATERIAL AND METHODS
Pieces of the 3 4 mm thick cuticle of the propodus of the cheliped of Cancer
were decalcified in 70% alcohol acidified with HC1, and cut into strips. From
these strips, squares, with varying orientation to the long axis of the strips,
were cut. The squares were then embedded in celloidin, and sectioned
obliquely at an angle of 45” to the four vertical faces, thus converting them
into truncated pyramids. The sections from the faces, 6 0 p m thick, were
stained in aniline blue and examined by phase-contrast microscopy.
The cuticle of the merus of the second pereiopod of Carcinus was also
decalcified and embedded in celloidin, and sectioned obliquely t o the
horizontal plane. 10 pm thick vertical paraffin sections were also cut.
Since Bouligand (1972) has cast doubt on the validity of observations made
directly by incident light microscopy on the continuity of laminae around a
right-angle break, cellulose acetate film replication was resorted to. The broken
surfaces of the undecalcified cuticle of Cancer were wetted with acetone, and a
strip of film applied. After allowing to dry for ten minutes the strip was peeled
off and attached, under gentle tension to straighten it, by “Cellotape” to a glass
slide. It was then examined dry by phase-contrast microscopy.
OBSERVATIONS
The following observations were made on the calcified zone forming the
main mass of the cuticle beneath the thinner calcified pigmented layer.
The appearance of sections from the four faces of the truncated pyramids
obtained from the cuticle of Cancer varied with the orientation of pyramids.
On a number of occasions the sections from all four faces presented the
appearance shown in Plate 1A. The laminae, being cut obliquely, are diffuse,
and macrofibres like those reported in Carcinus (Dennell, 197 3 ) are clearly
visible. No parabolic arcs are seen, but fine parallel fibres run obliquely
upwards across the laminae. Close observation shows that these fibres do not lie
precisely in the plane of the section, but the thickness of the section causes
them to be visible over considerable distances as they run obliquely to the
plane of the section. This and the close packing of the fibres gives a superficial
appearance of continuity.
On the other hand, if the pyramid is orientated at 45” to one giving the
above appearance, parabolic arcs are seen on one face only. The three other
faces again show fine fibres, but now two sets of fibres are apparent, crossing as
they pass obliquely upwards through the cuticle (Plate 1B). Again the fibres do
not lie preciseIy in the plane of the section. It was not possible to observe with
CUTICLE OF CANCER AND CARCINUS
243
certainty the presence of crossed fibrils on the face displaying parabolic arcs,
but their presence here also is to be inferred from a comparison of this pyramid
with that described above. These observations are summarized in Fig. 1 .
C
A
0
D
Figure 1. The appearance of the faces of cuticular pyramids from the propodus of the cheliped
of Cancer. The arrow indicates the long axis of the strip from which the pyramids were cut.
Laminae are indicated as solid lines and fine fibres as broken irregular lines.
Cuticular pyramids with intermediate orientation show laminae with
somewhat plumose appearance, but not complete parabolae, on two faces, and
fine fibres at varying angles on the remaining two faces.
Reexamination of the cuticle of Curcinus shows that it too possesses similar
fine fibres. In order to discover the angle at which the fibres pass upwards
through the cuticle, thick (60 pm) sections were cut at varying angles t o the
horizontal. In sections only slightly oblique to the horizontal the fibres were
found to be continuous (Plate 2A). Thin (10 pm) vertical paraffin sections gave
further information. Suitably orientated sections show fine fibres (Plate 2B, X)
running in the plane of the section and at an acute angle to the laminae. Other
fine fibres (Plate 2B, Y) pass obliquely upwards in a plane at right angles to the
plane of the section.
Cellulose acetate peels from the broken surfaces of pieces of undecalcified
Cancer cuticle are illustrated in Plate 3A, B. The laminae are clearly visible and
maintain their integrity in passing from one face to the other of a right-angled
break.
244
R . DENNELL
Figure 2. The disposition of fine fibres and parabolic arcs in a rectangular block of cuticle.
Except for the oblique face showing interlaminar arcs, which for the sake of clarity is
represented as opaque, the block is shown as transparent. For simplicity only the fibres in the
four diagonally inclined planes are indicated: additional fibres lie in planes parallel to these. The
position of laminae is indicated ohly o n the oblique opaque face and by broken lines at two
corners of the block. Sinuous macrofibres are not shown.
DISCUSS I 0 N
The observations recorded above show that the brachyuran cuticles
examined are more complex than the model proposed by Bouligand (1965,
1971, 1972) and do not support the view that the appearance of laminaeand
intervening parabolic arcs is due to optical and mechanical artifacts arising from
the sectioning of a pile of helicoidal fibres. The fine fibres described here have
no place in the Bouligand model, and the fact that the appearance of parabolic
arcs is not presented by all oblique sections is not in agreement with the model.
The conception of cuticle structure adopted here is illustrated in Fig. 2. The
fine fibres pass obliquely through the laminae in planes parallel t o the four
faces of a gently sloping pyramid. The parabolic arcs (interlaminar fibres,
Dennell, 1973) lie in a plane tilted at a steeper angle and oblique t o the planes
of the fine fibres. This disposition of arcs and fibres results in their arrangement
in a rectangular block of cuticle as shown in Fig. 2, and is in accordance with
the appearance of the sections obtained when such a rectangular block is
converted into truncated pyramids of differing orientation. It may be noted
that Drach (1953) not only described the parabolic arcs in Homarus as
continuous fibres but regarded them as lying in a plane oblique to the laminae.
The continuity of laminae as seen in replicas of right-angled breaks is in
agreement with the observations of Drach (1939), and is consonant with the
Zool. J. Linn. Soc., 53 (1974)
R. DENNELL
Plate 1
(Facing p . 244)
Z G OJ~. .L i m . SOC.,
5-l (1 971)
R. DESNELI,
Plate 2
Zoo!.
J. Linn. SOL.,54 (1974)
R. DENNELL
Plate 3
CUTICLE OF CANCER AND CARCINUS
245
view that the laminae are discrete sheets of cuticular material (Dennell, 1973)
and not artifacts as regarded by Bouligand (1965, 1971, 1972).
It seems from these and previous observations (Dennell, 1973) that the
brachyuran cuticle is to be regarded as a pile of laminar sheets tied together by
interlaminar parabolic fibres and obliquely interwoven fine fibres. The passage
of sinuous macrofibres from one interlaminar zone to another (Dennell, 1973)
may be an additional factor in binding the laminae together. The result of this
architectural arrangement of the components of the suticle is a fabric of
considerable mechanical strength even without the presence of the protein
constituents of the cuticle and the effect of calcification.
REFERENCES
BOULIGAND, Y., 1965. Sur une architecture torsadCe ripandue dans de nombreuses cuticules
d’arthropodes. C. r. hebd. Skunc. Acad. Sci., Paris, 261: 3665-8.
BOULIGAND, Y.,1971. Les orientations fibrillaires dans le squelette des Arthropodes. I. L’exemple des
crabes, I’arrangement torsadi des strades. J. Microscopie, 11: 441-72.
BOULIGAND, Y., 1972.Twisted fibrous arrangements in biological materials and cholesteric mesophases.
Tissue und Cell, 4 : 189-217.
DENNELL, R., 1973.The structure of the cuticle of the shore-crab Curcinus menus (L.).Zool. J. Linn.
SOC.,52: 15963.
DRACH, P., 1939. Mue et cycle d’intermue chez les Crustacks Dicapodes. Annk. Inst. ockunogr., Monaco
(N.S.), 1 9: 103-391.
DRACH, P., 1953. Structure des lamelles cuticulaires chez les Crustacks. C. r. hebd. Skunc. Acud. Sci.,
Paris, 237: 1772-74.
GREEN, J. P. & NEFF, M. R., 1972.A survey of the fine structure of the integument of the fiddler crab.
Tissue an# Cell, 4: 137-71.
EXPLANATION OF PLATES
PLATE 1
A. Thick (60 pm) celloidin section taken from one of the faces of a pyramid of cuticle from the
propodus of the cheliped of Cuncer pqgurus. Maaofibres (pale) and fine fibres are visible, but
no parabolic arcs. The remaining three faces of the pyramid have a similar appearance.
B. Thick (60 m)celloidin section from a face of a pyramid orientated at 45’ to that illustrated
above. Fine fibres are seen crossing. Two of the other faces have a similar appearance, and the
fourth shows parabolic arcs.
PLATE 2
A. Thick (60pm) celloidin section slightly oblique to the surface of the cuticle of the merus of
the second pereiopod of Cbrcinus muenus. Continuous fine fibres are visible.
B. Thin (10pm) paraffin vertical section of the cuticle of the merus of the pereiopod of
Carcinus. Fine fibres are seen at X and Y.
PLATE 3
A, B. Cellulose acetate replicas of vertical broken surfaces of the undecalcified cuticle of the
cheliped propodus of Cuncer. Surfaces X and Y meet at a right-angle in the centre of the
photographs. The laminae on the two faces are in register.