Zoological Journal o f the Linnean Society, 59: 59-67. With 1 plate and 4 figures
August 1976
The distribution and orientation of epizoic
barnacles on crabs
D. J . HEATH*
Zoology Department, University College o f North Wales,
Bangor, Gwynedd
Accepted f o r publication, October 1975
The distribution and orientation of Balanus crenatus on Carcinus maenas is described. Elminius
modestus was also found o n C. maenm b u t it was not common. Both species of barnacle were
also found o n Cancer pagums, although neither species of barnacle appeared t o be as common
on Cancer as on Curcinus. Possible reasons for this are discussed.
Treating the carapaces of similar sized C. maenas as sampling units it is clear that the
distribution of 8. crenarus over these sampling units is n o t random b u t is aggregated. This
probably arises because of the gregarious nature of the cyprids when they settle o u t but other
possibilities are considered. B. crenarus o n the carapace of C. maenas is found exclusively in the
grooves and depressions o n the carapace and this is because the cyprids preferentially settle in
concavities.
The orientation of B. crenarus also shows a consistent pattern, with the cirral nets facing
predominantly backwards. This pattern could arise because the barnacle cyprids orientate to
water currents, generated by the exhalant respiratory currents, flowing forwards over the
carapace.
CONTENTS
Introduction
Methods
.
Results
.
Discussion
References
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59
60
60
63
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INTRODUCTION
Barnacles are found as epizoites on a variety of marine animals. Among the
sessile barnacles, Coronula diadema is found on whales (Crisp & Stubbings,
1957) and Chelonibia patula is found on a variety of animals, including the
diamond back terrapin, the crab Callinectes sapidus and the king crab Limulus
polyphemus (Ross & Jackson, 1972). Balanus crenatus is often found on
mollusc shells and decapods (Darwin, 1854; Foxon, 1940; Dexter, 1955).
Balanus amphitrite niveus has been found on two species of shrimp, Penaeus
setiferus (Dawson, 1957) and Sicyonia dorsalis (Eldred, 1962). Stalked
Present address: Department of Biology, University of Essex, Colchester C 0 4 3SQ.
59
D. J . HEATH
60
barnacles are also common on whales and other large marine animals (Gotto,
1969) and on decapod gills (Humes, 1941; Dinamani, 1964; Walker, 1974).
Pearse (1932) and Henry (1954) list barnacles which are found associated with
crabs in the Gulf of Mexico.
Many of these records are casual observations of two species associated with
one another and give few details of the ecology of the epizoite.
This study investigated the distribution and orientation of the sessile
barnacles Balanus crenatus (Brug.) and Elminius modestus (Darwin) on two
species of crab, Carcinus maenus the shore crab and Cancer pagurus the edible
crab.
METHODS
Specimens of C. maenus and C. pagurus were caught in a baited crab trap in
the Menai Straits, North Wales in March, 1975. The size of the crabs and the
number, distribution and orientation of epizoic barnacles were recorded.
KESULTS
Table 1 gives the numbers of barnacles of the two species found on the
carapaces of different sized C. maenas. These figures include live and dead
barnacles and the calcareous bases behind by B. crenatus which have been
dislodged.
The carapaces of crabs of similar size can be treated as “sampling units” and
the distribution of barnacles over these “sampling units” can be tested for
non-randomness. The appropriate test is the variance to mean ratio (Elliot,
1971). Only two size classes (5.0-5.9 cm and 6.0-6.9 cm) have a sufficient
number of observations to apply this test satisfactorily. Values of d (normal
Table 1. Numbers of crabs of a given size with given numbers of barnacles on
carapace*
Size of
crabt
0
1
2
Number of barnacles o n carapace
3
4
5
6
3.0-3.9
4.0-4.9
5.0-5.9
6.0-6.9
1
12
31E
21
0
2
6
0
0
0
1
3
3(E)
(2E)
1(E)
0
0
2
0
0
0
1
4
3(E)
2
0
0
0
0
2
0
0
0
7.0-7.9
2E
8.0-8.8
0
9.0-9.9
1
3
4(E)
0
0
0
2(E)
(E)
1
0
0
0
0
0
1
4
Others$
0
0
0
7 , 7 , 8 , 9 ,1 1 , 11, 12, 1 3 ( 2 E )
21, 30
12, 14,
0
0
* The numbers in the body of the table give th e numbers of crabs of a given size with the given
number of B. crenaius, e.g. in th e size range 6.0-6.9 cm, 21 crabs had no B . crenatus on the carapace, 3
crabs had o n e B. crenatus on th e carapace, etc. The presence of one E. modestus on a crab which also has
B. crenatus is indicated by ( E ) ; two E. modestus on a crab which also has B. crenatus by (2E), etc. E.
modestus occurring in the absence of B. crenaius is indicated by E. Thus in the 6.0-6.9 size class, there
were three crabs each with two B. crenatus on th e carapace. Tw o of these crabs had E. modestus as well,
one crab had one E. modestus and th e other crab had two. In the 5.0-5.9 size class 3 1 crabs had n o B.
crenatus, but one of these had o n e E. modestus.
EPIZOIC BARNACLES ON CRABS
61
deviate) obtained for the distribution of B. crenatus are + 6.9 and + 19.2
respectively. The distribution of barnacles over the “sampling units” deviates
significantly ( P 40.01) from random in both cases and the positive sign
indicates that the distribution is clumped or contagious.
In Fig. 1 the distribution of barnacles of both species on the carapaces of all
crabs has been plotted. (This records live and dead barnacles but not the
calcareous scars. Each point indicates the centre of a barnacle.) These regions
/.
--
rk
\
_-
n
a
.o
a
a
a *
0
a
J
[
Figure 1 . The distribution of barnacles on the carapace of C. maenas. Each spot marks the
centre of a barnacle. 0, B. crenatus; 0,E. modestus.
correspond to the grooves and depressions on the carapace (Plate 1). Barnacles
do not occur on the raised areas of the carapace. The greatest concentrations of
barnacles appear at the posterior end of the cervical groove which separates the
gastric region of the carapace from the branchial region (see Bachau, 1966, for
a description of the areas of the carapace). Figure 2 shows the distribution of
barnacles on those crabs which had only one or two barnacles on the carapace.
The high density of barnacles in this region is again evident.
Barnacles were also found on the other parts of the crab and their
distribution is detailed in Table 2. Relatively little can be deduced from these
figures because the surface area of the different parts varies greatly but the 3rd
maxillipeds d o appear to have more barnacles than other parts with a similar or
greater surface area.
Fewer C. pagurus were caught and the numbers were too low for any
statistical comparisons between this species and C. maenas. The incidence of
infestation of barnacles was lower and the mean number of barnacles on
infested crabs was lower in C. pagurus than in C. maenas. (Incidence of
infestation: 4/16 (25%) in C. pagurus, 581125 (46%) in C. maenas. Mean
number of barnacles on infested crabs: 2.75 in C. pagurus, 5.17 in C. maenas.)
There were too few barnacles on the carapaces of C. pagurus to lead t o any
D. J. HEATH
62
Figure 2. The distribution of barnacles o n the carapace of C. maenas for those crabs that had
only one or two barnacles o n the carapace.
Table 2. Distribution of barnacles on other parts of C. maenas
Position
3rd maxillipeds
Chelipeds:
Propodite
Carpopodite
Meropodite
Basipodite
Coxopodite
Pereiopods
Branchiostegite :
Frontal
Lateral
No. of crabs with barnacles
in given position
12
3
5
6
0
0
2
12
11
Total no. of barnacles in
that position
14
3
8
8 + E
0
0
3
12 + 2 E
25 + E
Total number of crabs = 35.
Figures in Column 3 refer to numbers of B. rrenarus, E refers t o E. modesrus
as in Table 1 .
definite conclusions as to their distribution. Of the 1 1 barnacles, 8 were found
on the antero-lateral borders of the carapace, situated at the bottom of the
ridges which divide this border into 9 lobes with a pie-crust appearance. Two
barnacles were situated in the central region of the carapace and one was
against the postero-lateral ridge.
The orientation of barnacles on the carapaces of C. maenas is shown in
Fig. 3 . This figure records the orientation of live and dead barnacles but
excludes some very small barnacles whose orientation was not readily
discernible. Some of the arrows have been displaced slightly for clarity. The
EPIZOIC BARNACLES ON CRABS
63
Figure 3. Orientation of barnacles on the carapace of C. maenas. Arrows indicate the
rostro-carinal axis of the barnacle and point from the carina towards the rostrum (i.e. arrows
point in the same direction as the cirral net faces). Some arrows have been slightly displaced for
clarity.
arrows point in the direction in which the cirral net faces (i.e. from the carina
towards the rostrum).
Around the fronto-lateral regions of the carapace the orientation of the
barnacles is irregular. In the cervical grooves on the carapace and around the
lateral and poster-lateral borders of the carapace the orientation is more
regular. Most of the barnacles have their longitudinal (rostro-carinal) axis
running parallel to the length of the groove. On the posterior parts of the
carapace and on the postero-lateral edges the cirral nets face predominantly in a
posterior direction. In the more anterior portions of the cervical grooves the
orientation is reversed and the cirral nets face anteriorly and laterally.
Orientation of barnacles on the maxillipeds was also recorded. Of the 14
barnacles on the maxillipeds 1 3 were orientated with the cirral net facing in a
posterior direction (i.e. away from the distal end of the maxilliped) the
remaining barnacle lying at right angles to these.
DISCUSSION
Treating the carapaces of similar-sized crabs as sampling units it is clear that
the dispersion of B. crenatus on C. maenas is aggregated. This almost certainly
reflects the fact that barnacle cyprids are gregarious and tend to settle out on
surfaces where barnacles or the remains of barnacles are already present
(Knight-Jones, 1953). It is however possible that this clumped distribution is
caused by other factors. To apply the meadvariance ratio test the sampling
units should be identical, whereas in this study there was some variation in the
dimensions of the sampling units. The samples should also be drawn from the
64
D. J. HEATH
same population. The crabs sampled may have come from a mixture of
populations; some from areas where there were many cyprids and others from
areas where barnacle cyprids are rare. Shore crabs show seasonal migrations
(Naylor, 1962) so it is possible that different populations may become mixed.
The number of barnacles which settle on the carapace is dependent on the time
available for the cyprids to colonize the surface presented in the intermoult
period of the crab. Any heterogeneity in the growth rates of the crabs could
therefore lead to departures from the expected distribution of barnacles.
Humes (1941) notes that female blue crabs had more barnacles on their gills
than males. The time available for colonization of female crabs is greater than
that available for male crabs since the females do not moult on reaching
maturity. This results in larger numbers of barnacles occurring on females.
The most common species of barnacle was B. crenatus, E. modestus was
much less common. B. crenatus is a predominantly sublittoral species (Crisp,
1958) while E. modestus is more common in the low to midlittoral zone. This
may explain the predominance of B. crenatus on crabs. However, without
detailed knowledge of the proportions of the two species of barnacles and their
larvae in the area and the position on the shore of the crabs when the cyprids
settle out, it is impossible to say whether fewer than expected E. modestus
occur on the crabs.
The fact that the barnacles establish themselves in the grooves and
concavities on the carapace is to be expected. Cyprids are rugophilic and
preferentially settle in grooves and hollows (Crisp & Barnes, 1954). This
probably is of survival value t o the barnacle since it provides protection for the
cyprid and prevents it being washed away. The area of the carapace which is
first colonized by barnacles and carries the highest concentrations of them is
also the area of the carapace where the grooves are deepest and narrowest.
The maxillipeds are fairly smooth and may perhaps carry more barnacles
than expected on the basis of their surface area. They may be chosen as settling
sites by cyprids because of the presence of water currents and a rich supply of
food material falling off the crab’s mouthparts.
Although the numbers of edible crabs collected was too small for any
statistical comparison with C. maenas it seems that there may be some
differences in the incidence of infestation.
Fewer edible crabs were infested and those that were had fewer barnacles on
their carapaces, despite their larger size. The barnacles seem to settle
preferentially around the periphery of the carapace and were rarely found on
the central portions of the carapace. If these differences are real they could be
attributed to a number of factors. C. pagurus is a deeper water crab than C.
maenas (Crothers, 1969) and may therefore be exposed to lower concentrations of cyprids. This is perhaps unlikely since B. crenatus occurs abundantly
down to depths of 50 fathoms (Bassindale, 1964). The fact that the two species
of crab were caught in the same trap at the same time suggests that their ranges
overlap and that they would therefore be exposed to the same concentrations
of cyprids. I t may be however that their ranges do not overlap at the time of
year when the cyprids settle out; C. maenas is known to move up the shore in
summer (Naylor, 1962). Assuming that both species of crab are exposed to the
same concentrations of cyprids, a difference in the degree of infestation could
arise because of the smoother carapace of C. pagurus. The grooves and
.
EPIZOIC BARNACLES ON CRABS
65
depressions present on the carapace of C. maenas are much reduced on the
carapace of C. pagurtts. The only region where there are marked depressions is
around the latero-frontal periphery of the carapace where in fact most of the
barnacles were found.
Barnacles are known to settle with their axes parallel to grooves (Crisp &
Barnes, 1954) and this is well illustrated by Fig. 3 . I t is less easy to see why
they should be orientated with the cirral nets facing in the directions described
in Fig. 3 .
Three stimuli are important in determining the orientation of barnacle
cyprids; contour, light and water currents (Forbes, Seward & Crisp, 1971). Of
these contour is the most important and water currents the least important; the
effect of the latter may be overridden by a directional stimulus of either
contour or light. It is difficult to see how light could be the factor responsible
in this case since the crab will be constantly moving and the orientation of the
carapace t o the light will change. A consistent pattern of barnacle orientation
could not be established through the stimulus of light. Barnacles are known to
orientate with the cirral facing into the water current (Crisp, 1955; Crisp &
Stubbing, 1957) and it is possible that such a response could bring about the
orientations observed in Fig. 3 .
The respiratory current of the shore crab leaves the gill chamber in two
forwards directed streams issuing from apertures to each side of the mouth
(Arudpragasum & Naylor, 1964). This forward current may be enhanced by
movements of the mouthparts and it tends to draw water forwards over the
carapace as shown in Fig. 4. This can be demonstrated with a suspension of
44
44
Figure 4. Direction of water currents over the carapace of C. maenas.
5
66
D. J. HEATH
particles in the water or with dust floating on the surface of the water. These
water currents may be further channelled along the grooves on the carapace
which could act as gutters. The barnacles would then orientate t o face into
these currents as nearly as possible but their primary orientation would be
controlled by their rugotropic response to the orientation of the groove. The
irregular orientation around the antero-lateral edges of the carapace could be
explained by the weak and irregular currents caused by turbulence around the
projecting edges of the carapace. Support for this hypothesis comes from the
orientation of barnacles on the maxillipeds. Barnacles here are orientated
predominantly with the cirral nets facing in a posterior direction. The exhalant
respiratory current would tend to draw water over the maxillipeds in a forward
direction. The barnacles are thus orientated with their cirral nets facing into the
current.
Similar orientations of epizoic barnacles have been described by various
workers. Conchoderma auritum (Gotto, 1969) and Coronula diadema (Crisp &
Stubbings, 1957) on whales have their cirral nets facing into the water current
set up by the swimming of the host. Chelonibia patula tends to lie with the
rostro-carinal axis aligned parallel to the longitudinal axis of the terrapins and
kingcrabs on which it is found. When on the crab Callinectes sapidus, which
swims sideways, the rostro-carinal axis is parallel to the transverse axis of the
crab (Ross & Jackson, 1972). Such orientations are thought t o increase the
feeding efficiency of the barnacle since it can utilize the water currents set up
by the host and they have been termed phoretic associations by Gotto (1969).
The stalked barnacle Octolasmis stella on the gills of lobsters is also orientated
to face the water currents set up around the host gills (Dinamani, 1964).
I t is not clear whether the association described here comes into this
category. The currents set up over the carapace will be weak and shallow and it
seems unlikely that an adult barnacle will have appreciable quantities of water
drawn through its cirri by these currents. It is more likely that the orientation
of the adults merely results from the orientation of the cyprids to the weak
currents and that although very small barnacles may benefit slightly from the
water currents, the adult barnacles gain nothing.
Forbes, Seward & Crisp (1971) describe how Chelonibia patula which is
found around the edge of the carapace of Callinectes sapidus orientates with
the rostrum (and hence the cirral net) facing outwards. This, they suggest may
be an adaptation for detecting predator attacks since the light sensitive organ of
the barnacle is then directed towards the periphery of the host from which
direction most predatory attacks will come. This could equally well be
accounted for by the orientation of the cyprids to the water currents set up by
an exhalant respiratory current as suggested in this paper for barnacles on
C. maenas.
REFERENCES
ARUDPRAGASUM, K. D. & NAYLOR, E.. 1964. Gill ventilation and the role of reversed respiratory
currents in C. maenas (L.). Journal of Experimental Biology, 41: 299-307.
BASSINDALE, R., 1964. British barnacles. Synopses of the British Fauna, No. 14. London: Linnean
Society.
BAUCHAU, A,, 1966. La vie des Crabes. Paul Lechevalier. Paris.
CRISP, D. J., 1955. The behaviour of barnacle cyprids in relation to water movement over a surface.
Journal of Experimental Biology, 32: 569-90.
Zoolo~icnl.~ofr7no/
of the Linnenn Society, 59 (1976)
Plate 1