FIRST FINDING OF THE PELAGIC CRAB PLANES MARINUS
(DECAPODA: GRAPSIDAE) IN THE SOUTHWESTERN ATLANTIC
Eduardo Daniel Spivak and Claudia Cristina Bas
A B S T R A C T
Most grapsid crabs are benthic species that live in intertidal or subtidal marine or brackish water. Columbus crabs, Planes spp., which live associated with living or inanimate floating objects in
the open ocean are an exception. A group of crabs was found on flotsam stranded on a beach near
M a r Chiquita (37°45'S, 57°26'W), Argentina, and identified as Planes marinus. This is the first report of this oceanic species in the southwestern Atlantic Ocean. The crab was previously collected
in the Pacific Ocean, from British Columbia to New Zealand, and at St. Helena Island (southeastern Atlantic Ocean). Surface oceanic currents are postulated as a possible transport mechanism between the highly isolated collection sites. This paper includes morphometrical data that allowed
the identification of the species and several biological characteristics (sex ratio, relative growth
and sexual dimorphism, incidence o f limb autotomy and regeneration, epibiosis, and fecundity).
Both P. marinus and most littoral marine grapsids have strong sexual dimorphism (male chelae are
larger), a high incidence of autotomy, and a similar egg size and number (the latter related to female size). This information suggests that the life history and behavior of P. marinus, a typical
"pelagic" crab, is similar to its benthic littoral relatives.
The genus Planes includes three species of
oceanic grapsid crabs: P. minutus (Linnaeus,
1758), P. cyaneus Dana, 1851, and P. marinus Rathbun, 1914. These "Columbus crabs"
live on a variety of floating objects such as
drifting kelp (mainly Sargassum), seaweed,
timber, Uelella, shells of Spirula, inanimate
flotsam (ranging from buoys and a wooden
box to feathers, tarballs, and pumice), and
marine turtles (Caretta caretta (L.)). Occasionally, the crabs were found on beaches and
even under rocks (Chace, 1951, 1966; Davenport, 1992; Dellinger et al., 1997).
In March, 1997, a local fisherman collected
a group of Planes from a piece of rope stranded
on a beach near Mar Chiquita (37°45'S,
5 7 ' 2 6 ' W ) , Argentina. On the same substratum there were one specimen of Lepas anatifera L., 1758, and several Caprella andreae
Mayer, 1890 (Amphipoda: Caprellidae). This
is the first report of this species in the southwestern Atlantic Ocean.
MATERIALS AND METHODS
The sample included 40 specimens. They were collected alive, but they died shortly after arrival at the laboratory. The carapace maximum width (CW) and length
(CL), chela length, width, and height, pleon width (fourth
segment), length of 3 distal segments of the second pair
of walking legs, and length of 4 distal segments of the
male pleon were measured with vernier calipers. The
length and width of the male terminal segment (telson)
were measured with a stereomicroscope (6.7x). Crabs
were sexed and females assigned to 3 categories (immature, ovigerous, and nongravid) on the basis of pleon
width and the presence of eggs under the pleon. Females
were considered immature if the pleon did not reach the
coxae of walking limbs. The number and type of autotomized legs, as well as the number of regenerating buds,
and the presence of epibionts, were registered. Fecundity (total number of early eggs per brood) was calculated
from total dry weight of egg masses and average dry
weight of individual eggs; the latter was determined in 9
replicate samples of 4 0 0 eggs collected from different
females. The samples were briefly rinsed in distilled water, put on preweighed aluminum cartridges, subsequently
dried in an oven for 48 h at 80°C, and weighed to the
nearest 0.01 mg. Dry weight measurements of whole egg
masses were made in the same way. Previously, the stage
of embryonic development was determined microscopically; early eggs were completely filled with yolk and had
no sign of differentiation. The diameter of 10 eggs per female was measured under a microscope (lOx) and the egg
volume was calculated assuming a spherical shape.
Sex ratio was tested for deviations from a hypothetical 1:1 ratio employing a goodness-of-fit (chi-square) test.
Quantitative relationships between size and fecundity, as
well as between morphometrical variables, were described
with least-square regressions (after G tests of homogeneity for normal distribution). Log-log regression slope
values lying between 0.9 and 1.1 were considered indicative of isometry ("conservative definition"; Clayton,
1990: 285).
RESULTS
T h e crabs collected from the M a r Chiquita
seashore have a subquadrate carapace, the
b r a n c h i a l r e g i o n s s h o w distinctly striate lateral surfaces, and the walking legs have welldeveloped natatory fringes. Their carapaces
are w i d e r than l o n g ( m e a n ratio o f 1.12 ±
0.03;
range:
1.08-1.25).
This
relationship
d o e s n o t vary w i t h size. T h e m a g n i t u d e s o f
Table 1.
Morphological and morphometrical characters used for the distinction of species of Planes.
(*) Morphometrical data from Chace (1951); the regressions were recalculated on the basis o f mean values available in tables 2 and 3 (p. 73).
the length of the four distal segments of the
male abdomen are 1.08 ± 0.07 times the magnitude of the basal width of the fourth segment
(range: 0.93-1.19). The proportion of the
length of three distal segments of the second
pereiopod to carapace length is 0.81 ± 0.05
(range: 0.65-0.90). The proportion of length
and basal width of the terminal segment of
the male pleon is 0.85 ± 0.05, ranging from
0.75-0.95 (Table 1). Six specimens (3 males,
3 females) had a large white spot that covered most of the anterior half of the carapace.
The sample included both males and females
(immature, nongravid, and ovigerous) ranging
from 10.2-20.7 mm (CL) and from 11.3-23.3
mm (CW) (Table 2). The sex ratio was 15:25
and did not differ from the expected 1:l r a t i o
(X2 test, P = 0.3673). The largest immature female measured 12.2 mm (CL) and 13.5 mm
(CW); the smallest mature female measured
12.9 mm (CL) and 14.1 mm (CW).
The growth of the chelae is isometric
(length) or negatively allometric (height and
width) in females, but shows positive allometry in males (Fig. 1 a-c). The width o f the
pleon grows isometrically in adult females
and shows negative allometry in males. However, data from 3 immature females suggest
a positive allometric growth of their pleon before maturity (Fig. 1 d).
Forty-five per cent of the crabs (9 males, 9
females) showed limb autotomy. Among these,
Table 2. Number, sex, and size range of Planes murinu.s at Mar Chiquita.
12 crabs had lost a single limb, 5 two limbs,
and 1 three limbs. Lost limbs included the
chelae (3 times, always a single autotomy) and
the walking legs (first to fourth: 5, 7, 4, and 6
times, respectively). Fifteen crabs had regeneration buds. The carapace and limbs of 13
crabs had colonies of Hydrozoa attached; one
crab had remains of an unidentified barnacle.
The number of early eggs carried by a female ranged from 5 , 0 0 0 - 2 0 , 0 0 0 . There was
a positive relationship between fecundity and
size, best described by a linear model: number of eggs = 1,852 * C W - 23,125, r2 = 0.81,
F = 28.9, P = 0.01, N = 9. The diameter of
early eggs (4 females) was 337 ± 13 11m; the
estimated volume was 0.020 ± 0.002 mm 3.
DISCUSSION
The present knowledge of the geographic
distribution of Planes spp. is summarized in
Table 3. Apparently, P. minutu.s lives only in
the Atlantic Ocean and the Mediterranean; the
southern limit of its distribution does not exceed 10°S. Planes cyaneus inhabits the Pacific, the Indian, and the Atlantic Oceans. On
the other hand, P. marinus is found in the Pacific Ocean and, rarely, in the Atlantic Ocean.
However, Chace (1966) stated that "This species is probably more common on floating objects than the rather sparse records would indicate." The species identification of these
crabs is based both on morphological and
morphometrical characters. The most relevant
of them are summarized in Table 1. There is
good agreement between the characteristics of
P. marinus described by Chace (1951, 1966)
from St. Helena, those described by Dell
(1963, 1964) from New Zealand, and the characteristics of the Mar Chiquita crabs. Consequently, the distribution of this species should
now be extended to the southwestern Atlantic.
The characteristics of oceanic communities
Fig. 1. Allometric relationships and sexual dimorphism in Planes marinus. CW = carapace width, ChL, ChH, and
C h W = chela length, height, and width, respectively, PW = pleon (fourth segment) width. In a-c, both immature and
mature females are represented by closed circles, males by open squares.
Table 3.
Present knowledge of the geographical distribution of Planets spp.
that live on a variety of floating objects have
been reviewed by Jokiel (1990). Many species of benthic invertebrates, as well as fishes,
are found associated with floating objects, but
there are few "exclusive" residents, among
them Lepas spp. and Planes spp. Jokiel
(1990) emphasized long-distance dispersal of
benthic or shallow-water species by rafting,
since floating objects can drift over distances
of thousands of kilometers. In addition,
oceanic currents may help to explain the wide
geographic distribution of neustonic species
(e.g., Planes and Lepas).
In our sample, the crabs coexisted with several specimens of Caprella andreae, a caprellid previously reported from the North Atlantic, the Mediterranean, and the North Pacific (Takeuchi, personal communication).
The absence of reports of C. andreae and P.
marinus from the southern Atlantic could be
attributed to the scarcity of collections from
floating substrata in this area. However, this
is the first finding of both species on the
southwestern Atlantic shores. This floating
community may have traveled a long distance
before arriving on the coast of Mar Chiquita.
The nearest locality reported for P. marinus is St. Helena, >5,000 km northeast of
Mar Chiquita. The surface currents of the
southern Atlantic (Benguela, Southern Equatorial, and Brazil Currents; Strahler, 1960),
and the western branch of subtropical waters
that arrives at Argentine coasts north of 40°S
in the spring and summer (Piola and Rivas,
1997) may be postulated as transport mechanisms between Mar Chiquita and St. Helena.
However, most findings of P. marinus are in
the Pacific, from British Columbia to New
Zealand. The west wind drift could carry
crabs from New Zealand to the southern Atlantic. There, surface waters going north
along with the Falklands Current could reach
the coast even at the latitude of Mar Chiquita.
If this group of P. marinus found at Mar Chiquita had come from the Pacific, it should
have passed through the cold waters of Drake
Strait (average: �6°C in summer; Hoffmann
et al., 1997). Planes spp. are distributed mostly
in tropical and temperate waters, but some of
the collection sites are characterized by cold
water (Table 3). Unfortunately, there is no information on the resistance o f Planes to low
temperatures. Although surface currents could
explain its transportation from distant places,
the piece of rope where the crabs were found
probably belonged to a ship, and the contribution of human activities cannot be ruled out.
Except Planes, most grapsid crabs live in
shallow marine, brackish, or rarely fresh-water habitats, some of them being semiterrestrial (Anger, 1995). It would be interesting
to compare the life history traits of Planes
with those of their benthic relatives, in order
to detect modifications related to the floating
habitat.
Available information about the biological
characteristics of Planes marinus is scarce.
Chace (1966) reported that the two ovigerous females collected at St. Helena measured
13.4 and 15.1 mm CL. In New Zealand, females of P. marinus are ovigerous year round,
the incubation period lasts about 6 weeks,
with probably more than one egg batch per
year (Wear, 1970). On the other hand, the
ecology and behavior o f P. minutus is better
known (Davenport, 1992; Dellinger et al.,
1997). The populational characteristics of the
group of P. marinus found at Mar Chiquita
are similar to those of P. minutus living on
inanimate flotsam (Dellinger et al., 1997); it
is a mixed group which includes juveniles,
males, and females with the sex ratio close
to 1:1. The only remarkable difference is the
higher incidence of autotomy observed in the
crabs from Mar Chiquita.
Anger (1995, table 1) summarized some of
the available information about life-history
traits of several western Atlantic grapsid species from different habitats. The egg size and
number (the latter in relation to female size)
are similar in P. marinus and many littoral marine species that inhabit rocky shores. In addition, males of P. marinus have significantly
larger chelae than females. This intense sexual
dimorphism is observed in most grapsid crabs,
mainly those that dig burrows in estuarine
muddy beaches or occupy holes or crevices in
rocky shores. A high incidence of autotomy
and regeneration of pereiopods characterize
the group of P. marinus, as well as some intertidal grapsid species (Spivak and Politis,
1989; Luppi and Spivak, 1996). All these
characters suggest that the life history and behavior of P. marinus, a typical "pelagic" crab,
are very similar to its benthic littoral relatives.
ACKNOWLEDGEMENTS
We are indebted to Mr. "Toti" Barragan, who found
the crabs, to the staff of the laboratory of ecology for
bringing them to us, to Dr. Ichiro Takeuchi for the identification of Caprella andreae, and to Dr. Christoph
Schubart for stimulating talks and a critical reading of a
first draft of the manuscript. This study was funded by
the Universidad Nacional de Mar del Plata, Argentina, by
Grant 15/E082 to E.D.S.
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RECEIVED: 13 February 1998.
ACCEPTED: 13 April 1998.
Address: Departamento de Biologia, Facultad de Ciencias Exactes y Naturales, Universidad de Mar del Plata,
Casilla de Correo 1216, 7600 Mar del Plata, Argentina.
(e-mail: [email protected])