2009 NORTHEASTERN NATURALIST 16(4):637–646
Notes on the Unusual Megalopae of the Ghost Crab
Ocypode quadrata and Related Species (Decapoda:
Brachyura: Ocypodidae)
John J. McDermott*
Abstract - The postlarval stage, or megalopa, of Ocypode quadrata (Atlantic Ghost
Crab) has distinctive morphological characteristics, with large size being its most
recognizable character. Nevertheless, there is little biological or ecological information
on this critical stage in the crab’s life cycle. Over a 20-year period (1980–2000)
of field work, only five specimens have been collected incidentally along the sandy
beaches of southern New Jersey, near the northern limit of its geographic range.
Megalopae were obtained via seine, hand net, and sediment core samples from September
6 to October 6, even though some of the locations were sampled throughout
the year. Mean carapace width (CW) and length (CL) for the five megalopae were
4.85 ± 0.59 mm by 6.14 ± 0.68 mm, respectively; CW ranged from 4.01–5.43 mm
and CL from 5.18–7.01; mean CW/CL ratio was 0.790 ± 0.023. Carapace dimensions
of New Jersey specimens were compared with megalopae recorded in the literature
from the shores of Long Island, NY and North Carolina, as well as data from two
eastern Pacific species, Ocypode gaudichaudii and O. occidentalis. Morphology of
the megalopa appears to be adaptive for survival through metamorphosis into the first
crab stage. Recruitment of O. quadrata to sandy beaches in the northern part of its
range is probably in the fall.
Introduction
“The large size and peculiar structure of this megalops render it one of the
most interesting forms of the group of larvae to which it belongs.” S.I. Smith
(1873a) reference to the megalopa of Ocypode quadrata.
Ghost crabs, Ocypode spp., are well-known, semi-terrestrial, generally
warm-water inhabitants of worldwide marine sandy beaches, where older
animals live in burrows primarily well above mean high-tide level, and juveniles
occur closer to the water line. Ocypode quadrata (Fabricius) appears
to be the only species of this genus found on beaches of the western Atlantic
Ocean, ranging from Long Island Sound, NY to Brazil (Cowles 1908; Haley
1969, 1972; Milne and Milne 1946; Williams 1984). Other species of Ocypode
are found on the eastern coast of Africa (Monod 1956) and the eastern
Pacific and Indo-West Pacific coasts (Crane 1940, Poore 2004).
Laboratory-raised zoeal stages (five instars) and the megalopa of
O. quadrata from North Carolina were described by Diaz and Costlow
(1972). They distinguished these larvae from those of four other
*Department of Biology, Franklin and Marshall College, Lancaster, PA 17604-3003;
jmcdermo@fandm.edu.
638 Northeastern Naturalist Vol. 16, No. 4
species, O. cordimanus Desmarest, O. gaudichaudii (Milne Edwards and
Lucas), O. occidentalis Stimpson, and O. platytarsis (Milne Edwards).
Although complete zoeal series were not known for the latter four species,
the gross morphological characteristics of their distinctive, large megalopae
were very similar to those of O. quadrata.
Little biological information is available for the Ocypode megalopa, considered
by Hines (1986) to be among the largest of all known brachyurans
(Fig. 1). However, the megalopae of the vent crab Bythograea thermydron
Williams are larger (Williams 1980). Few megalopae of O. quadrata have
been reported from the planktonic zone or at the surf-beach interface where
metamorphosis into the first crab stage takes place (Say 1817; Smith 1873a,
b; but see Smith 1880). They are easily recognized in plankton samples,
sieved sediment, or in seine samples because of their extraordinary size,
globose body, large blue eyes and often white or mottled coloration. Thomas
Say (1817), confused as to the nature of preserved museum specimens of
the O. quadrata megalopae, described them (without illustrations) as adult
crabs new to science. The megalopa was named Monolepis inermis (genus:
one scale, which refers to caudal lamellae on each side of the telson; species:
unarmed or defenseless). Smith (1873a, b) recognized the megalopae
he collected in 1870 on Fire Island beach (south shore of Long Island) as
those of O. quadrata, the largest of which was 5.6 mm by 6.4 mm in CW
Figure 1. Photograph of a living megalopa of O. quadrata, antero-dorsal view, collected
2007 from a sandy beach in Fort Macon State Park, NC, courtesy of Park
Ranger Randy Newman. Specimen not measured, but was approximately 5 mm wide
(R. Newman, pers. comm.).
2009 J.J. McDermott 639
and CL, respectively. He did not give the dimensions of any other specimens.
Megalopae of O. quadrata were apparently much less numerous than the justmetamorphosed
first-crab stages in the same location. The latter were 6.1 to
6.5 mm CW and 5.6 to 6.0 mm CL (width now greater than length). Smith
(1873a, b) described the basic morphological characters of the megalopa
(without illustrations) as follows: large eyes; carapace convex above, deep
body, posterior broader, no dorsal spine; front sharply deflexed, with long medial
tooth; sides of body impressed to receive the first three pairs of walking
legs; small last pair of legs folded to fit into grooves on the latero-posterior
sides of the carapace. He further described the first crab, also without illustrations,
as follows: carapace broader than long and convex; front broader than
in the megalopa; margin of the orbit inclined obliquely backward; posterior
pair of walking legs proportionately smaller than in the adult.
Megalopae of O. quadrata were found initially on the shore of Long
Island by Smith (1873a, b) in August, and by the middle of September, they
were particularly common. Later Smith (1880) reported that “… a great
number of the megalops have been taken in Vineyard Sound by V.N. Edwards,
and, early in September, 1875, I took some specimens swimming at
the surface in the same region,” but no size data were provided. A specimen
kept alive for “some days”, burrowed in sand and produced a partial molt.
Presently there is not much published information on the role of the megalopae
in the natural history and recruitment of the ghost crab to its sandy
beach habitat.
Crane (1940) described and illustrated the megalopae of O. quadrata and
those of the eastern Pacific species O. gaudichaudii and O. occidentalis. Her
drawings of O. quadrata correspond in basic morphology to illustrations of
Diaz and Costlow (1972) for laboratory-raised specimens. Diaz and Costlow
(1972) showed that megalopae of the eastern Pacific species and those of two
Indo-West Pacific species (Raja Bai Naidu 1951, 1954) were very similar,
concluding that the Ocypode megalopa was a distinctly unique brachyuran
larval stage.
The purpose of this paper is to provide information on the occurrence
and carapace dimensions of Ocypode quadrata megalopae collected over
a 20-year period (1980–2000) from sandy beaches on the southern coast of
New Jersey. These data will be compared with those from the literature on
the same and other species within the same subfamily (Ocypodinae), and the
adaptive morphological and behavioral features of these brachyuran megalopae
will be discussed.
Methods
Data on ghost crab megalopae were obtained during organized yearlong
ecological studies of the sandy beach surf zone, primarily in early fall,
on the Cape May peninsula (McDermott 1983, 2001). Using a beach seine
640 Northeastern Naturalist Vol. 16, No. 4
(15 x 1.5 m; 6.5-mm mesh), a sediment corer, and a hand net, specimens
were collected from Stone Harbor and Hereford Inlet on the east shore of
the peninsula, and Delaware Bay on the west shore. Mean surface water
temperature was 21.9 ± 1.3 °C, similar to fall temperatures measured over
many years. All specimens, except one, were preserved in 10% seawater
formalin and stored in 70% ethanol. All were measured (CW x CL) by ocular
micrometer to 0.01 mm, and CW/CL ratios were determined.
Results
Only five megalopae of Ocypode quadrata were collected during a 20-
year span (Table 1). All were found in the months of September and October.
Two were collected in a seine on an exposed sandy beach at Stone Harbor,
another in a sediment core sample from the same beach, one with a hand net
in sand inside of Hereford Inlet, and one from a sandy beach in Delaware
Bay. One early crab stage was collected in a seine haul on the Stone Harbor
beach during the same time period.
Mean carapace measurements and the mean CW/CL ratio for the five
megalopae are given in Table 1. Ranges in CW and CL were 4.01–5.43 mm
and 5.18–7.01 mm, respectively. All larvae had the morphological characteristics
described by Crane (1940) and Diaz and Costlow (1972) for
O. quadrata. A megalopa collected 7 September 1999 at Stone Harbor,
maintained in the laboratory with periodic feeding of Artemia nauplii, died
after 34 days without molting. An early crab stage (possibly a first crab)
collected 9 September 1994 was 7.52 mm CW and 6.68 mm CL with a 1.13
CW/CL ratio.
Discussion
Morphometric comparisons
A comparison of carapace width and length measurements of Ocypode
quadrata with data from the literature (Table 2) shows that the CW/CL
ratio for the one cultured specimen taken from the drawing of Diaz and
Table 1. Carapace dimensions and mean width/length ratios for megalopae of Ocypode quadrata
(Atlantic Ghost Crab) collected on sandy beaches in New Jersey.
Megalopa Date of Collection Carapace
number* collection site** Width (mm) Length (mm) W/L ratio
1 6 Sep 1980 DB 5.30 6.48 0.82
2 19 Sep 1981 HI 5.01 6.18 0.81
3 13 Sep 1985 SH 4.01 5.18 0.77
4 7 Sep 1999 SH 5.43 7.01 0.78
5 6 Oct 2000 SH 4.51 5.85 0.77
Mean ± SD 4.85 ± 0.59 6.14 ± 0.69 0.790 ± 0.023
* Listed chronologically.
** DB = Delaware Bay, HI = Hereford Inlet, SH = Stone Harbor.
2009 J.J. McDermott 641
Costlow (1972) corresponds to the mean value for the five megalopae
from New Jersey, but not those of Smith (1873a, b) and Crane (1940).
Carapace dimensions for the two eastern Pacific species of Ocypode (Table
2) show that CL is also greater than CW, but the ratios appear slightly
higher than for those of O. quadrata. Smith (1873a) reported that the
largest specimen of O. quadrata from Fire Island beach, NY was 5.6 mm
CW by 9.4 mm CL for a ratio of 0.596. Smith (1873b) recorded the largest
megalopa from Fire Island as 5.6 by 6.4 mm. As his two papers were
nearly identical except for the 9.4 mm value, most likely a measurement
or typographical error was involved.
Carapace measurements of the small juvenile Ocypode quadrata from
New Jersey implied that at metamorphosis (or shortly thereafter) the carapace
width became the greater dimension as is also the case in the adult
(Williams 1984). However, it is possible that this crab was not the first
crab instar. On Fire Island beach, Smith (1973a, b) measured the carapace
of the smallest crabs, which ranged from 6.1 to 6.5 mm CW by 5.6 to 6.0
mm CL (number of crabs measured not given). Thus, my specimen (7.52
x 6.68 mm) was most likely a second instar. In contrast, Haley (1972)
measured what he considered to be a first crab instar from a summer
brood of O. quadrata specimens along the middle Texas coast at a CW of
7 mm in October. However, Haley measured only one of these first crabs,
and unless megalopae of O. quadrata are larger in this southern part of
their range, the 7-mm CW individual may also represent a second instar.
Alternatively, 7 mm CW could have been the upper CW limit of firststage
crabs at his field site.
Hines (1986) sought correlations between larval patterns and life-history
characteristics among seven families of brachyurans, which included the
Ocypodidae. In his analysis, megalopa size (CW) was positively correlated
Table 2. Carapace dimensions and mean width/length ratios, determined by other authors, for the
megalopae of Ocypode quadrata and two species within the same genus from the eastern Pacific.
Carapace
No. of Width Length
Species specimens (mm) (mm) W/L ratio References
O. quadrata* 1 5.6 6.4 0.88 Smith (1873a, b)
3** 4.9 5.9 0.83 Crane (1940)
1 5.9 7.4 0.79 Diaz and Costlow (1972)***
O. gaudichaudii 1† 4.7 5.6 0.85 Crane (1940)
5**†† 4.6 5.3 0.87 Crane (1940)
O. occidentalis 1††† 4.3 5.0 0.86 Crane (1940)
*all specimens from northwestern Atlantic Ocean, United States.
**size ranges given from which “mean values” were determined.
***measurements from their line drawing, Figure 9.
†specimen from Costa Rica.
††specimens from Colombia.
†††specimen from Mexico.
642 Northeastern Naturalist Vol. 16, No. 4
with adult crab size only if data for the unusually large megalopae of three
species of Ocypode species were eliminated from the other crab species, i.e.,
Uca spp., within the Ocypodidae.
Natural history of the megalopae
Say (1817) collected “several specimens” of the megalopae of Ocypode
quadrata on the eastern sandy shore of Maryland. Deposited on the beach
as the tide receded, they burrowed into the sand, protecting them from sun
and minimizing water loss, presumably prior to metamorphosis. Smith
(1873a, b) collected megalopae in late August at Fire Island beach. He
observed “many more young crab instars” that had metamorphosed from
megalopae. He conjectured that molting as soon as possible would tend to
minimize mortality in a terrestrial environment. Smith further noted that
the megalopae of late August were only half the size of those collected in
the middle of September, suggesting that the disparity in size might be due
to multiple broods during the reproductive season. The present data from
New Jersey beaches and those of Smith for the beaches of Long Island suggest
that early fall may be the usual period for recruitment O. quadrata in
this section of the Atlantic coast.
Crane (1940) studied the morphology of megalopal stages of three species
of Ocypode. She concluded that the obese megalopae of Ocypode had a
thicker exoskeleton than megalopae of other brachyuran species. This feature
and the curious behavior of folding their five pairs of legs close to the
cephalothorax (Fig. 2) where grooves accommodate these folded legs (Crane
1940, Diaz and Costlow 1972), led Crane to see these as adaptive characteristics
to coming ashore in the turbulent surf zone of sandy beaches. With legs
folded, the animal becomes semispherical and compact (Fig. 2b), preventing
legs from injury as it is rolled by the surf onto the beach. The thick exoskeleton
would give added protection to the delicate internal organs, and the
capacity of the megalopa to burrow at least superficially into the sand would
serve to minimize water loss. Once on shore, it would seem imperative that
megalopae transform as soon as possible into the semi-terrestrially adapted
first-crab stage with its functional walking legs and feeding appendages.
Are the megalopae of ghost crabs that are deposited on the shore competent
to metamorphose, i.e., have they reached the developmental stage,
as megalopae, necessary for a successful molt? Diaz and Costlow (1972)
showed that the time for development of the megalopa in the laboratory was
a minimum of 34 days following hatching as first zoeae. Their megalopae,
however, failed to metamorphose into first crabs. Possible chemical or physical
cues to induce metamorphosis were not considered by Diaz and Costlow.
Therefore, the length of time required for the megalopa to reach competency
is not known.
Presumably, megalopae are tossed onto less favorable shore locations
than the preferred sandy beaches. Can competent megalopae of Ocypode
2009 J.J. McDermott 643
Figure 2. Line drawings of the megalopa of Ocypode quadrata by Crane (1940) from
a preserved museum specimen (USNM No. 10995) collected off Woods Hole, MA in
1885. A. lateral view with folded legs and anterior spine of carapace on left projecting
ventrally; length of carapace = 6.4 mm. B. ventral view showing chelipeds and
pereopods folded inward under the body. Permission to reproduce Crane’s image
courtesy of the Wildlife Conservation Society.
644 Northeastern Naturalist Vol. 16, No. 4
postpone metamorphosis after landing in such unfavorable locations? Will
they reenter the water column to be carried elsewhere to metamorphose? I
have not been able to find any information on delayed metamorphosis in
Ocypode. The fact that one of the megalopae collected in New Jersey was
able to postpone metamorphosis for 34 days indicates that such a process
is possible and may be adaptive. Certainly, the adaptive nature of delayed
metamorphosis has been demonstrated for many crustaceans and other major
invertebrate groups (Bishop et al. 2006, Butman 1987, Pechenik 1990).
Do megalopae have some mechanism for remaining planktonic in order to
minimize being swept ashore before competency? Chemical or physical environmental
cues on sandy beaches that might control metamorphosis should
be considered (Steinberg et al. 2007, 2008).
There is much to learn about the enigmatic, bug-eyed megalopa of Ocypode
and its critical role in the recruitment of new generations to sandy
beaches. Metamorphic data are needed on the size range of this larval stage
in relation to its metamorphosis into the first crab. How do megalopae manage
to return from a planktonic existence to populate sandy beaches? Are the
unusually large eyes of megalopae of particular importance in this navigation?
How do they stay out of the surf zone until they are competent to molt
into first crabs? More intense field studies and laboratory observation of
cultured larvae are needed to answer these and other questions dealing with
the natural history of ghost crabs.
Acknowledgments
I thank the following for checking their crustacean collections for megalopae of
Ocypode quadrata: J. Dineen, Smithsonian Marine Station, Fort Pierce, fl; A.H.
Hines, Smithsonian Environmental Research Center, Edgewater, MD; and D.M.
Knott, Southeast Regional Taxonomic Center Marine Resources Research Institute,
Charleston, SC. D.M. Knott also provided me with a photo of an O. quadrata
megalopa, which was taken by R. Newman, a park ranger at Macon State Park, NC.
Critical reviews of an early draft of this paper by J. Dineen and D.M. Knott were most
helpful, as were the comments on the manuscript by two anonymous reviewers. I appreciate
discussions with W.S. Johnson, Goucher College, Towsen, MD, concerning
his zooplankton studies along the mid-Atlantic coast. J. Norenburg and C. Walter,
Smithsonian Institution, Washington, DC, were helpful in retrieving information
from files of the US National Museum, and J.T. Thompson, Franklin and Marshall
College, provided technical assistance. Franklin and Marshall College continues to
aid and encourage my research.
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