The Biology of the Mussel Crab, Fabia subquadrata, from the Waters of the San Juan Archipelago

33 Pages·2009·17.15 MB·English

The Biology of the Mussel Crab, Fabia subquadrata, from
the Waters of the San Juan Archipelago, Washington
JACK B.PEARCE!
THEMUSSELCRAB,Fabiasubquadrata,described sults of the present study indicate that F. sub
by Dana in 1851 from material collected in quadrata passes through stages comparable to
Puger Sound, is placed in the subfamily Pinno those reported for P. pisum by Atkins and P.
therinae Milne-Edwards, one of the two sub ostreum byStauber and byChristensen and Me
families of the Pinnotheridae indigenous to the Dermott (1958:150). There are, however, im
Americas. portant differences in the developmental cycle
Most species of pinnotherids live in associa of P.subquadrata.
tion with a host organism. The relationships The present known range of the mussel crab
have been variously described as parasitism, is from the coast of Alaska to southern Cali
commensalism, or mutualism. Although the fornia.Rathbun (1918:102) noted it in waters
mussel crab is usually found in association with 250 m deep and Wells (1940:47) found it in
the horse mussel, Modiolus modiolus, several mussels dredged at a depth of 220 m. Hart
other pelecypod hosts as well asa tunicate have (personal communication) has found it in M.
been reported (Wells, 1928:289). The present modiolus taken intertidally near Victoria, Van
research has revealed additional bivalve host couver Island, British Columbia.
species.It is frequentlyreported from both spe While it is true that pinnotherid crabs have
cies of MytiltIS in waterssouth of Puget Sound, been known from ancient times, only recently
although in the latter waters it does not appear have there been any comprehensive studies of
to frequent these hosts. any member of this family. Atkin's early ob
The life cycle of F. subquadrata is complex servations (1926) on the moulting stages of
P.pisum laid thegroundwork for future studies.
and,aswithmanypinnorherids,includesseveral
This was followed by Hart's investigations
developmental forms subsequent to the typical
(1935) in which she reported success in hatch
decapod larval stages (i.e., the zoea and mega
ing the eggs of Pinnotheres taylori and rearing
lops) and before the definitive adult stage is
them through the first true crab stage. Sandoz
reached. As in most marine decapods, the early
and Hopkins (1947:250) were able to rear
pinnotherid zoea and megalops are planktonic.
P. ostreum to this same stage. These investiga
Upon moulting from the megalops into the first
tions extended the earlier work of Atkins, in
true crab stage the animal, it is thought, leaves
which the hard and posthard stages subsequent
the plankton and becomes associated with its
to the first crab stage had been described. At
host. kins (1955) later raised two species of Brit
Posrplankronicdevelopmentalstages ofapin ish pinnotherids, P. pisum and P. pinnotheres,
norherid were first described by Atkins (1926: through the megalops stage.
475) for Pinnotheres pisum, which is common Most of the workers cited above were con
to the coast of the British Isles. Later Stauber cernedlargelywith theearlydevelopment of the
(1945:269) found that the developmental cycle crabs rather than with their ecology or associa
of the North American east coast pinnotherid, tion with the hosts.Wells'studies (1928, 1940)
Pinnotheresostreum, wasverymuch the sameas wereamong the firstpublished papers concerned
that previously described for P. pisum. The re- with the biology of American speciesof pinno
therids. Later Stauber (1945) investigated the
Department of Zoology, University of Washing
1 postlarval development and habits of the oyster
ton, Seattle 5. Present address: Marine Laboratory,
crab,P.ostreum. Thisworkwasfollowed bythat
Humboldt State College, Arcata, California 95521.
Manuscript received June 25, 1964. of Christensen and McDermott (1958) which
3
4 PACIFIC SCIENCE, Vol.XX,January 1966
represents the most comprehensivestudy of this versity of Washington, Friday Harbor, SanJuan
or any other species of the Pinnotheridae. Island,Washington.Observationswere made on
Except for purely taxonomic studies, these the contents of host mussels collected at least
papers are the main reports concerning the pin oncebutfrequently twice per month throughout
notherids despite Rathbun's (1918:10) early the period of study. In addition, materials were
admonishment concerning the lack of knowl collected once a week during the summer
edge of this family and the inherent rewards to of June, July,and August of 1958 and 1959. A
be found in its study. total of 3,480 host mussels were examined dur
Other than Wells' data (1928, 1940) there ing this period.
have been no extensive reports concerning rhe The mussels were collected by dredging in
biology of F. subquadrata. It is, therefore, one localities where they are known to occur. The
ofthe leaststudied species of the Pinnotheridae, dredging gear included either a standard rock
Until Wells' work of 1928 the male of the spe dredge or beam trawl, depending upon the type
cies had not been recognized and was, in fact, of bottom from which the mussels were to be
described as a separate species in a different removed.
genus, Pinnotheres concharum. Several areas within the San Juan Archi
The present paper is concerned with the bi pelago, selected as dredging sites, were chosen
ology of this neglected species. The principal as being representative of a variety of depths
study was conducted over a period of one and and bottom types. The deepest stations were
one-half years, from June, 1958 to January, located in PresidentChannel northwestof Orcas
1960, but many observations made subsequent Island (48°39'45"N,123°1'W), where the wa
to the main investigation have been incorpo ter is 195 m in depth.The shallowest station is
rated in this paper. Information on the develop offPointLawrence, Orcas Island (48°39'30"N,
mental cycle,reproductive biology, relationship 122°44'45"W), where the water is 22-30 m
to the host organism, distribution and size in in depth.Other stations were located near Point
relation to water depth, and ecdysis is reported Caution, San Juan Island (48°34'N, 123°0'48"
here. W) in water 130 m indepth; offMineral Point,
San Juan Island (48°35'10"N, 123°3'35" W)
The author is especially indebted to Dr. Dixy
in waters 55 and 130 m deep; and in East Pea
1. Ray, as well as to Dr. Robert Fernald, Dr.
Vine Pass (48°35'30"N, 122°47'30"W) in 48
Ernst Florey, and Dr. Paul Illg, whose valuable
m of water.
assistance and helpful criticisms were most use
Upon being brought to the surface the mus
ful in the preparation of this paper. The many
valuable suggestions of Dr.J. F.1. Hart of Vic sels were immediately placed in live boxes with
toria, British Columbia, and the critical reading circulating sea water. The drains of these boxes
of the manuscript by A.M.Christensen are also are covered with screening of a gauge sufficient
to insure that any swimming stage crabs would
acknowledged.
be retained should they leave their hosts. The
Finally,Ishould like to acknowledge the sum
mussels were then brought into the laboratory
mer cooperative fellowship provided me by the
where they were opened and examined alive for
National Science Foundation in 1959, the Na
the presence of crabs.In those mussels that were
tional Institutes of Health predoctoral fellow
infested, any damage which may have occurred
ship (GF 10,872) awarded me duringthe years
as a result of a crab's presence was noted. A
1960 through 1962, and the NIH postdoctoral
dissecting microscope was always used in these
fellowship (GPD-10, 872-C3) given for study
examinations. Each mussel was measured the
at the University of Copenhagen's Marine Lab
larger ones (greater than 10 mm in le;gth)
oratory, Holsinger, Denmark.
with a vernier caliper, the smaller with a dial
caliper. The larger mussels were measured to
MATERIALS AND METHODS
the nearest 0.5 mm, the smaller (less than 10
All fieldwork involved in this study wascar mm) to 0.1 mm. All crabs collected after July
ried out at the Marine Laboratory of the Uni- 15,1958 were measured with an ocular microm-
Biology of Fabiasubquadrata-PEARCE 5
eter to the nearest 0.01 mm.Thegreatest width stages a very anomalous instar appears. First de
of both carapace and abdomen were noted. scribed by Atkins (1926:478) for P. pisum as
Thecrabswere then placedin standardhouse the Stage I crab, this instar is,in its morphology
hold polyethylene ice cube trays. Each tray con and behavior, entirely different from any of the
sists of 14 cubicles and 1crab was held in each stages preceding or following it. The exoskele
of these. A "vaporize" pen was used to number ton iswell calcifiedand very hard. It is,in many
each cubicle with the crab's respective catalog species, highlypigmented with definite patterns
number. In this manner several hundred crabs on the carapace. Above all it is highly modified
could be retained, facilitating observations on for a temporary, freeswimming planktonic ex
their behavior, ecdysis,and subsequent changes. istence. The setal ornamentation found on the
The crabs were kept at temperatures approxi pereiopods is extensive and, in addition, these
mating those of their natural environment, and appendages are broad and flattened in contrast
either aflowof water from the seawater system to the cylindrical condition noted in the pre
or several daily changes were used to maintain hard instars. They thus serve as very effective
adequate environmental conditions. swimming appendages. It has been reported for
P. pisum (Atkins, 1926:475) and P. ostreum
DEVELOPMENTAL STAGES IN F.subquadrata (Stauber, 1945:272; Christensen and McDer
morr, 1958:152) that at this stage of develop
As previously noted the life cycle of F. sub
ment the males leave their host to seek out
quadrata issimilarto that described for P.pisum
females, copulating with them in their host.
(Atkins, 1926:475) and P. ostreum (Stauber,
To this point of development the male and
1945:272; Christensen and McDermott, 1958:
female crabs have paralleled each other. The ex
150).The typical planktonic zoeal and rnegalo
ternal morphology of both sexes is very similar
pal stages are followed by a series of true crab
throughout the prehard series and the Stage I
instars, The first of these is the invasive crab
instar. Only by the examination of the external
(Christensen and McDermott, 1958:150). Fol
genitalia can the two sexes be distinguished.
lowing the invasionof the host organismseveral
Followingthis stage adichotomyoccurs in the
instars occur which are collectively designated
developmental cycleof the two sexes.The male
as prehard stages. These prehard crabs have a
is thought to remain in the hard stage, dying
soft, membranous exoskeleron. With the excep after breeding. The female, however, moults
tion of the initial invasive stage there is little soon after copulation and the new posrhard in
setal ornamentation on the pereiopods, which star is soft, with a membranous exoskeleton
are cylindrical in shape. comparable to that of the earlier prehard stages.
Whiletheprehardinstarswere thought to oc The first posthard stage is referred ro as the
cur cheywere not described for any pinnorherid Stage II female. It is followed, both in P. pisum
until the investigation of P. ostreum by Chris and P. ostreum, by Stages III, IV, and V.These
tensen and McDermott (1958:147). The total stages are characterized byan overall increase in
number of prehard instars isstill not known for size,greater complexity of the pleopods, and an
any species, although Hart (personal commu increase in the width of the abdomen relative
nication) has found up to five prehard instars to the carapace width. The Stage V crab is the
in F. subquadrata. Since the terminal one of terminal adult female. Although subsequent
these is smaller than the smallest Stage I crabs moults may occur, they result largely in an in
of this species it is certain that at least seven crease in size and there is little morphological
prehard instars normally occur between the change.
megalops and the Stage I crab. Because there is TheStageII crab isverysimilarin appearance
a considerablevariation in the sizeof the termi to the terminal prehard instar, In F.subquadrata
nal prehard instar it may be assumed char the there is little increase in carapace or abdomen
total number of prehard insrars also varies width during the terminal prehard-Stage I and
somewhat. Stage I-Stage II moults. This stage (II) is very
Subsequent to the series of prehard crab difficult to identify unless the actual Stage 1-
6 PACIFIC SCIENCE,Vol. XX, January 1966
Stage II moult is observed. Stages following it phology, become highly modified. The signifi
can be readily distinguished, however, on the cance of this suddencransformation isdiscussed
basis of the differential growth of the abdomen in a later portion of this paper.
and increased complexity of the pleopods. The pleopods of the first crab stage and the
SinceAtkins (1926:475) recognizedonlythe first few prehard instars subsequent to it are
Stage I-Stage V crabs and did not describe the merely small knobs protruding from the ven
prehard series, the nomenclature originally ap tral surface of the abdomen. At this time there
plied by her to the pinnotherid developmental is no differentiation into endo- or exopodites.
instarsisnolongeradequate.However,asallthe In the later prehard insrars immediately preced
prehard stages have not been described for any ing the Stage I or hard instar, the pleopods
pinnorherid crab it would be difficult to rename become very conspicuous and show clear differ
or renumber these forms at this time. For this entiation into endo- and exopodire portions.
reason her original terminology, with some The smallest F. subquadrata found within a
modifications made by Christensen and Mc mussel measured 0.85 mm in carapace width.
Dermott (1958), has been retained in this This crab is somewhat larger than the first crab
investigation. stage of this species reared by Hart (personal
communication), which hadacarapacewidthof
Invasive and Prebard Stages 0.76 mm. This difference in size may be ac
counted for by assuming that the formerly
The carapace of the invasive first crab stage
planktonic first crab stage undergoes a moult
ofF.subquadrata ismore square in outline than
very soon after entering the host mussel. Con
are the later prehard stages, which tend to be
sequentlyit would be difficultto find atrue first
ovoid in shape.The eyestalks andpereiopods of stage crab in a host mussel.
the first crab stage are also proportionately
However, comparison of the supposed first
larger in relation to the rest of the body than crab stages removed from mussels with the
are those of succeeding prehard instars. The
known first crab stage raised by Hart indicates
pereiopods of this instar arecovered with swim
that morphologically they are very similar or
ming'hairs or setae. The distributional pattern identical.
of thesehairs isdifferent,however,from that of As earlier noted, Hart has reared F. sub
the Stage I or hard crab, which also has similar quadratathrough fiveprehard insrars,thelargest
setae. The pereiopods of the first crab stage oc of these still being somewhat smaller than the
cur with the hairs distributed over much of the smallest Stage I insrar yet observed (1.3 mm).
surface,giving the appendagea bottle-brush ap For this reason it may be suspected that several
pearance. The hairs of this stage are also much instars intervene between the aforementioned
more sparse and the entire structure does not forms.Christensen and Mcffermotr (1958:150)
appear to be as efficient an arrangement for .found that the smallest P. ostreum collected
swimming asthatof the pereiopods of the Stage measured 0.59 mm.Theysuggested that at least
I crab.Since the firststage crab apparently seeks four moults occur before a crab would rnouk
out or in some manner becomes associated with into the Stage I instar. The smallest Stage I
a host immediately after moulting from the insrar in their collection also measures 1.3 mm.
megalops, appendages well adapted to extended Since, however, the Stage I F. subquadrata is
swimming activities are not necessary. This in normally somewhat larger, it is suspected that
star isable toswim,however,asisdemonstrated at leastseven moults occur between the invasive
by its activities in the laboratory. first crab stage and the average Stage I instar,
As an individual crab progresses through the When the method used byHiatr (1948:165) to
series of prehard moults the swimming hairs extrapolate the number of intermoult periods in
found on the pereiopods, as well as the swim Pachygrapsus crassipes was applied to F. sub
ming abilities and activity, are lost until the quadrata it was confirmed that approximately
Stage I or hard instar is attained. At this point seven,to eight moults occurred between the first
the swimming hairs,aswell as the general rnor- crab stage and the average size Stage I instar.
Biology of Fabiasubquadrata-PEARCE 7
The abdomen width of the smallest F. sub and wellornamented with functionalswimming
quadrata removed from a mussel was 0.26 mm hairs; in F.subquadrataand P.ostreum only the
or approximately one-third the carapace width. second and third pereiopods bear the long plu
This is a ratio that is approximated in all de moseswimming hairs, whereasthey are present
velopmental instars through the Stage II post onallthe walking legsofP.pisum (Christensen
hard. With the exception of a few abnormal and McDermott, 1958:152). While Darbishire
females it is true for the hard Stage I form. (1900) isquoted (Christensenand McDermott,
Apparentlyit israreforamaleStageIF.sub 1958:152) as stating that the Stage I P. pisttm
quadrata to moult into a posthard,soft carapace usesthe third and fourth pereiopods for swim
crab. As will be discussed later, however, such ming, in contrast with P. ostreum which uses
malesdo occasionallyoccur and, in fact,may be the second and third, recent observations by
morecommonchansuspected.Inorder to obtain Christensen (personal communication) confirm
an approximation of the size of the terminal that P. pisum uses primarily the second and
prehard instars, both males and females, each third pereiopods,as doesF. subquadrata.
individual collected was measured. If, within a The carapace of the Stage I F. subquadrata,
week, the crab moulted intoa Stage I insrar the like that of P. pisum, is quite convex. The sur
previousdimensions were recorded aschose of a face of this structure has a distinct pattern of
terminal prehard. bright orange markings (see Maerz and Paul,
The carapace width of 19 male prehard crabs 1930, plate 10, E-12). This pattern is very
varied from 3.0-5.3 mm, with a mean width constant andisfound in almostallStageI crabs.
of 4.3 mm. The abdomen width ranged from Thebackground isa brilliant white.Theorange
1.1-2.1mm and averaged 1.7mm.It should be pattern tends co fade to a dull brown (Maerz
kept in mind, however, that males may occa and Paul,plate 13,G-ll) some weeks after be
sionally moult into a soft instar from the hard ing removed from the host mussel.Otherstages
Stage I form. Furthermore, as will be discussed of this species, both pre- and posthard, do not
later, thissoft instar maysubsequentlyrevert to present any indication of this pigmenrarion.
the hard form. Such a moulting sequence may Macroscopically the exoskeleton in these latter
thus invalidate these measurements.
forms appears colorless, although microscopic
Since the female 'regularly moults from a examination reveals isolated black and red
terminal prehard form into the Stage I instar chromatophores. Finally, as reported for the
any dimensions of these forms can be accepted comparable stage of P. ostreum by Christensen
as valid. Thirteen such moults were observed and McDermott (1958:152), the Stage I F.
and the individuals involved ranged from 2.7 subquadrata was noted to have two cylindrical
5.1 mm in carapace width prior to the moult.
rods conneoring ,the dorsal and ventral sides of
The average carapace width of these terminal
the body. These structures, along with the al
prehard females was 4.1 mm.
ready discussed exoskeletal rigidity, may be
modifications for a freeswimming existence.
TheStageI (Hard) Crabs
Finally, in addition to these differences, the
This is one of the stages originally described Stage I F. subquadrata varies from the other
for P. pisum by Atkins (1926:478) and sub stages in having a heavy pubescence along the
sequently applied to the comparable instar of antero-lateral margins of the carapace. This
P. ostreum by Stauber (1945:272-276). The pubescence appears somewhat heavier in the
latter suggested that it was during this stage male,butsuchdifferencesare hard toquantitate.
that P. ostreum invaded its oyster host. The average carapace width of 54maleStage
The Stage I instar of F. subquadrata is in I crabs, selected at random from collections
many waysmorphologicallysimilar to the Stage made onJuly29, 1959,is4.1mm,with a range
I form in both P. pisum and P. ostreum. Inall of from 1.3-6.8 mm. The mean of 29 female
three species the exoskeleton is well calcified Stage I crabs collected on the same date is 3.5
and very hard. The pereiopods are flattened mm,with a range of 1.5-6.2mm.This does not
8
PACIFIC SCIENCE, Vol.XX,January 1966
appear .w be as large a size difference between where tapering occurs, ,they are, in P. ostreum
sexes as was found for P. ostreum by Christen and F.subquadrata,slender and lanceolate.
sen and McDermott (1958). In addition to a
sexualdimorphism in sizethe StageI crabs have The Stage II Females
other sexual differences. The abdomen of the
There is no apparent increase in body size of
female is different in shape from that of the
this insrar over theStage I female. The average
male;.the lateral margins ofchemale'sabdomen
carapace width of seven Stage II female crabs
are concave, whereas those of the female are
straight.It hasalsobeen noted thatan occasional which were observed to moult from the Stage I
instar is 3.4 mm, with a range of 2.9-3.9 mm.
Stage I female will have an abdomen which is
The average abdomen width is 1.1 mm, with a
relatively wider than the 1:3 abdomen-carapace
range of 0,9-1.3 mm.
ratio which ischaracteristic ofmost oftheStage
The exoskeleton of the Stage II crabs is soft
I crabs both male and female. Finally, the ab
and membranous, as is that of the prehard in
domen'of the female bears four well developed
stars. There are few swimming setae or hairs
pairsof pleopods which contrast markedly w!th
to be found on che pereiopods, nor is there
the two pairs of highly modified reproductive
any pubescence along .the anterolateral carapace
appendagesborne by the male.
margins.
Both the male and female Stage I crabs have
The appendages are subcylindrical, not flat
much stouter chelipeds than either the pre- or
tened as in the Stage I instar. The carapace is
posrhard growth forms. The merus and carpus
ovoid; the angles of thesubpencagonal carapace
are heavier and both fingers of the chela are
of the Stage I form have become rounded. It is
swollen.
during this stage that the lateral carapace sulci
As observed by Stauber (1945:274) in P. (one of the definitive characters of the genus;
ostreum, the Stage I F. subquadrata possesses a Rathbun, 1918:101) become pronounced. They
locking mechanism whereby the abdomen may
appear faintly in che prehard stages and are
be 'secured in the sternal groove. On the fifth hardly present at all in the Stage I instar.Wells
thoracic segments of the sternal groove chere (1928:289) notes that these sulci are present
are pairs of antero-venrrally directed knobs. in the newly moulted Stage I crab but are lost
These knobs hook under shelves found on the with subsequenthardening.Aspreviouslynoted,
opposing ventral surfaces of the abdomen in the typical pigmentation of the Stage I instars
such a manner as 'to become securely locked is lost in rhe Stage II forms.
when any attempt is made to lift forcibly
Stauber (1945:275) indicates large differ
the abdomen of the living crab. Consequently,
ences between the pleopods of Stages I, II, and
whereasit iseasyto displacetheabdomen of the III in P. ostreum. Christensen and McDermott
pre- and posthard instars it is very difficult to
(1958:152) suggest that Stauber's series of
freeehe abdomen from the sternal groove in the
Stage II crabs may have included some prehard
Stage I crabs. individuals. At any rate, no such marked dif
The reproductive appendages of the male ferences could be found between the pleopods
Stage I mussel crab are very similarto thosede of terminal prehards and Stages I,II,and III of
scribed for P. ostreum by Stauber (1945:276), F.subquadrata.
and quite dissimilar from the reproductive ap
There is little or no widening of the abdo
pendages of P. pisum as described by Atkins
men relative to the carapace in the StageII crab.
(1926:476). Atkins described the first copula
The ratio between the two is approximately the
tory appendage of P. pisum as blade-like and
same as that of the Stage I forms. The sternal
hairy.RecentexaminationofpreservedP.pisum
groove remains deep and is only as wide as the
material by the present author verified a con abdomen. The locking mechanism which was
siderable difference. While the appendages of present and functional in the Stage I crabs no
P.pisumare broad withalmost parallel margins, longer operates. As was surmised by Stauber
except for the distal one-fourth of its length (1945:278) for P.ostreum, this may be due to
Biology of Fabiasubquadrata-PEARcE 9
.rhe diminished rigidity of the exoskeleton in The Stage V Females
the posrhard ins-tars.
This is the definitive adult female crab and
the stage most commonly found throughout the
year. As in previous posthard forms the ex
The Stage III Females
oskeleton is membranous and, while the body
This is the first stage subsequent to the Stage shape is similar to the Stage IV crabs, the rela
I instar in which there is an increase in cara tively large growth of the abdomen causes this
pace widthover.thatof theprecedinginstar, the ins-tar to become very awkward in its move
Stage II form.Theaveragewidthof 41StageIII ments, especiallywhencomparedwith the earlier
females was 5.4 mm, with a range of 4.0-5.9 stages. The abdomen is as wide or wider than
mm. This instar is also the first in which the the carapace and normally it protrudes laterally
abdomen is more than one-third as wide as the beyond the coxopodires and anteriorly to the
carapace. The average abdomen width of rhe mouth parts.
above Stage III crabs was 3.6 mm. The range There is a great deal of variability in this
was 3.2-4.2 mm. stage, especially in the size and width of the
Except for the relatively wider abdomen and abdomen relative to the carapace. From ob
larger overall body size the Stage III crab is, servations made on moulting Stage V crabs it
externally, morphologically similar to the Stage has been found that this stage consists of not
II form. The carapace is soft and membranous, just one instar, as is usually true in the previous
the pereiopods are slender and subcylindrical stages, but rather of a series of growth instars,
and devoid of swimming hairs. The sternal in which the general morphology remains the
groove, however, is shallower and che abdomen same but with each succeeding insrar become
no longer lies within the confines of this de somewhat larger than the one preceding. This
pression. Rather, it extends both laterally and results in a wide range of size within this one,
anteriorly beyond the bordersofchegroove. The arbitrarily designated stage. The smallest Stage
pleopods are almost identical in both structure V crab observed measured only 4 mm in cara
and serarion with those of the Stage II instar. pace width, whereas several Stage V crabs were
found to measure 14mm.The average carapace
width of all observed Stage V crabs (831) was
The Stage IV Females 9.5 mm, and the average abdomen width was
10.3 mm.
The average carapace width of 33 Stage IV
Christensenand McDermott (1958:162) dis
crabs is 5.8mm.They range in width from 5.3
6.1 mm. The average abdomen width of these cuss the effect of the presence of P. ostreum in
crabs is 5.4 mm with a range of from 4.8-5.9 slow-growing spat. They suggestthat,while the
growth of the crab is retarded in such host
mm. This stage is not only larger than the Stage
oysters, the development is not affected to a
III instars but in addition obvious external
similar extent.Thedata gathered on che F. sub
changes indicate that it issexually more mature
quadrata-M. modiolus relationship would sug
than thosestages which precede it. Ovaries con
gest that a similar situation prevails. The very
taining large numbers of developing eggs were
small, below average in size, Stage V F. sub
observed in 29% of the Stage IV crabs. Also,
while no ovigerous Stage IV crabs have been quadrata are usually found in relatively smaller
rioted, it issignificant that at this time the pleo host mussels. In a more recent study Houghton
(1963:254) reports a similar situation for P.
pods undergo the greatest change since cheir
pisum.
initial appearance. These modifications in the
pleopods involve changes in size, proportion, In addition to a positive correlation between
and setal decoration. This is in preparation for crab and host size it has been determined that
the deposition and attachment of eggs. Finally, there is a negative correlation between the size
the abdomen is now nearly as wide as the cara ofrhe Stage V crabs and .thedepth of the water
paceand ismoreconcave than inpreviousinstars. from which they were removed. Crabs reaching
10 PACIFIC SCIENCE,Vol.XX, January 1966
90
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DEPTH IN METERS
FIG. 1. Curves showing: dash line, correlation between mean width of Stage V crabs and depth of water
from which their host mussels were collected; solid line, correlation between per cent of infestation of host
mussels and depth of water from which they were removed.
maturityin relativelyshallowwaterswere,larger StageV instar isreached,that egg deposition oc
on an average,than crabs which have developed curs.The smallest oviger found measured 5mm
and are collected from deeper waters (Fig. 1). in carapace width;thelargest was 13.4mm.The
This relationship was noted to exist throughout largestcrabscollected (i.e.,those14mm incara
che entire one and one-half years that the crabs pacewidth),were notovigerous.However,rheir
were studied. A further discussion of these cor gonads did contain large numbers of well de
relations is deferred to another par.e of this veloped eggs and it appeared ,that these were
paper. about to be spawned.
The highly colored eggs contained in che The average carapace width of 187 ovigers
gonads show clearly rhrough the thin mem collectedfrom twodepthsoffMineral Point (55
branous exoskeleton.Thecolor varies duringde and 130 m) was 8.5 mm. These crabs were re
velopment; initially appearing chrome yellow, moved from mussels colleoted during a period
they appear coffee brown immediately prior to (November, 1959) when the ovigerous females
their deposition (see Maerz and Paul, plate 9, constituted almost 60% of the total population.
K-2 and plate 15,A-ll).Unless a crab has just These ovigers were, on the average, 1 mm less
become ovigerous, eggs are almost always pres in carapace width than the average of all the
em in some stage of development. It has been Stage V crabs collected during the period of
observed,thatwithinaweekafter egg deposition study. This is undoubtedly due <to the fact that
new eggs begin to form and become visible in many ofthese crabs were still in their first year
the gonadal tissues. and had not attained full adult size.In addition,
While the gonads of Stage IV crabs occa part of this sample was taken, as noted, from
sionally contain developing eggs no Stage IV relatively deeper waters where the average size
ovigershavebeen found.Ir isnot until the adult of the Stage V is smaller.
Biology of Fabiasubquadrata-PEARCE 11
AbnormalInstars subcylindrical and with few swimming hairs.
Such moults are not accompanied by significant
Although the sequence of developmental in
growth. In no case has a posthard male been
stars,asalreadydescribed, represents the normal
observed to undergo further moulting, as was
situation, investigationspreceding this one have
observed by Atkins in P. pisum.
revealed occasional deviations from this general
These posrhard male F.subquadrata were ob
pa~tern by other pinnotherid species. Orton
served only during the summer months of July
(l921:533) described a single male P. pisum
and August.Thisdoes not necessarily mean that
which was morphologically similar to a soft,
rheydo not occur atothertimes, since theycould
posthard female.Stauber (1945:280) discussed
easilyhavebeen mistaken for prehard forms had
a second stage,posthard male P. ostreum which
theynot been observedmoulting from theStage
appeared externally to resemble the Stage II or
I instar.
III females. He notes chac they were found in
Apparent abnormalities are found not only in
"appreciable" numbers and that their size distri
the males but also in Ithe morphology of the
bution was somewhat greater than that of rhe
Stage I females. In these cases Stage I females
typical Stage I males. He suggested that these
are noted whose abdomens are precociously
atypical males might be ". .. the result of some
widened. This increase in widthover that of the
SOrtofparasitism asMercierand Poisson (1929)
normal individuals isquite large,the abdomen
havereportedfor P.pisum."Stauber furthersug
carapace ratio approaching that found in the
gested that rhese posthard male forms were
Stage III females. Other morphological aspects
copulatory partners for zhe larger posrhard fe
ofcheseindividuals tend to benormal,although
males.Christensen and McDermott (1958:152)
these forms are invariably larger than the aver
suggest thatcheabnormalP.ostreum referred'to agefemaleStageIinstar, Of 183StageI females
by Stauber were actually prehards and that the
examined, 5 were of this anomalous type.
greatersizerange ofStauber'ssecond stage,pOSt
Christensen and McDermott (1958:152) re
hard male over his Stage I series of male crabs
pONsimilar anomalies in the Stage I females of
was probably due 'to a sampling error. They do P. ostreum.In two cases they found individuals
make the reservation that a hard Stage I male
considerably larger than the normal Stage I
may, "now and rhen," moult to a soft-shelled
females.Both thesecrabshadabnormallyformed
form. Atkins (1958) presents evidence that, at
pleopods. However, they do not mention any
least in P. pisum, the hard or Stage I males do
extraordinary increase in the relative abdomen
quite frequently undergo a metamorphic moult
width of these crabs. It was their opinion that
intoasofrposthard form.Shehasrepeatedlyob
they had been retarded in their development,
served the same crab change from one form 'to
another with usually two or three soft forms in
ECDYSIS IN F. subquadrata
tervening between hard instars.These soft post
hard malesare usually found during the summer Ecdysis is one of the most significant events
months, June-August inclusive, in southwest in the life history of any crustacean. In a few
England. It is during this period that the males crustaceans it has evolved to be primarily a
moult and young crabs are found in mussels. mechanism allowingan increase in sizetooccur.
Becauseof this she suggested that the soft post This is true both in rhe freshwater decapods,
hard males occur during the periods of rapid the Potamonidae, crabs which hatch from the
growth. egg as a replica of the adult (Rathbun, 1918:
A similar situation has been found with re 11),and in aspeciesof the Oxyrhyncaor spider
gard to F. subquadrata. During the summer of crabs, Nacioides serpulifera (Rathbun, 1914:
1959 eightStageImaleswereobserved tomoult 653). In most marine crustaceans, however,
into sofcposthard forms. The latcer are similar moulting is accompanied not only by increased
in bodyshape to the Stage I insrars but are soft size but also by considerable morphological
and membranous. The pereiopods of these soft change. In no group is this more true than
posthard males are,asthoseof posrhard females, in the family Pinnotheridae. Certainly other
12 PACIFIC SCIENCE,Vol.XX,January 1966
crustacean groups have representatives which held. These authors, therefore, had co resort to
undergo extensive changesrhrough ecdysis, but other techniques in order to obtain moulting
few others, particularly among the brachyuran specimens.
families, have fiued into the postlarval (post No external change in color or opacity her
planktonic) portion of their life cycle such alds approaching exuviation in the hard Stage I
complex morphological changes as accompany crabs. Only the somewhat more flexible nature
ecdysis in the pinnotherids. oftheexoskeletonand theappearanceof acrack
While previous investigators have described alongchepostero-lateralmarginsofthe carapace
ecdysis and accompanying phenomena in other indicates chat ecdysis is under way. The cara
brachyurans (Drach, 1939;Hia~t, 1948; Guysel paceof theStageI formdoesnotbecomeassoft
man, 1953;and Knudsen,1957),Iietle informa or decalcified as is indicated for some other
tion is available concerning these processes in brachyuran species (Hiatt, 1948:156); however,
the Pinnotheridae. For this reason careful notes a recent paper by Knudsen (1957:134) states
were made of any moulting activities of F.sub that in the California Xanthidae che exoskele
quadrata during the period of this study. Sub ton does not become fragile prior to ecdysis.It
sequent studies of ecdysis in F. subquadrata as may be that, a-t least in the case of F. sub
wellasotherWestCoastpinnorheridshavebeen
quadrata, since che following posthard instars
made (Pearce, 1962b). These studies involved
are not heavily calcified che hardening salts re
theuseofbothlightandelectronmicroscopesin
main in the exuviae of the Stage I instar rather
determining cissue changes which occur during
than being retained in the crab's tissues to be
ecdysis.Thesedatawillbeincluded inaseparate
subsequentlyredepositedinthenewexoskeleton.
paper, the present work noting only the macro
Abom one dayafter the onset of the opaque
scopic aspects of ecdysis in F.subquadrata.
appearance in the pre- and posrhard crabs a
Two distinct phases of ecdysis can be recog
crack appears along rhe epimeral line, and at
nized in all brachyurans.The first ispreparaorory
thistime theactive phasebegins.Thebodynow
and, to all outward appearances, is passive in expands due to the uptake of water (Drach,
na-ture al-though there can be no doubt that
1939; Guyselman, 1953:129). This in effect
physiologically the animal is very aotive. The
lifts and freesthe posterior portionsofthe cara
second, or active phase, involves the actual ex
pace. In the pre- and posrhard stages the old
uviation of rhe cast. This phase is characterized
integument being shed has the consistency of
by a great deal of movement by the crab.
heavy, wet cellophane. Further, because of its
Mostobservationsconcerningthemoulting of
supple nature, it is never lifted oro the extent of
F. subquadrata were made on animals recently
a 30° angle as was noted in Paehygrapsus eras
removed from a host mussel. A total of 134
sipes byHiaor (1948:157) or asisfound in the
moultswere recorded.In 61of thesethe dimen
StageI musselcrabs.Rather,theold integument
sions were noted both before and after ecdysis.
lies free upon the dorsal surface of the new
Prehard and posrhard crabsrhat are about to
integument of the carapace.
mouLtcan be easily detected. One to two days
As is noted byKnudsen (1957:136) for the
prior ro exuviation animalsin,thisstate become
xanthid crabs,it is evident that muscular move
"creamy" andveryopaque inappearance.Unlike
other species which have been studied (Hiatt, ments occur during this period since the new
1948:155), they do remain quite active. All integument can beobservedto bepulled inward,
stages of Fabia have moulted under laboratory formingsurfacedepressions.
conditions-someafter being heldaslongassix Following the freeing of che posterior por
weeks.Christensen and McDermott (1958:150) tions of the carapace the last pair of thoracic
found it difficult to obtain moulting P. ostreum appendages,the fourth pereiopods, and the ab
under the laboratory conditions in which they domen are simultaneously freed from rhe old
worked. Unless crabs were "obviously ready to integument. This is a procedure intermediate
moulton arrivalIto the laboratory" no moulting betweenthatobserved byKnudsen (1957:136),
occurred in the Petridishes in which theywere who insists that the abdomen is freed first in