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 ------ II 80 ~ ~ o " IuLIl lO -," 70 " , cl " Qcl. 60 a« ac:l '0...................... z o 9 - 50 f"T'lI ou, ..... - 1-Ill :to :c 40 -l l- .......... o e 8 .....- '"0- 0 z ~ 30 Z et ILl ~ 7 20 10 - 20 40 60 80 100 110 120 140 160 180 200 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