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Relative growth in the New Zealandmud crab Macrophthalmus hirtipes(Brachyura: Ocypodidae)M. J. Simons aa Department of Zoology , University of Canterbury ,Christchurch 1, New ZealandPublished online: 22 Sep 2010.

To cite this article: M. J. Simons (1981) Relative growth in the New Zealand mud crabMacrophthalmus hirtipes (Brachyura: Ocypodidae), New Zealand Journal of Marine and FreshwaterResearch, 15:2, 193-200

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New Zealand Journal of Marine and Freshwater Research, 1981, Vol. 15: 193-200 193

Relative growth in the New Zealand mud crabMacrophthalmus hirtipes (Brachyura: Ocypodidae)

M. J. SIMONSDepartment of ZoologyUniversity of CanterburyChristchurch 1, New Zealand

Abstract The relative growth of chela, abdo-men, and male first pleopod is described forMacrophthalmus hirtipes (Jacquinot, 1853) (Brachy-ura: Ocypodidae) from the Avon-Heathcote Estu-ary (43°33´S, 172°44´E) and from a marine inlet atGovernors Bay (43°38´S, 172°39´E), Canterbury.Marine and estuarine crabs had a similar estimatedsize of maturity of between 10 and 13 mm carapacewidth. Coincident with sexual maturity was anacceleration in the growth rate of the male chela, anenlargement and change in shape of the femaleabdomen, and a change to negative allometricgrowth of the male first pleopod. Statisticallysignificant (P < 0.05) intraspecific differences insome body proportions were found between the 2populations. Relative growth of the chela of M.hirtipes is similar to that of Australian macrophthal-mids.

Keywords Macrophthalmus hirtipes; Brachyura;growth; habitats; estuaries; marine ecology; marinecrustaceans.

INTRODUCTION

The secondary sexual characters of Brachyura(chela, abdomen, and pleopods) have differentialgrowth rates before and after the attainment ofsexual maturity (Hartnoll 1978). Changes in thegrowth of these skeletal structures at puberty usuallytake place over a limited size range, often showingup as sharp inflections or discontinuities in relativegrowth analyses. The significance of these changeshas been related to their functional commitments inreproduction, e.g., the use of the male chela incourtship and agonistic displays, and the importanceof the size of female abdomen for egg attachment(Hartnoll 1974).

Received 29 October 1980; revised 2 February 1981

Intraspecific differences in relative growth charac-teristics of different populations of Nephropsnorvegicus in different areas have been shown byFarmer (1974). There is further evidence to suggestthat populations of marine crustaceans inhabitingestuarine and marine habitats show different growthpatterns (Jones 1980). This paper describes andcompares the relative growth of the chela,abdomen, and male intromittent organ of theendemic mud crab Macrophthalmus hirtipes (Jac-quinot, 1853) from the Avon-Heathcote Estuary(43°33'S, 172°44'E) and from Governors Bay, amarine inlet at the head of Lyttelton Harbour(43°38'S, 172°39'E).

M. hirtipes is common on the lower regions ofmud flats of harbours, lagoons, and estuariesthroughout New Zealand (Morton & Miller 1973,Jones 1977); it constructs temporary burrows (Nye1974) and feeds primarily on detritus sifted from thesurface sediment (Beer 1959, Fielder & Jones 1979).Males have no courtship displays, but have a threatbehaviour which involves extending the chela to givethe impression of greatest width (Beer 1959).

METHODS

Crabs (4.0-25.8 mm carapace width) were collectedfrom mud flats in the Avon-Heathcote Estuary andGovernors Bay between March and December1978. Each specimen was sexed (based on abdomenshape and pleopod morphology), and maximumcarapace width (CW) (Fig. 1A), chela propoduslength (Fig. IB), and abdomen width (Fig. 1C, D)were measured; for males, the length of the firstpleopod was also measured (Fig. IE). Measure-ments were taken with hand-held vernier calipers (»10 mm) and a micrometer eyepiece in a stereomicroscope (< 10 mm). Chelae of most, individualswere equal in size, but when they were different thesize of the larger chela only was recorded. Chelaewith damaged or regenerating tips were notmeasured. Immature females had small, triangularabdomens in which the first or third segment was thewidest. The abdomen widened in the mid-region ofthe third and fourth segments to produce asemicircular form, such that in mature individualsthe fourth abdominal segment was the widest. Thusmeasurements of the first, third, and fourthabdominal segments were made in females (Fig.1C); in males only the fifth abdominal segment (Fig.

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194 New Zealand Journal of Marine and Freshwater Research, 1981, Vol. 15

B

Fig. 1 Measurements used toanalyse relative growth in Mac-rophthalmus hirtipes: A, carapacewidth; B, length of chela prop-odus; C, maximum width offemale abdomen (e.g., 4th seg-ment in mature specimens); D,width of male 5th abdominalsegment; E, length of male 1stpleopod.

ID) was measured. The pleopods of each femalewere examined for the degree of morphologicaldevelopment. The female was then dissected andovarian condition related to morphological evidenceto ascertain the size at sexual maturity. The femalepleopods became larger and more setose as pubertyapproached, and, although these changes wereobvious when an immature female was comparedwith a large mature one, specimens of the same sizenear puberty could not be easily separated. Thesmallest size of mature females recognised thus was9.4 mm CW at the estuary and 10 mm CW atGovernors Bay. Similarly, females of the same sizeoften showed different states of gonad maturity.Ripening ovaries were found in crabs larger than 8.0mm CW, and they may develop before the crab ismorphologically mature. Ovarian maturity cannotbe regarded as a reliable character for determiningsize at puberty in this species.

Analysis

Measurements were grouped into 1 mm size classesand mean chela and abdomen data plotted againstcarapace width on arithmetic coordinates (Fig. 2-4).Male pleopod data showed changing rates of relativegrowth and were graphed on log-log coordinates toimprove interpretation (Fig. 5). Allometric growthconstants (b in the allometric equation Y = aX*;where b is the slope and a is the value of Ywhen X= 1) were calculated for all lines by transforming alldata to logarithms. Fifty randomly selected pointsfrom a number of these logarithmic lines werecompared by analysis of covaiiance to test forsignificant differences bei:ween pre-puberty andpost-puberty animals, the sexes, and the 2populations.

1

la le

ngth

Che

l

14-

12-

10-

8 -

6-

4 -

2-14-

12-

10-

8 -

6 -

4 -

2 -

-0

o

(a) V

\(0=165^) J

(n=93c) / ' ^

(n=138cf) '•/I

/f

(n=1139) , / i l^f/ b^

i i i i i i i4 6 8 10 I i 16 18 21

Carapace width (mm)Fig. 2 Relative growth in length of chela propodus ofMacrophthalmus hirtipes from Avon-Heathcote Estuary(a) and Governors Bay (b) (closed circles, males; opencircles, females; bars, standard deviations; lines fitted byleast squares).

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Simons—Relative growth in Macrophthalmus 195

6-

^ 4-

-£ 2-

> n-| 6 -

I4"2-

(a)

(b)

^ ^ . - • ' • ' * * (n=110)

6 8 10Carapace width (mm)

12 14 16 18 20 22 24

Fig. 3 Relative growth in width of 5 th abdominal segmentof male Macrophthalmus hirtipes from Avon-HeathcoteEstuary (a) and Governors Bay (b) (bars, standarddeviations; lines fitted by least squares).

2 -

Carapace width (mm)Fig. 4 Relative growth in maximum width of abdomen offemale Macrophthalmus hirtipes from Avon-HeathcoteEstuary (a) and Governors Bay (b) (closed circles, widestabdominal segment; open circles, parasitised females;bars, standard deviations; lines fitted by least squares).

RESULTS

Chela propodus length

Males. There was a marked inflection in therelative growth of the male chela (Fig. 2). Forestuarine crabs (Fig. 2a), this occurred between 10.0and 13.0 mm CW, and therefore regressionequations were calculated for crabs 11.0 mm andsmaller, and for those larger than 12.9 cm CW(Table 1). Similarly, regression lines were calculatedfor bay crabs equal to and less than 12.0 mm CWand larger than 13.9 mm CW (Fig. 2b, Table 1).Allometric growth of the male chela of M. hirtipeschanged significantly (P < 0.01) from near isometryto positive allometry after the inflection (Tables 1 &2).

Females. The rate of growth of female chela wasconstant with increasing carapace width (Fig. 2),and therefore regression equations were calculatedusing all the data. Chela growth was similar in bothpopulations and remained negatively allometricthroughout post-larval life (Table 1).

Abdomen widthMales. Relative growth of the male fifth abdomi-nal segment was linear over the entire body-sizerange measured and similar in crabs from the 2 studyareas (Fig. 3). Data for all sizes were used tocalculate the regression lines and growth wasnegatively allometric (Table 1).

Females. The abdomen showed a significantchange in allometric growth (P < 0.01) from nearisometry in small crabs to positive allometric growthat puberty for both estuary and bay crabs (Fig. 4). Inestuarine females this discontinuity in relativegrowth occurred between 10.0 and 11.0 CW, whilstfor bay crabs it was between 11.0 and 12.5 mm CW.Regression equations were calculated separately fordata from above and below the discontinuity foreach population (Table 1).

The marked change in abdomen-width relativegrowth coincided with a change in abdomen shape inwhich the widest segment changed from the third tothe fourth. Equations were formulated usingabdomen width data grouped according to whichabdominal segment was the widest (i.e., crabs withfirst or third abdominal segments as the widest wereseparated from those with a prominent fourthsegment; crabs with the third and fourth segmentsequal were excluded) (Table 1). The resultingequations were virtually identical to those based onpre-puberty and post-puberty crabs shown in Fig. 4.

Developmental changes in abdomen shape wereanalysed by plotting the percentage of females withimmature (lst-3rd segment), intermediate (3rd-4th), and mature (4th segment) abdomens against

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196 New Zealand Journal of Marine and Freshwater Research, 1981, Vol. 15

O)

_©

O

aJa.

Carapace width (mm)Fig. 5 Relative growth in length of 1st pleopod of maleMacrophthalmus hirtipes from Avon-Heathcote Estuary(a) and Governors Bay (b) (bars, standard deviations; linesfor pre-puberty specimens drawn by eye; other lines fittedby least squares).

carapace width (Fig. 6), Half the estuarinepopulation showed mature abdomen configurationsat the 11.1-12.0 mm CW size class, whilst for baycrabs it was between 12.1 and 13.0 mm CW.Females mature over a considerable size range, butestuarine crabs apparently mature at a smaller sizethan their marine counterparts.

Pleopod lengthThe relative growth pattern of the male first pleopodwas similar for estuarine and marine crabs (Fig. 5).Early growth rates changed constantly until thecrabs were between 10.0 and 12.0 mm CW,whereupon pleopod growth became negativelyallometric (Table 1). Some calcification of thepleopods was observed in mature males.

Sexual and interpopulation comparisons

Allometric growth equations were compared byanalysis of covariance and revealed some differencesin the relative growth of identical structures ofmature and immature animals, between the sexes,and sometimes between crabs from the twopopulations (Table 2). Clear sexual dimorphism ofthe adult chela propodus length and abdomenproportions was confirmed (Table 2). Significantintraspecific differences in the form of the allometricgrowth equation were detected for adult male andfemale chela length and female and male abdomenwidth (Table 2).

DISCUSSION

The relative growth and changes in form of thesecondary sexual characters of M. hirtipes reportedhere are typical of the general pattern found in otherbrachyurans (e.g., Haley 1969,1973, Hartnoll 1974,Jones 1978). Before sexual maturity, the size andshape of the chela and abdomen are similar in bothsexes and approximate isometric growth. However,at maturity there is a sharp increase in the relativegrowth of these characters in one sex (i.e., malechela length and female abdomen width), but in theother sex growth continues much as before (i.e.,female chela length and male abdomen width).Sexual dimorphism of the pleopods is apparentearlier than puberty, although further morphologi-cal changes appear in these organs at sexualmaturity. The differential growth of the male chelaand pleopods and female abdomen is correlatedwith their roles in reproduction (Hartnoll 1974).Those structures (such as female chela and maleabdomen) which are not involved in mating orbrooding of the eggs show no significant changes inallometric growth.

The chela of mature male M. hirtipes issignificantly larger than that of a similarly sizedmature female. Larger chela size may be importantto the male in subduing a female before copulationor in agonistic displays among conspecifics (Beer1959). Barnes (1968) examined the relative growthof the chela in 13 Australasian species (representing4 genera) of the subfamily Macrophthalminae,including M. hirtipes. A direct comparison of the

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Simons—Relative growth in Macrophthalmus 197

Fig. 6 Frequency of 3 abdomenconfigurations in female Macroph-thalmus hirtipes from Avon—Heathcote Estuary (a) and Gover-nors Bay (b) (open triangles,immature; closed circles, inter-mediate; closed triangles, mature;lines fitted by eye).

710 12 14

Carapace width (mm)16

male chela length data was possible only afteranalysis of the present data using Barnes's (1968)procedure. Despite the limited numbers used byBarnes, there is remarkable agreement between thechela length/carapace width relationships obtainedin each study (Table 3). Formulae for chela relativegrowth in male and female M. hirtipes fall wellwithin the ranges for other members of theMacrophthalminae shown by Barnes (1968), andsuggest similarities between this endemic NewZealand species and the Australian macrophthal-mids.

The male pleopod illustrates a distinct puberty-related change at about 10-12 mm CW, at whichtune it shows strong, negative allometric growth.This body size compares favourably with theestimate of size at female puberty, based onabdomen growth. Standardising the attainment ofsexual maturity at the same minimum size for eachsex would be advantageous for maximising repro-ductive output. The changing pre-pubertal develop-ment of the first pleopods of M. hirtipes is similar tothat reported for Ocypode quadrata and O.ceratophthalmus (Ocypodidae), but is different inmature specimens as post-puberty growth of themale pleopods in these species of Ocypode is

isometric (Haley 1969, 1973). Hartnoll (1974)suggested that negative allometric growth is anadaptive feature which reduces the variation in thesize of the copulatory organs between mature malesof different carapace widths. The conclusion is thatstandardisation of the size of the organs facilitates agreater overall sexual compatability between malesand females. An ability to mate with as wide a sizerange of females as possible would allow each maleto maximise his reproductive potential.

The growth of the female abdomen in M. hirtipesshows a marked change in size and shape correlatedwith sexual maturity. This increase in size occurs at acarapace width between 10.0 and 12.5 mm CW, andis associated also with ripening of the ovaries,changes in pleopod development, and the size atwhich egg deposition begins. The smallest ovigerousfemales collected at the Avon-Heathcote Estuaryand at Governors Bay were 10.0 mm and 10.5 mmCW respectively. Development of the abdominalflap and pleopods serves to bring them to anefficient size and shape to carry and protect theincubating eggs. Some large females were found tohave smaller abdomens than predicted by theabdomen width relationship (Fig. 4), and wereinfected with an undescribed parasitic epicarid

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198 New Zealand Journal of Marine and Freshwater Research, 1981, Vol. 15

Table 1 Constants for linear regression equations (Y = m X + c) and allometric equivalents (Y = a J&) (X, carapacewidth; Y, appendage length) for the relative growth relationships of Macrophthalmus hirtipes from Avon-HeathcoteEstuary (Estuary) and Governors Bay (Bay) (n, sample numbers; -, not applicable).

CHELA PROPODUS

Females

Estuary

Bay

Males

Estuary ($11.0

Estuary (513.0

Bay ($12.0

Bay (>14.0

LENGTH

mm CW)

mm CW)

mm CW)

mm CW)

97

113

80

57

43

112

0.426 +0.118 0.994

0.424 +0.268 0.983

0.538 -0.552 0.976

1.185 -8.279 0.927

0.475 -0.023 0.987

1.246 -10.159 0.942

0.87

0.84

1.00

1.78

0.90

1.81

0.676

0.739

0.524

0.074

0.661

0.064

0.995

0.989

0.981

0.925

0.988

0.941

ABDOMEN \

Males

Estuary

Bay

Females

Estuary

Estuary

Estuary

Estuary

Bay

Bay

Bay

Bay

PLEOPOD

Males

Estuary

Bay

/JIDTH

($10.0 mm

(segments

(511.0 mm

(segment

($11.0 mm

(segments

(512.5 mm

(segment

LENGTH

(511.0 mmC

(511.0 mmC

CW)

1-2')

CW)

4)

CW)

1-3)

CW)

4)

i CW)

1 CW)

110

83

0.

0.

277

252

+0.

+0.

110

386

0.

0.

972

994

0.

0.

0.2

77

0.

0.

538

624

0.

0.

969

997

68

74

87

103

50

54

104

115

0.556 -0.788 0.961

0.556 -0.827 0.968

0.993 -4.076 0.938

0.978 -3.881 0.945

0.512 -0.582 0.976

0.537 -0.768 0.978

0.986 -4.660 0.954

0.992 -4.754 0.963

0.7

1.07

1.41

1.37

1.00

1.03

1.45

1.47

0.961

0.441

0.235

0.260

0.507

0.467

0.200

0.190

0.961

0.972

0.907

0.920

0.979

0.980

0.946

0.956

29

31

0.

0.

73

74

0.

0.

634

626

0.

0.

919

970

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Simons—Relative growth in Macrophthalmus 199

Table 2 F-tests of significance of allometric growth equations for relative growth relationships of Macrophthalmushirtipes from Avon-Heathcote Estuary (Estuary) and Governors Bay (Bay). F values for b (slope of line) and a (Yintercept where X = 1) (d.f., degrees of freedom; level of significance: •*, 1%; *, 5%; NS, not significant).

Pairs of lines tested

d.f. Sign. d.f. Sign.

CHELA PROPODUS LENGTH

Estuary males Ull.O)/Estuary males U13.0)

Estuary males <<11.0)/Estuary females

Bay males («:12.O)/Bay males (S14.0)

Bay males (£12.0)/Bay females

Estuary males U11.0)/Bay males (<12.0)

Estuary males (Sl3.0)/Bay males (S14.0)

Estuary females/Bay females

287.17

1.5166

300.17

18.306

0.1144

4.4803

10.868

1/98

1/98

1/91

1/91

1/91

1/98

1/98

**

NS

**

**

NS

*

**

15:153

0.0108

2.3047

1.5171

0.9393

13.614

0.0516

1/99

1/99

1/92

1/92

1/92

1/99

1/99

**

NS

NS

NS

NS

**

NS

ABDOMEN WIDTH

Estuary females (1-3)/Estuary females (4)

Estuary females (1-3)/Estuary males

Estuary females (4)/Estuary males

Bay females (l-3)/Bay females (4)

Bay females (1-3)/Bay males

Estuary females (1-3)/Bay females (1-3)

Estuary females (4)/Bay females (4)

Estuary males/Bay males

139.34

36.434

244.79

361.43

10.682

9.9592

2.9162

5.0335

1/98

1/98

1/98

1/98

1/98

1/98

1/98

1/98

**

**

**

**

**

**

NS

*

15.198

545.47

215.53

25.723

508.70

0.9714

12.662

1.4517

1/99

1/99

1/99

1/99

1/99

1/99

1/99

1/99

**

**

**

**

**

NS

**

NS

PLEOPOD LENGTH

Estuary males/Bay males 0.2198 1/58 NS 0.0023 1/59 NS

Measurements are mm CW.

2Figures refer to abdominal segments.

isopod (M. B. Jones and G. Hewitt, pers. comms;Simons 1980) which appeared to retard develop-ment of their ovaries and abdomen. Infection beforepuberty may have interrupted the endocrine balanceof the female before and during puberty, resulting injuvenilisation of the abdomen (Fig. 4). A few largecrabs with normal-sized abdomens (for theircarapace width) were also infected and haddegenerate ovaries, but may have escaped juvenil-isation of their abdomens by being parasitised after

puberty. Males were also infected by the isopod, butno deformities were detected.

Statistically significant differences in the relativegrowth of identical structures of M. hirtipes at theestuary and at the bay were found in this study(Table 2). An indication that maturity of the skeletalstructures of the estuarine crabs takes place at asmaller size than in their marine neighbours isevident in the male chela and in development of thefemale abdomen. This agrees with the general

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Table 3 Relationships; between chela propodus length ( I ) i^i -wapace width (X) in Macrophthalmushirtipes from Barnes (1!>68) and present Study (Estuary, Avon-Heathcote Estuary; Bay, uovemons bay; n,sample size).

SourceSizeran"e

(mm CW)

Regressionequation

Males

Estuary

Bay

Barnes (1968)

Females

Estuary

Bay

Barnes (1968)

165

138

22

97

113

15

4.8

5.0

10.5

4.0

4.6

9.3

- 20.3

- 23.6

- 30.2

- 25.8

- 19.6

- 29.3

log Y = 1.33 log X - 1.33

log Y = 1.43 log X - 1.65

log Y = 1.53 log X. - 0.86

Y = 0.43 X + 0.12

Y = 0.42 X + 0.27

Y = 0.44 X - 0.11

principle that estuarine animals are often smallerthan members of the same species from marinehabitats (Barnes 1976, Jones 1980). It is difficult toenvisage any adaptive significance for theseintraspecific differences, although environmentaldifferences may stimulate such a condition. On theother hand, the differences may indicate theexistence of genetic variants at the two localities, assuggested by Fanner (1974) to account fordifferences in body proportions of the Norwaylobster, Nephrops norvegicus, from different areas.

ACKNOWLEDGMENTSI thank Dr M. B. Jones fo:r his assistance and criticalappraisal of the manuscript.

REFERENCESBarnes, R. S. K. 1968: Relative carapace and chela

proportions in some ocypodid crabs (Brachyura:Ocypodidae). Crustaceana 14: 131-136.

Beer, C. G. 1959: Notes on the behaviour of two estuarinecrab species. Transactions of the Royal Society 86:197-203.

Farmer, A. S. 1974: Relative growth in Nephrops nor-vegicus (L.) (Decapoda: Nephropidae). Journal ofnatural history 8 : 605-620.

Fielder, D. R.; Jones, M. B. 1979: Observations offeeding behaviour in two New Zealand mud crabs(Helice crassa and Macrophthalmus hirtipes). Mauriora 6: 41-46.

Haley, S. R. 1969: Relative growth and sexual maturity ofthe Texas ghost crab, Ocypode quadrata (Fabr.)(Brachyura, Ocypodidae). Crustaceana 17(3):285-297.

1973: On the use of morphometric data as aguide to reproductive maturity in the ghost crab,Ocypode ceratophthalmus (Pallas) (Brachyura:Ocypodidae). Pacific science 27(4): 350-362.

Hartnoll, R. G. 1974: Variation in growth patternbetween some secondary sexual characters in crabs(Decapoda, Brachyura). Crustaceana 27(2):131-136.

1978: The determination of relative growth inCrustacea. Crustaceana 34(3): 281-293.

Jones, M. B. 1977: Limiting factors in the distribution ofintertidal crabs (Crustacea: Decapoda) in theAvon-Heathcote Estuary, Christ church. New Zea-land journal of marine and freshwater research 10:577-587.

1978: Aspects of the biology of the big-handedcrab, Heterozius rotundifrons (Crustacea: Brachy-ura), from Kaikoura, New Zealand. New Zealandjournal of zoology 5: 783-794.

1980: Reproductive ecology of the estuarineburrowing mud crab Helice crassa (Grapsidae).Estuarine and coastal marine science 11 : 433-443.

Morton, J.; Miller, M. 1973: The New Zealand sea shore.2nd ed. Auckland, Collins.

Nye, P. A. 1974: Burrowing and burying by the crabMacrophthalmus hirtipes. New Zealand journal ofmarine and freshwater research 8 : 245-254.

Simons, M. J. 1980: A comparative study of the biology ofMacrophthalmus hirtipes (Brachyura: Ocypodidae)in marine and estuarine environments. UnpublishedMSc thesis, University of Canterbury, Christ-church, New Zealand.

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