145

A CONTRIBUTION TO THE STUDY OF RELATIVEGROWTH OF PARTS IN INACHUS DORSETTENSIS

BY M. E. SHAW, M.Sc.(From the Zoological Laboratory, King's College, University of London.)

(With Eleven Text-figures.)

{Received July 1928.)

1. INTRODUCTION.

THE work which forms the subject of the present paper was carried out under thedirection of Professor J. Huxley to supplement work of a similar nature whichhe himself carried out on Maia squinado (see Huxley 1927) where, inter alia, hefound that certain facts could be accounted for by postulating a growth-gradientor graded distribution of what may perhaps be called growth-potential, from acentre in the chelar propus downwards along the chela and then backwards alongthe body.

2. MATERIAL AND METHODS.

The crabs were obtained from Plymouth, as many various sizes as possible beingselected, with approximately equal numbers of 33 and $$, and the investigationwas carried out by taking linear measurements of the different appendages and partsof the body. The method of measuring as large a number of crabs as possible, takenat random from a population but including a large range in size, was adopted be-cause of the difficulty of keeping crabs in captivity and collecting the successivemoults. 162 crabs (75 3 and 87 $) were measured, ranging in size from 6 to 34 mm.in carapace length1.

The measurements carried out were as follows (see Text-fig. 1):1. Carapace length (this was the standard measurement with which all others

were compared). From a point between the two anterior spines to the median pointof the posterior border of the carapace.

2. Carapace breadth. Greatest breadth of the carapace, between the bases ofthe first two pereiopods.

3. Cheliped. Only the propodite was measured since the folding back of thecheliped in the $ makes a total length measurement unreliable.

(a) Propus length (see diagram).(b) Propus breadth (maximum).

1 All crabs measured were free from Sacculina externa and any showing obvious regenerationof an appendage were rejected.

BJEB'Vlii I O

146 M. E. SHAW

4. Pereiopods. These were removed from the body at the breaking joint, butthe junction of the merus with the ischium was found to be a more convenientpoint from which to measure, so that the measurement taken was the length ofthe posterior border of the pereiopod from the proximal end of the merus to thetip of the dactylus. The pereiopod is, of course, flexible structure but it is a simplematter to straighten the limb and take the length measurement accurately to thenearest \ mm.

5. Third maxilliped. A length measurement was taken from the point of junc-tion of the ischium and basis on the inner side to the most distal point of the merus(see diagram).

m ' Carapace

D E

Cara-pwce

Text-fig. 1. Diagrams showing measurements taken. A. Carapace; ab, median length; cd, maxi-mum breadth. B. $ Chelar propus; ef, length; gh, breadth. C. 3rd maxilliped; ik, lengthof ischius and merus. D. $ abdomen; Irn, length; no, breadth of 6th segment; rs, length ofindividual segment e.g. 4th; pq, maximum breadth of individual segment e.g. 4th; E. <J abdomen;tu, length; vzv, breadth (6th segment).

6. Abdomen.A ?. (1) Length. Greatest length of the abdomen in the extended position,

from the median point of the posterior border of the carapace to the tip of the sixthabdominal segment.

(2) Length of 3rd, 4th, 5th and 6th, abdominal segments.(3) Breadth of 3rd, 4th,^th and 6th, abdominal segments.

B <?. (1) Length. Greatest length in extended position.(2) Breadth of 6th abdominal segment.

Measurements on the pereiopods were carried out by placing the leg in thefully extended condition on a metal mm. rule; on the carapace (length and breadth)and abdomen (length, <J and $) with fine accurately adjustable dividers. The third

Relative Growth of Parts in Inachus Dorsettensis 147maxiUiped, chelar propus, and breadth of the abdominal segments were measuredby making use of the travelling stage of a microscope with a cross-wire in the eye-piece. The measurements on the pereiopods were probably accurate to the nearest1 mm. and the abdomen and the carapace to about \ mm., all others to aboutJ^mm.

The biometric constants for the measurements have deliberately not been calcu-lated since this piece of work is intended only as a general mapping of the groundwhich may serve as a basis for more detailed work on certain parts in the future.Some of the smaller differences obtained are doubtless not statistically significant.The original data have been deposited at the British Museum (Natural History).

The results were analysed as follows:The S3 were divided into six classes and the $$ into eight classes according to

carapace length, and the mean absolute sizes and relative sizes of the differentappendages and parts of the body calculated for these classes. Graphs were thenconstructed showing:

A. A comparison of the mean relative lengths of the appendages in the <$ andthe ? (constructed on data from all classes of crabs together). Graph VII.

B. A comparison of the percentage increase in size of the appendages andabdomen in the $ and $ for a given percentage increase in carapace length.Graph VIII.

C. A comparison of the ratio <?/$ for the absolute size of all the appendagesand for the abdomen in large and small crabs, i.e. change in this ratio with changein size. Graph IX.

D. Graphs where the relative length of the part in question was plotted againstthe carapace length. These showed growth changes in the proportions of the variousparts. Graphs III, IV, V.

E. Graphs where the logarithms of the mean sizes for the different classes wereplotted against the logarithms of the mean carapace length for the classes. Thesewere of value in determining k in the heterogony formula y = bxk (y = measure-ment of part in question, x = standard measurement), and so in comparing therelative growth-rates of the different parts of the body. Graph I.

3. RESULTS.

A. GROWTH OF THE PROPODITE OF THE CHELIPED.

(1) Male. The points on the log. log. graph showing the relative growth inbreadth (Graph I B) form a rough approximation to a straight line whose inclina-tion gives k = 17. However, when we look closer, we find that the relative growthis at first less than this (from 8-15 mm. carapace length k = approximately 1-4),followed by an acceleration of relative growth (k = approximately 2-5), withfinally a marked falling off for the last size class. The log. log. graph showing thechanges in relative growth-rate for the length of the chelar propus is exactlysimilar to that for the breadth, but k calculated for the general slope of the lineis only 1-3, and the various changes in relative growth-rate are not so marked.

10-2

148 M. E. SHAW

The deviation from a simple straight line series of points is in all probabilitydue to the phenomenon of "facultative" high and low dimorphism, first describedby G. Smith (1906) for / . scorpio. In this species the normal strong heterogonyof the $ chela is replaced by the $ isogonic type of growth in the non-breedingseason. This period is usually passed through in the winter months, but not neces-sarily, so that during the breeding season there may be three classes of <J<J, " high,""low," and "middle" or "female" type. The "high" and "low" <$<J are largebodied and small bodied respectively and most of the "middle" type are of inter-mediate size. The "middle" <$<$ have the $ type of chela, the others the S type butof different relative size (see Huxley 1927 for analysis). In the summer (breedingseason) the number of middle <J<J is small and depends on when the last moulttook place. In the winter months there are a few "high" $<$ but no "low" SSjall the small crabs having the flat ? type of chela.

DText-fig. 2. A. Dorsal view with abdomen in the extended position showing origin of the 4th

pereiopod opposite the ist abdominal segment. B and C. Extreme variation in shape of chelarpropusin&Jof 18 mm. carapace length; B, "high"c? type; C, "$"typeofcj. D. Chelar propusof ? of 18 mm. carapace length.

The measurements which form the subject of the present paper were carriedout on specimens of / . dorsettensis collected at three different times, namely April1926, October 1926, October 1927. Thus the first batch was collected at the verybeginning of the breeding season and the other two batches in the non-breedingseason, so that one would expect only a small number of " low " SS to be represented.In this species there is evidence that the $ chela does not go over to the $ type ofgrowth, in the non-breeding season, nearly so completely as in / . scorpio. An in-spection of the chelae of all the small and medium sized 3 crabs (under 20 mm.carapace length) showed that a small number of crabs had the definite $ type ofchela, a small number the full S type (see Text-fig. 2), but that in the majoritythe chela was intermediate in shape between the <£ and $ types, and that all grada-tions between the two extreme conditions existed. A plot of mean chelar breadthagainst mean carapace length on the absolute scale showed no interruption of the

Relative Growth of Parts in Inachus Dorsettensis 149curve at intermediate body sizes, such as was found by Huxley (1927) on analysisof G. Smith's results on / . scorpio. In view of these facts one would expectGraph I B to show only a slight trace of the phenomenon of "facultative" highand low dimorphism, and examination of the graph proves that this is the case.This graph may be interpreted as follows: from 8-15 mm. carapace length the<J chela is of the $ type but relatively larger than in $$ of the same size; from15-17 mm. carapace length the positive heterogony is slightly more marked andthis part of the graph probably represents the "low" S$ taking part in the firstbreeding season; from 17-21 mm. carapace length there is an almost imperceptibledecrease in relative growth-rate which may be explained as being due to reversionto the $ type of growth, in a certain number of cases, during the non-breeding

7 8910 20 30

7 8 910 20 30 '-7 8 910 20 30

Carapace length in mm.

Text-fig. 3 (Graph I). Logarithmic plot of means for $ chelar propus length (A), breadth (B);$, chelar propus length (c), breadth (D); <J, pereiopod length (E); $, pereiopod length (F) ;against carapace length.

season; from 21-25 mm. carapace length there is strong positive heterogony, thispart of the graph probably representing the "high" $$ taking part in the secondbreeding season; the final falling off may be due to a second non-breeding seasonin old animals but as it is based on one class only this cannot be pressed in anyway. It is more probable that it has no real significance as a similar large decreasein slope is commonly found towards the end of graphs showing the relative growth-rate of heterogonic organs, and has a purely mathematical explanation.

There is however dimorphism of a sort in the <$ chela as it shows a bimodalfrequency curve (Graph II), the two modes occurring at 4 mm. and n mm.chelar breadth, and representing true "high" and "low" forms. Thus there aretwo phases of <? chelar growth, and the conversion from the "low" type to the"high " type takes place suddenly and presumably at a single moult, which occurs

M. E. SHAW

most frequently at about 20 mm. carapace length. A similar bimodality for chelarpropus breadth is shown in the correlation table given by G. Smith (loc. cit.page 97) for / . scorpio, the modes occurring at 3 mm. and 10 mm. chelar breadth.It is, however, not referred to by him in his text. A plot of the modes for the chelarbreadth at different carapace lengths, on the log. log. scale, for / . scorpio, showed

HA

20 25 30 35 40

Relative breadth $ chelar propus

IIB

3 0 -2 9 -28272625242322

•a 9

00

uoGj

IOS

- T T T

T - -r -r -it

20

19181716,L15U-

i

T T .

T T

13L

f12fT91

,. T

0 t 2 3 4 5 6 7 8 9 10 11 12 13 14 15 If.

cJ Chelar propus breadth in mm.

He

0 1 2 3 4 5 6 7 8 9 10 II 12 13 14 15

S Chelar propus breadth in mm.

Text-fig. 4 (Graph II). II A. Frequency of relative <? chelar propus breadth for all specimens.II B. Absolute frequency of <J chelar propus breadth at various carapace lengths. II c. Absolutefrequency of <J chelar propus breadth for all specimens taken together.

that there is the same enormous variation in relative growth-rate as in / . dorsettensis.Comparison of G. Smith's normal and infected ^ (no data for the $ available)showed that in / . Scorpio, when there is reversion to the $ type of growth, the <Jchela in a certain number of specimens actually goes over to values equal to thosefor the infected $

Relative Growth of Parts in Inachus Dorsettensis 151(2) Female. The growth of the chelar propus in the $ shows slight positive

heterogony; the points on the log. log. graph (Graph I c and D) conforming toapproximately straight lines, for which k= 1-16 for length and 1-15 for breadth, sothat, in contrast to the #, the relative increase in length is greater than in breadth.

B. GROWTH OF THE PEREIOPODS.

(1) Male. The relative growth of the pereiopods is shown in Graph I E andGraph III. Graph III shows that the changes in relative growth-rate are in ageneral way similar to those in the chelar propus but, owing to the much greaterlength of the pereiopods, all changes are much more marked. The peculiar "backkink" of the curve for all the pereiopods is hard to interpret. It may well be thatthe relative growth-rate of the pereiopods decreases at the same time as that of

PereiopodsChelar 3rdpropus maxilliped

0 20

Mean rela-tive breadth

160 180 200 220 240 260 280300 320 340 360 380 I 40 60

Mean relative lengths20 40

Text-fig. 5 (Graph III). <?. Mean relative length of 3rd maxilliped, chelar propus, pereiopods 1-4,and mean relative abdomen breadth plotted against mean carapace length.

the chelar propus decreases {i.e. there is a similar slowing down of growth in thenon-breeding season) and that after this period when the relative growth-rate ofthe chela rapidly increases, this acts as a drain on the pereiopods, so that thesenever again attain to their former relative size. On the other hand it may beentirely due to a "draining" or retarding effect of the chela, the effect of whichis most obvious just before the period of strong positive heterogony in "high"&J rather than while this is actually taking place. As the log. log. graph (Graph I E)shows, there is an actual decrease in absolute size (between 19-20 mm. carapacelength) for all the pereiopods except the 3rd which shows a trivial increase, k inthe heterogony formula calculated for classes 1-5 gives the following series: P1,1-22; P 2 , 1-22; P 3 , 1-26; P 4 , 1-27; where P = pereiopod, and after the periodof size decrease k does not again reach its previous values for the remainingclasses (6-8).

(2) Female. The growth of the pereiopods is shown in Graph I F and Graph IV.The graphs are rather irregular in spite of the fact that there are larger numbers

M. E. SHAW

of individuals in each class than in the case of the <J, and the meaning of this isnot at all clear. Only to an extremely limited extent can the irregularities be saidto follow those of the chelar propus. On the other hand the general form of thecurve is similar for all the pereiopods. An interesting point brought to light bythe log. log. graph (Graph I F) is that, over the whole size range covered, thepereiopods show positive heterogony although this is not so marked as in thecase of the <J, the k series being P i, i - n ; P 2, I-IO; P 3, 1-06; P 4 , 1-08 (the lastclass being neglected for all the pereiopods in the calculation of k). The significanceof the rise in k for P 4 will be referred to again later.

C. 3RD MAXILLIPED.

The maxilliped is very slightly negatively heterogonic in both sexes (see GraphsIII and IV), k for the $ being -98 and for the <J -95. This difference between the <?and the $ may not be significant, but it is clear that there is no positive heterogony.

PereiopodsK

Abdomen 4th. 3rd 2nd 1stChelarpropus

3rdmaxilliped

!_ . I . 1 ! I I ( ! I I I lit II40 60 80

Mean rela-tive breadth

KQ160 180 200220 240 260 280 300 I 40 60 80

Mean relative lengths20 40

Text-fig. 6 (Graph IV). 2. Mean relative length of 3rd maxilliped, chelar propus and pereiopods 1-4,and mean relative abdomen breadth plotted against mean carapace length.

D. ABDOMEN.

Graph V is constructed from measurements of the 6th abdominal segment inboth <$ and $, and shows that in the $ there is a period of slight positive heterogony,then one of very strong positive heterogony followed by a period of isogony.The first period represents the narrow flat abdomen of the adolescent crab,the last the broad abdomen with convex ventral surface of the adult crab, andthere is evidence that these two types are separated by only a single ecdysis asin the case of / . scorpio as mentioned by G. Smith (loc. cit. p. 68). This has notbeen actually observed in the case of / . dorseitensis but is proved by the existenceof a discontinuously bimodal frequency curve for the abdomen breadth (seeGraph VI). The case is somewhat similar to that of the <$ forceps in Forficula(see Huxley 1927), but the "low" type (adolescent abdomen) and the "high" type(adult abdomen) are more distinct (see Graph VI). All the small $ crabs are of the"low" type and all the large $ crabs of the "high" type, but crabs of mediumsize (12-17 mm- carapace length) fall into one or other of the two equilibrium

Relative Growth of Parts in Inachus Dorsettensis 153positions. Within the "low" group there is slight positive heterogony (k = 1-4)and the "high" group is isogonic or even slightly negatively heterogonic (k = ap-proximately -97). The case is different from that of Forficula in that the "high"type follows the "low" type in time, and during the persistence of each type anumber of moults may take place. Further the relative abdomen width in the"low" type increases slightly with increasing body size and in the "high" typeremains constant, whereas in Forficula the relative forceps length taken separatelyfor each type slightly decreases. In Inachus it seems clear that the abdomen growthconsists of two long periods, one of slight positive heterogony, the other of isogony,

10 20 30 40 50 60 70 80 90Relative abdomen breadth

Text-fig. 7 (Graph V). Mean relative abdomen breadth plotted against mean carapace length foro (A-A) , $ (©-®). Limits for each carapace length class plotted for both <J and ?.

separated by a short period of violent heterogony, which presumably begins directlyafter a moult, since its effects are shown completely by the next moult. The rangeof size over which this may occur is 13-17 mm. carapace length. The linear sizesare as 1 : 1*3, which would correspond closely to a doubling in volume and ac-cording to Brook's and Przibram's law and experimental data on Carcinus wouldimply that there is a range of one whole instar for the particular moult at whichthe adult abdomen is assumed. Similar considerable variation in the size at whichthe full relative width or adult abdomen is attained is found in the Fiddler crabUca (see Huxley 1927). Whereas the $? of both Uca and Inachus acquire a definitiverelative abdomen breadth, unpublished data, by Huxley and Richards, on Carcinusmoenas, show that this does not occur in Carcinus, the abdomen (like the 3 chelaof Uca etc.) becoming relatively larger with increased absolute size throughoutthe whole of life.

154 M. E. SHAW

As in the case of / . scorpio, the adolescent abdomen appears to be retaineduntil the first brood of eggs is produced, as (with the exception of four specimens)all the crabs with "adult" abdomen are in berry. The four specimens mentionedabove are of carapace lengths 15*2 mm., 15*7 mm., 16-5 mm., 20-2 mm., and havethe full relative width abdomen. There is of course the possibility that they wereabout to pair or that their previous brood of eggs had hatched.

As regards the individual segments of the abdomen in the $, it was found thatin the adolescent crabs showing the "low" type of abdomen, k was the same forall the segments, both as regards length and breadth (k = 1-3), with the excep-tion of the 5th segment where k for length was 2-2. (Only the 3rd, 4th, 5th and6th segments were measured as the 1st and 2nd segments are too small to bemeasured with sufficient accuracy.) k for this period was calculated from classesU and V, Table III. For the period when the rapid transition from the "low"

2 2 | -2lL - T T

IT18r

as

ocsa,«

a

17-

13-

IT

T T .T T

35 40 45 50 55 60 65 70 75 80.-40-45-50-55-60-65-70-75-80-85'

Relative abdomen breadth

Text-fig. 8 (Graph VI). $. Frequency of relative abdomen breadth at different carapace lengths.

type of abdomen to the "high" type is taking place k is greatest for the 6th segmentbreadth and is approximately 3-15; for the breadth of the remaining segments itgradually decreases (seg. 5, k = 2-8; seg. 4, k = 2-6; seg. 3, k = 2-5). The greatestincrease in relative length again takes place in the 5th segment \k = 3-1). The valuesof k for the lengths of the other abdominal segments are as follows: seg. 6,k= 2*9;seg. 4, k = 2-9; seg. 3, & = 2-1. From this it will be seen that apart from the highvalue of k for the 5th segment, there is a gradual decrease in k from the distalto the more proximal segments. On the whole the k values for the breadth arehigher than those for the length, and this is what one would expect as in the.transition from the "low" to the "high" type the main alteration is an increase inrelative breadth.

Relative Growth of Parts in Inachus Dorsettensis 155Graph V shows that the abdomen in the <J is isogonic in young crabs, but be-

comes slightly negatively heterogonic in older crabs.

E. GRAPHS OF RELATIVE GROWTH OF PARTS.

A series of graphs were constructed to compare the $ and $ as regards relativegrowth of parts (list already given, p. 145 etseq.).

(1) Graph VII. The mean relative lengths of the third maxilliped, chelarpropus, and pereiopods 1-4, were plotted for the <3 and ?, and it was found, aswas to be expected, that the chela in the <Jattains to a much greater relative size than inthe ?. The result for the pereiopods was ratherunexpected and the exact opposite of the resultobtained by Huxley (loc. cit.) for Maia squinado.In Maia the differences in relative length of thepereiopods in the £ and the $ fall regularly fromP i to P 4 and, what is more important, thedifference expressed as a percentage of the $value decreased from P1 to P 4 in the same way,indicating that the strongly heterogonic growthof the chela in the 3 was correlated with thegrowth of the pereiopods, P 1 being affectedthe most and P 4 the least. The difference inthe relative size of the 3rd maxilliped in the <Jand $, expressed as a percentage of the $ valuewas —4*33, indicating a retarding effect of theactive growth of the <J chela on the appendageanterior to it.

In Inachus, on the other hand, the differences between the mean relative lengthsof the pereiopods of the <J and $ do not decrease regularly in this way (P 1,15-2; P 2,16-2; P 3,147; P 4,13-3) and the percentage differences show exactly the oppositeresult (P 1, 5-0; P 2 , 7-1; P 3 , 7-8; P 4 , 8-2) so that relative to its length the 4thpereiopod is affected most. The 3rd maxilliped, however, showed the same resultas in Maia, namely a negative percentage (see above) of 3-48.

(2) Graph VIII. This graph was constructed by finding the percentage in-crease in abdomen and appendage size for <J and $ crabs taken over the same rangeof increase in carapace length. It shows that in none of the $ appendagesis the percentage increase in size so great as in the <?, but for the abdomen it isvery much greater in the $. In the $ pereiopods the percentage increase is greatestfor P 3 and P 4 as was to be expected from the values for k. In the $ pereiopodsthe percentage increase is practically the same for P 1, P 2 and P 4, but considerablyless for P 3 . This is accounted for by the low value of k for the 3rd pereiopod,but it is difficult to say with certainty what is the meaning of this. It may perhapsbe explained by assuming that as the 4th pereiopod lies opposite the 1st abdominalsegment it is included within the region of active growth which is responsible for

3rd Chelar 1st and 3rd 4thmaxilliped propus " ?' I"""

(x io ) (x6) PereiopodsText-fig. 9 (Graph VII). Mean relativelengths of various appendages in $ and $.

156 M. E. SHAW

3rdmaxilliped

Chelarpropus

CO

O

a

ist

and

O I i

J3 3rd -

\4th

Abdomen

50 60 70 80 SQ 100 110 120 130 140 153 160 170 180 190 2Q3

% increase

Text-fig. 10 (Graph VIII). Percentage increase in size of various appendages and abdomen in <$ and <for given percentage increase in carapace length (shown by vertical lines, 2, £).

A —Large <?#Large 2 2

Small o* o* (v' Small 2 2 / o

3rd Chelar 1st and 3rd 4th Abdomenmaxilliped propus '— •-• ~

PereiopodsText-fig, i i (Graph IX). Ratio <J/2 for various appendages and abdomen in large and small crabs.

Relative Growth of Parts in Inachus Dorsettensis 157the great increase in relative size of the abdomen (see Text-fig. 2 A). If this is thecorrect explanation then k for the 4th pereiopod is abnormally high ov/ing to itsproximity to the abdomen, rather than k for the 3rd pereiopod abnormally low.This would agree with the k series for the 2 pereiopods, as k falls regularly fromthe 1st to the 3rd pereiopod. For the 3rd maxilliped the percentage increase isgreater in the $ than the ?. It is worthy of note that in the posterior part of thebody the graph is almost the exact reverse in the <J and the $. Graph VIII maybe said to give part of the "growth profile" of the two sexes.

(3) Graph IX was constructed by plotting the ratio ^ Va]Ue (as a percentage)

Table I. 3.

Mean absolute measurements in mm.

Referenceletter of

class

ABCDEFGH

Carapacelengthclasses

27-3424-2720-2418-2016-1814—1611—146-11

Averagecarapacelength

29-725-22I-I19-016-814-912-68-2

No. inclass

1 1

71111111086

Chelar propus

length

26-122-2156i3'8u*79*4764'7

breadth

11-4IO-25-84-8392-82-31-2

Pereiopod length

1

92-280-966066-755*148-237-024-0

2

70-66o-85O'45O-543*336-527-6l8"2

3

58-95°'441-941-835-830-622*9I4'5

4

50-643'436-136-33O-826-119-412-5

3rdmaxilliped

length

7-66-25'45'°4'4397-6

Abdomenbreadth

5'34-43'93-63*4292-4i-8

Table II. +'.

Mean absolute measurements in mm.

Refer-enceletter

of class

ABCDEFGHJK

Cara-pace

lengthclasses

20-2319—2018-1917-1816-1715-1614-1513-1412-137-12

Ave-ragecara-pace

length

20-819418317-316-215614-313-212-3103

No. inclass

131415141594544

Chelar propus

length

121

US10-7999-28-88-57'36-85'5

breadth

3'23-i2-92-72-42-32-119i-8i'5

Pereiopod length

1

58657'953-849247'544'743"637-234'927-2

2

44'343'34°"336-635"333'532'527-626-321-3

3

36-335-633-i3O'428-927-926-622-422'OI9-2

4

31630-929-126-325'424-422*9199189150

3rdmaxil-liped

length

6 15*75'55-24-84'74-34-i3-83'i

Ab-domenbreadth

14-4I3'3I2'612'I10-7 |10-8 S8"57-15'23'9

Abdomenbreadth

"low"type

7-0

6-i5"6

"high"type

II-2

IO-I

9'3

for the 3rd maxilliped, chelar propus, and pereiopods 1-4 in (a) small S3 (classesG and H Table I) and small ?? (classes J and K, Table II) and (b) large <?<? (classesA-F, Table I) and large $? (classes A-G, Table II). In the small crabs the chelarpropus and 1st and 2nd pereiopods are larger in the S than the $ but the reverseholds for the 3rd maxilliped and the 3rd and 4th pereiopods, and of course forthe abdomen. The value for the 4th pereiopod is high because of the high valueof k in the Q. In the large crabs the ratio for the chelar propus has increased con-

158 M. E. SHAW

siderably and the ratios for all the pereiopods have increased to approximately thesame value so that pereiopods 3 and 4 have ceased to be longer in the $ than the 3.The ratio for the 3rd maxilliped is slightly nearer 100, showing possibly that the3rd maxilliped in the <$ starts by being smaller than that of the $ and, althoughalways remaining slightly relatively smaller, more nearly approaches it in size inlarge crabs. The difference, however, may not be significant.

Table III.

Mean absolute measurements for abdominal segments in $.

Refe-

renceof class

UVwXYZ

Cara-

pacelengthclassesin mm.

7-1212-14

14-1515-1719-2020-21

Mean

cara-pacelength

in mm.

9 912-5

I4-31 5 9i9'520-5

6th seg.

2-o2-74 - i5"55"97.9

Length

5th seg.

0 91 - 22-O

2-52-93*2

Abdominal segments

in mm.

4th seg.

0-7i - oi -62 - O

2-32-3

3rd seg.

o-60-91 2

i -5i -81-9

6th seg.

3'95"38-8

11-313-214-4

Breadth

5th seg.

4 - i5-68-6

II-I

12-713-8

in mm.

4th seg.

4-0

5'37-89 9

ii-612-5

3rd seg.

3'7 |5-o7*49 - 1

10-511-4

GENERAL RESULTS AND DISCUSSION.

1. Axial Gradients and the Relative Growth of Parts.

Analysis of the growth-rates of the different appendages seems to show that itis not possible to interpret the greater relative length of the pereiopods and smallerrelative length of the 3rd maxilliped in the <J as simple stimulating and retardingeffects of the enlarged chela of the <$ acting on a pre-existing growth gradient. Thatthere is a graded effect on the pereiopods is obvious but that it is the very reverseof a stimulating effect is equally obvious as the percentage increase in length isconsiderably greater for the 3rd and 4th pereiopods than for the 1st and 2nd pereio-pods, and the difference in mean relative length between the pereiopods in the <Jand the $ expressed as a percentage of the $ value show a steady increase frompereiopods 1-4. The k values for the pereiopods corroborate these results, k inthe <£ being approximately the same (1-22) for the 1st and 2nd pereiopods butgreater for the 3rd and 4th pereiopods (1-26 and 1-27). It is possible that boththe $ and the $ may be supposed to have the same growth mechanism (a gradientin slight positive heterogony with k greater for the cheliped and least for the 4thpereiopod, but that the draining influence of the very active growth of the chelipedin the <J is such that the gradient is reversed in the <J, k being least for the pereiopodsimmediately posterior to the cheliped. The original gradient is not affected in the ?where the chelar propus is only very slightly positively heterogonic. An explanationof the facts would be provided by the assumption that in the <£ there is a generalgrowth promoting effect and at the same time a draining effect of the large chela,in antagonism.

The 3rd maxilliped in both sexes is slightly negatively heterogonic and more

Relative Growth of Parts in Inachus Dorsettensis 159so in the 3 than the $. This indicates that there is a different growth mechanismin the appendages anterior to the chela, and that possibly the negative heterogonyis more pronounced in the 3 because the chela has the same retarding effect onthe growth as in the case of the pereiopods. This retarding effect must not bestressed in any way as the difference between the 3 and the $ may well not besignificant.

2. Distribution of Growth-Potential.

One definite result of this work on Inachus dorsettensis has been to show thatthere are marked regional differences in the relative growth-rates of the differentparts of the body, and that the two sexes differ in this respect, the distributionof what we may call "growth-potential" being in favour of the hinder thoracicappendages, especially the cheliped in the 3, and in favour of the abdomen inthe $. Thus greatest heterogonic growth is found associated with the developmentof the secondary sexual characters, but neighbouring parts, not usually thought ofas secondarily sexual, may be involved in this. Somewhat similar results wereobtained by Kunkel and Robertson (1928).

3. Growth-Centres.

Recent work on heterogonic growth in Crustacea has brought to light the factthat in parts of the body showing positive heterogony there is a growth-centre,which in appendages is usually towards the distal end. In the 3 Inachus dorsettensisit is obvious that the propus is the growth-centre for the cheliped; in the $ thegrowth-centre for the abdomen is also near the distal end, the greatest increasein breadth occurring in the 6th (last, most distal) segment, and the greatest increasein length in the 5th segment. Huxley (1927) showed that in Uca and in Maiathe growth-centre for the chela (based on weight measurements) was in the propus.

SUMMARY.1. The relative growth-rate of the various parts of the body was investigated

by analysis of linear measurements carried out on the carapace, appendages, andabdomen.

2. The results were as follows:(a) The chelar propus shows strong positive heterogony in the 3 and very

slight positive heterogony in the $. The 3 chela shows dimorphism, the 33 beingdifferentiated into "high" and "low" forms and the dimorphism in all probabilitybeing due to the fact that the 3 chela assumes the $ type of growth to a greater orless extent in the non-breeding season.

(b) The pereiopods are more positively heterogonic in the 3 than the $ andin both 3 and $ there is a graded k series, but whereas in the 3 k increases fromP 1-P4, in the $ the series is reversed k being greatest for P 1. In both sexesthe heterogony in the pereiopods is not so marked as that of the chelar propus.In the 3 the pereiopods suffer an actual decrease in absolute size at the time whenthe relative growth-rate is least for the chelar propus and after this period neveragain attain to their original relative size.

160 M. E. S H A W

(c) The third maxilliped is negatively heterogonic in both sexes and slightlymore so in the <5 than the 2, although this difference may not be significant.

(d) The abdomen in the 2 shows marked positive heterogony in young crabsbut is isogonic after the attainment of sexual maturity; in the 6* the abdomen isisogonic in young crabs but becomes slightly negatively heterogonic in old animals.The abdomen in the 2 is dimorphic, the "low " type (characteristic of the adolescentcrab) being separated from the " high " type (characteristic of adult crab) by a singlemoult. There is considerable variation in the relative abdominal growth-rate inadolescent crabs and consequently in the time of attainment of sexual maturityand the full relative abdominal width. During the period of adolescence the in-crease in relative length and breadth is the same for all segments except the 5th,which forms a growth-centre for length. At the period of rapid conversion of the"low" type of abdomen into the "high" the 5th segment remains the growth-centre for length but a growth-centre for breadth is established in the 6th segment,and increase in relative breadth decreases from the distal to the more proximalsegments.

3. A comparison of the <$ and 2 as regards relative growth of parts was madeand brought to light certain facts.

All the pereiopods are relatively longer in the $ than the 2. This is agraded effect—the actual difference in relative length (in £ and 2) decreasing fromP i-P 4, but the increase in relative length relative to the actual size of the pereio-pods increasing from P i-P 4. These facts are interpreted as meaning that thereis a common stimulating effect in the <£, and also a retarding effect of the 3 chelaon the appendages posterior to it. It is tentatively suggested that the facts relatingto the 3rd maxilliped are explained if we assume that the <$ chela has the sameretarding effect on the anterior appendages as on those posterior to it.

4. Certain general conclusions wrere drawn:(a) There is a different distribution of growth-potential in the two sexes,

and strong positive heterogony is found associated with the development of thesecondary sexual characters.

(b) The results on the relative growth-rates of the appendages in <J and 2and on the growth-rates of the individual segments of the 2 abdomen, indicate thepresence of definite gradients in the body, in relation to which growth takes place.

(c) The growth-centre for the heterogonic organs is situated towards thedistal end of the organ; this is in agreement with results obtained for otherCrustacea.

REFERENCES.HUXLEY, J. S. (1927). Biol. Zentralb.47, 151-63.KUNKEL, B. W. and ROBERTSON, J. A. (1938). jfourn. Marine Biol. Ass. 15, 655-81.SMITH, G. (1906). Fauna u. Flora Golf Neapel, Monog. 29, 65-76.