Ind ian Journal of Marine Sciences Vol. 29. December 2000, pp. 333-335

Short C011l11lunication

/

Morphometric characteristics in the hQrseshoe crab Iachy~{~us_giJ5as _ (Arthropoda: Merostomata)

Jl Yijayakumar , s Das, A Chatterji &. A H Parulekar

Nati onal In stitute of Oceanography, Dona Paula, Goa - 403 004, india e-mai l : rathod @csnio .ren.nic.in

rat hod @darya.nio.org

Received 29 Febl'llary 2000; revised 25 September 2000

The relationships observed between total carapace length wi th carapace width, carapace weight, tel son length , body weight were found stati sticall y significant (p< 0.05) in the male and female horseshoe crab, Tachypiells gigas (Mu ll er). In females, increase in weight was found to follow the cube law strictly. The carapace width grew faster at higher carapace length in females whereas, carapace weight in males was more at lower carapace. In general, a proportional increment in morphometry was seen with increase in age .

Allometry is described as the study of relationship between difference in one body parameter to the other. The allometric relationship provides an important information regarding comparative growth of various body parameters. A proper understanding of allometry in shell and soft body palts of horseshoe crab is essential to define the growth of a spec ies. The morphometric characters and their allometric re lationship are to a large ex tent influenced by age, local environmental conditions and population density of the spec ies i.". In this study we have applied allometric re lationship to the horseshoe crab, T. gigas to establish the ratios between any two parameters at one time. It is known that animals changing their body proportion may also change their shape. The use of regression anal ys is to ex plain such re lationship between various morphometric characters are found to be most suitable3

.4. The present study was therefore, taken up primari Iy to exa mine how best the concept of allometry fits to this species.

The specimens of the horseshoe crab, Tachyplells gigas (Mulle r) (Arthropoda: Merostomata ) were collected from Balaramgari, Orissa (lat. 21 0 17 ' N ; long. 87 0 00 ' E) from June , 1995 to June, 1996. Fortnightly samples of 12-15 speci mens were collected and exa mined for various morphological characters. A total number of 220 specimens were examined for the study. Specimens were measured with the help of Vernier Ca liper to 0.1 mm accuracy, fo r length of prosoma (a); le ngth of opithosoma (b); length of tel son (c); and width of prosomCl (d). T he a llometric and morphometric anal ys is were fo ll owed as described by Chatterji et ([/. 5.

Carapace length - total length relationship (Fig 1 A) was linear, with a high degree of correlation (r = 0.87), indicating that the carapace length increases proportionately with the gradual increase of total le ngth. Carapace width - total length relationship is linear showing that the carapace width increase proportionate ly with advancement of total length (Fi g IB). Telson length-total length relationship highlighted a proportionate increase in tel son length to the total le ngth (Fig. lC). Body weight - tota l length relationship was observed to be linear (Fig. ID). The rate of body weight increase was of higher magnitude than that of total length. The body we ight increased very sharpl y in the length range of 300-400 mm. The relati onship between carapace width and carapace length was linear with a very hi gh degree of correlation (r=0.95), implying that the increase in carapace width to carapace le ngth was proportionate (Fig. 2A). Body weight - carapace length shows that sharp increase in body weight was observed , the carapace length increased marg inall y in the specimt ns in the size range of 100-200 mm and the relationship was linear (Fig. 2B). Body weight - carapace width relationship (Fig. 2C) was found to be of same nature as in the case of body weight and carapace length. The increase in body weight was associated with carapace length and width with eq ual degree o f correlation.

The concept of a llomet ry was first postulated by Huxley & Tessie r6 and s ince then it has been ex tens ively applied by Illany biolog ists to estimate the popu lation growth characte ri stics of multitude organi sms. Chatte rj i ef 0/

5 re ported the length and

334 INDIAN j MAR SCI, VOL. 29, DECEMBER 2000

300 n= 137

E 5 I

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300 n= 137 © E E

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. . / ....

/ ... , ..

:' ,1:-.. .. -.. . .:... . .. .

O~--~ ____ ~ ____ ~ ____ ~ o 100 200 300 400

TOTAL LENGTHlmm)

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600

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r=+ O· 83

, \

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g 200U·:·./~ . .'

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o --~I~--~-J o 100 200 300 400

TOTAL LENGTHlmm)

Fig. I - Regress ion relationship betwee n - A: Carap:lce length and total lenglh , B: Carapace width and total length, C: Telson length and total length. 0 : Body weighl and tOlal length .

n= 137 300 y=085x + 37·09

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i 200 f-o ~ w o « Cl. 10 0 « a:

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o

j J ' 100 200 300

CARAPACE LENGTH(mm)

600

f-

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>-~ 200 m

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~ u" >- , ' g 200 ,:" ,': m " .. ~ .

, ' /

0100

--2~0~0:-------C;3';'60

CARAPACE WIDTH(mm)

Fig. 2 - Regression relali,lIl ship between- A: Carapace widlh and carapace length. B: Body wei ght and carapace lenglh. C: Body weigh t an d carapace width.

weight relationship of Cll rc il/.oscorpius ro llilldic(l[u/u (Latreill e). This study suggests that a llometric re lationship in har:> ' shoe crab plays an important role in understanding comparati ve morphometry. Thi s study al so emphasizes that care must be taken to :I pply morphometric for a uniform size group of

horseshoe crab popul at ions. It is known that the changes in the form of an animal cannot be described sa ti sfactorily by such relati onsh ips. They are, however, useful in comparison of carapace/shell dimension of ani mals of known size within a locality which is a primary concern of such a stud y. An

SHORT COMMUN ICATION 335

understanding of the all ometric relationship in shell/carapace and soft body parts is therefore, essential to understand the growth of a spec ies.

The carapace length-total length suggests a linear relationship between these parameters . Carapace width-total length relationship was found to be proportionate and uniform as also indicated by linear relationship . It appears, therefore that the energy gained by these crabs from ingested food gets evenly distributed in the body building processes.

Telson length-total length relationship indicated a linear rel ati onship and found proportionately increasing, thus suggesting uniform growth pattern of body dimensions with the advancement of growth. The rel ationship between body weight and total length was linear. In general , the rate of increase of body weight was of highest magnitude than that of the total length. In the length range 300-400 mm, the weight increased sharply. The increase in the soft body parts could probably be due to increased feeding effici ency and food ava il abi lity to horseshoe crabs.

Carapace width and carapace length relationshi p also showed a proportionate increment in these parameters. The ratio of carapace width length increas ing with the increase in the age of horseshoe crab, suggests that as the anima ls grow older, they become wider and many a times carapace width exceeds carapace length . Higher increase in body weight was assoc iated in the anima l's groups with carapace length range of 100-200 mm. This could probably be due to increased soft ti ssue (e.g. ovary and testes) and also increased avai labi lity of food.

Although the data analysed during thi s study was not of a greater magnitude, it is still possible to infer that the al lometric relationship among various parts of the body is functionally important in the biology of the organism and particularly serve as a predicti ve tool for ecological investigations . The changes in body dimensions of the T.gigas population indicate that the relationship could indirectly be influenced by popu lation density, feeding efficiency, food avai lability and loca l environmental cond itions.

Authors are thankful to Director, National In stitute of Oceanography, Dona Paula , Goa for encourage­ment and to the Department of Biotechnology, Government of India, New Delhi for fin ancial sUPPOtt .

References I Hi ck man R W, All ometry and growth of thc green tipped

mussel Perna callalicu/lls in New Zealand, Mar. Bioi, 5 1 ( 1979) 3 11-327

2 Schaefer R, Trutschler K & Rumohr H. Biometric stud ies on the bivalves Astarte elliptica, A. borealis and A. Il lOntaglli in Kiel bay (Weastern Balti c sea),. Helgolallder MeereSlllllers, 39 ( 1985) 245-253.

3 Gould S J. Allometry and size in ontogeny and phylogeny, Bioi. ReI'. 4 1 (1966) 587-597.

4 Paulik G T & Gales L E, All ometric growth and Beverlon and Holt equation , TrailS Allier Fish Soc. 93 ( 1964) 369-381.

5 Chattelj i A. Vij ayakumar R & Paru lekar A H, Growth and morphomctric characteristic in the horseshoe crab. CarcilloscorpillS rotlllldicallda (Latreill e) from Canning (W st Bengal), Indi a, Pak. 1. Sci. Illc/. Res, 3 1 ( 1988) 352-353 .

6 Huxley J S & Tessir G. Terlllillology of re/a1ive gro\l·th. Na ture. 137 ( 1936) 780-78 1.