j STOMACH CONTENTS OF THE BERING SEA KING CRAB

by

Patsy A. McLaughlin and James F. HebardBiological Laboratory, Bureau of Commercial Fisheries

Seattle, Washington

5

ABSTRACT

This preliminary study of the stomach contents of the BeringSea king crabs indicates that the major food items of these crabsare molluscs and echinoderms. The frequent occurrence ofechinoderms in the stomachs in large amounts is in contrast tothe findings of Russian workers who feel that echinoderms playonlya very minor dietary role in the king crabs of the Kamchatkaregion. Differences in food preferences apparently do not existbetween the sexes.

This report is the result of the analysis of king crabstomachs collected during the summer of 1957 insoutheastern Bering Sea by biologists of the KingCrab Investigation of the United States Fish and Wild­lifeService. A study of the food habits of the crabisof considerable importance because food availabilityand utilization may play important roles in the dis­tribution, migration and molting patterns of thecrabs.

Very few reports on the food habits of the kingcrab, Paralithodes camtschatica (Tilesius), are found inthe literature. Among the available papers, the workof Feniuk (1945) is the most recent and perhaps themostcomplete. For the most part, his methods wereused in this analysis. Quantitative studies on thespeciesof food of the crab are made exceedingly dif­ficult because, in contrast to fish which for the mostpart swallow food whole, the crab breaks up his foodwith the chelae before ingesting it; hence reconstruc­tion of individual organisms is usually impossible.Methods which are .1sed in quantitative food studiesofvertebrates which masti.-;~tetheir food are not adapt­able to the food studies of u<! bs, since the quantityof.crab stomach contents is usualiy small. As yet, noSUitablemethod of quantitative analysis of thesestomachcontents has been devised.

Because of this lack of an applicable method, thesmallnessof samples from anyone area and the limitedscope of available related data, this report intends----

Received for publication August 18, 1959. Original, English..Source: ContributionfromtheBureauof CommercialFish-

lcrlCSU S F'

, '. Ish and Wildlife Service, SSR-F 291. INPFC Docu-ment No. 280.

BUllet' 5 I .In , nt. North Pac. F,sh. Comm., August, 1961.

I

primarily to indicate the types of organisms whichcontribute to the diet of crabs. However, it will showquantitatively the fullness of the stomachs examined.More intensive studies, designed to compensate forthe inadequacies noted here, will be pursued in 1958.

The crabs from which stomach samples were takenwere from catches made by the chartered trawlerMitkrif during the months of June and July, 1957(Figure 1). Of the 329 stomachs obtained, the sexeswere almost equally represented-160 females and169 males. Sizes ranged from 63 mm. to 193 mm. incarapace length. All types of presently recognizedshell conditions were also represented, as the samplesvaried from newly molted, soft-shelled crabs to thosehaving barnacle:encrusted, very old shells. Themajority of the crabs, however, possessed hard newshells, indicating a molt within the season.

Anatomically, the stomach of the king crab is dividedinto two sections, referred to as the cardiac and pyloricsections (Marukawa, 1933). This conventionalnomenclature will be used in this report. However,as pointed out by Pearson (1908), these terms maytechnically be misnomers when applied to the Mala­costraca, since the cardiac section of the stomach is

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PACIFIC OCEAN

162"' .~

FIGURE I. Stations at which stomach samples were takenduring 1957.

6 BULLETIN 5 - NORTH PACIFIC COMMISSION

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"-1

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G.R.

~ !\ II, "-

A,~. ~A '/

A

i.~ {

~. I'- A. L

i>

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" c '~y

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C.RY

,B.R .

vy y

B.R

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FIGURE2. Carapace of king crab (after Marukawa).G.R., Gastric region;C.R., Cardiac region;B.R., Branchial region.

to maximum capacity in both sections, the rati. a ofthe amount of food m each of the three parts is 6:2.\respectively (Feniuk, 1945). Each percentage".fullness was assi.gneda :o~e~ index (Table 1). Ea~part was consIdered mdlvldually and :~'" fullnessestimated visually.

In order to eliminate variations due to the different 1sizes of the parts of the stomach, a common indexof .Kx was obtained for each part by multiplying its inde i

of fullness (kx) by its factor taken from the ratio o~the parts to the whole. The total K factor representsthe sum of the Kx factors, i.e., K =6kl +2k2 (+ 1k

not used in these samples). With the entire cardia~stomach filled to maximum capacity, the maximumX obtainable in our samples is 48 units. The per­centage of fullness of the stomach, both parts con-'sidered, is thus calculated from this K factor. Thequantity of food present in the stomachs varied con­siderably; however, only 7.6 per cent of the stomachswere found to be completely empty. Table 2 showsthe distribution of stomach fullness.

TABLE 2. Distribution of stomach fullness in 329 crabs.

Stomach fullness

TABLE 1. Code for relative fullness of stomachs.

farthest from the heart and lies under the region of thecarapace defined as the gastric region (Figure 2) byMarukawa (1933). Figure 3 shows the position ofthe stomach with relation to the carapace and therest of the body.

The cardiac section of the stomach was dissectedout of each crab and preserved in 4 per cent formalinin an individual cloth bag. The pyloric section andthe intestine were not used in this study. Anatomi­cally, the cardiac section of the stomach is divided intotwo parts. The anterior part, which comprises thelargest area of the entire stomach, is guarded at itsentrance by a hairy filter and at its exit by the teethof the gastric mill. The smaller part of the cardiacstomach lies between the teeth of the gastric mill andthe pyloric valve which is the entrance to the pyloricsection of the stomach. When the stomach is filled

Percentage offullness

EmptyTrace of food

1-2021-4041-6061-8081-100

Index (kx)

Number of crabsUpper limit

Equivalent(in per cent)of Kpercentage

0

Empty 7.68

Trace 14.416

1-20 42.7.24

21-40 18.632

41-60 10.040

61-80 4.948

81-100 2.4

The contents of each stomach were examined micro­

scopically. Although the components occasionallywere completely unidentifiable masses of torn tissues;for the most part, relatively large pieces of recognizablematerials were present in the anterior part of the cardi­ac stomach: for example, pelecypod shell fragments,legs and dactyls of crabs, gastropod shell fragmentsand opercula and legs of brittle stars. Occasionally,complete individuals were found-among these, smallbrittle stars and amphipods were the most common.Remains in the posterior part of the cardiac stomachwere often still recognizable, but were more finelybroken from the action of the teeth of the gastric mill.Because no bottom fauna from the sampling area wascollected or available for species identification andcomparison with organisms present in the stomachs,these organisms, in most cases, were identified onlyto the class or order level. We express our apprecia­tion to Dr. Paul Illg of the Department of Zoology of

.

Cardiar.; region\

Post. card j fiG

FIGURE 3. Left side of sagittal section of a king crab .

c

8 BULLETIN 5-NORTH PACIFIC COMMISSION

SUMMARY

LITERATURE CITED

Chi-square tests show no statistically significantdifferences in the frequency of food group Occurrencebetween the sexes, with the exception of the Coelen­terata, and this exception cannot be considered reliablebecause of the smallness of the sample. Limitationsof this preliminary study prohibit further comparisonof food preferences, if any, exhibited by newly moltedcrabs and crabs approaching the molt, or by varioussize groups of crabs.

Our study, like Feniuk's, shows that the Sexexhibit no food preferences. Table 3 gives the p eser.

centages of frequency of occurrence of food groupsfor I

males and females. \TABLE 3. Frequency of occurrence of food groups in 169

male and 160 female crabs. I-==Percentage of frequency of Ioccurrence

(

I.

1

-Female-­60.6

35.6

19.416.9

16.315.6

5.6

76.9

48.526.0

18.312.411.80.5

MaleFood group

MolluscaEchinodermata

Decapod crustaceaPolychaetaJ~ h;aeOther .:rustaceaCoelenteratA

It appears that the major food items of the south­eastern Bering Sea king crab, of the sizes studied, aremolluscs and echinoderms. On the basis of sex, thesecrabs do not seem to show a differential preference forparticular food items. This preliminary study in­dicates that a number of differences in feeding habitsexist between the southeastern Bering Sea king craband the king crab of the Kamchatka region. Morecomplete studies are planned to answer the manyremaining questions pertaining to the food habits andneeds of the king crab.

FENIUK,V. F. 1945. Analysis of stomach contents of Kam­chatka crab. (in Russian.) Tikhookeanskii nauduw-issledovatel'skii

institut rybrwgo khoziaistva i okeanografii (Vladivostok), Izvestia,19: 71-78.

HYMAN,LIBBtEH. 1955. The invertebrates: Echinodermata.Vol. IV, McGraw-Hili Book Co., 763 pp.

MARUKAWA,H. 1933. Biological and fishery research onjapanese king crab Paralithodes camtschatica (Tilesius). Journal

of the Imperial Fisheries Experimental Station (Tokyo), 4 (Paper37). 152 pp.

PEARSON,JOSEPH. 1908. L.M.B.C. Memoirs on typical Britishmarine plants and animals. XVI. Cancer. Liverpool Marini

Biology CommIttee. London, Williams and Norgate. 209 pp.

Among the food groups represented, molluscs,present in 69.0 per cent of the stomachs examined, arethe major food item; pelecypod remains dominatingthose of gastropods. Echinoderms rank second, ap­pearing in 42.2 per cent of the crabs. Representativesof the phylum include asteroids, ophiuroids, and echi­noids of the sub-order Clypeastrina. In contrast toreports in the available literature, no regular echinoids(Hyman, 1955) were present. This lack may be onlycoincidence, as regular echinoids are most commonlyfound in rocky areas, and the majority of our sampleswas not taken from these areas. The third mostabundant food group is the decapod crustaceans,which were present in 22.8 per cent of the stomachs,primarily represented by the sub-order Reptantia.Other groups in the order of their abundance includepolychaete worms (17.5 per cent), algae (14.3 percent), crustaceans, excluding decapods, (16.0 percent), and coelenterates (3.0 per cent). Foraminifera,nematode worms, tunicates, echiuroids, fish and othergroups were present occasionally, but not often enoughto be considered, on the basis of present information,foods of any relative importance. Possibly these or­ganisms, in some instances, had been carried into thecrab stomachs as food of other food organisms.

Contrary to the findings of Feniuk (1945), we foundrelatively large amounts of algae present in many ofthe stomachs. At least with regard to king crabs ofsoutheastern Bering Sea, this causes us to question hisproposition that algae occur only as a food of otherorganisms. Sand, which appeared in practically allof his samples, was present only occasionally in oursamples, thus leading us to believe that sand may notbe a chosen item, but rather one of coincidence. Also,Feniuk's report indicates that the echinoderms playonly a minor role in the diet of the king crab of theKamchatka region, which is in contrast to our dataon the southeastern Bering Sea crabs.

In addition to the estimation of percentage of full­ness, tabulations were made of the irequency of oc­currence of the major food groups. In the deter­mination of this frequency, each food group presentin a stomach, even when represented by sever?.!organisms, was considered to have had an occurrenceof one.

the University of Washington for his verifications ofmany of our identifications of the food organisms.Remains of organisms which apparently, because ofdecomposition rather than digestive action, had beendead prl0;' to being ingested by the crabs, as well asthe remains of .,~jmals which were alive prior tocapture, were found ill :he stomachs.