Feeding Habits of the Sand Crab Hippa Pacifica (Dana) 1

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Feeding Habits of the Sand Crab Hippa pacifica (Dana) 1

DONALD C. MATTHEWS2

BURROWING SAND CRABS, Hippa pacifica My appreciation is extended to Dr. Max (Dana), [Remipes pacifica Dana ofEdmondson, well Doty, University of Hawaii, for the phy 1925; Emerita pacifica (Dana) of Edmondson, cological determinations and to Dr. Fenner 1946, and Bonnet, 1946], were observed feed Chace, Jr., United States National Museum, ing on Portuguese man-of-war (Physalia utri for the identification of the Hawaiian sand culus Eschscholtz) at Waimanalo Beach, Oahu, crabs. during the high surf ofJune, 1954. However, METHODS AND TECHNIQUES these sand crabs completely ignored another colonial hydrozoan, Velella pacifica Esch A. Sand crabs collected during the high scholtz, present in the surfat the time. Viewing surfwere usually attached (singly or in groups these two colonial hydrozoans from above, as offive or six) to their captured Physalia. These tute man could easily differentiate between the crabs were removed and taken immediately gas filled float of Physalia and the flattened, to the laboratory where antennae, antennules elliptical float and oblique sail of Velella, but and the contents of the dissected digestive less sagacious sand crabs, viewing these colo systems were examined microscopically for nial hydrozoans from below, could not be the presence of minute plants and animals expected to discern subtle differences of color (exclusive ofbacteria), an abundance ofwhich or length of suspended zooids. This would would indicate unselective, filter feeding. be especially true during a high surf because B. Physalia and Velella disappeared from the of increased turbidity, yet, in no instance did surf as the wind subsided but, washed high I observe even a tentative approach to Velella. on the beach, stranded colonies formed an Even though H. pacifica has well developed almost unbroken line as evidence of their eyes, the likelihood that selection of Physalia previous abundance in the sea. These served and rejection of Velella could result solely as a ready source of food for subsequent ex from visual discrimination is unlikely. periments. Sand crabs likewise disappeared Whereas unselective feeding by antennal from the surf as the sea calmed, leaving the straining has been investigated in Emerita, wave-washed region of the beach bare. Upon insofar as could be ascertained, no published this region of the beach I tossed (1) pieces of work on selective feeding in H. pacifica has algae (VIva fasciata Delile, Turbinaria ornata been reported. Therefore the experiments re J. Agardh, Sargassum obtusifolium J. Agardh, ported on here were performed to investigate Polyopes clarionensis Sethchell & Gardner) bro the method by which selective feeding is ken to approximately the size of Physalia; accomplished. (2) Velella; (3) pieces of commercial shrimp discarded by fishermen; and (4) Physalia. 1 Contribution No. 69, Hawaii Marine Laboratory. Sand crabs which responded to any ofthese 2 Department of Zoology and Entomology, Uni versity of Hawaii. materials were collected and taken to the

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Feeding Habits of Hippa-MATTHEWS 383

laboratory where antennae, antennules and female; each in a separate paper cup. In no the contents of the dissected digestive sys case was the filtrate introduced until all four tems were examined microscopically for evi sand crabs had buried themselves in the sand. dence of filter feeding. Ten drops (approximately 0.5 Cc.) of filtrate C. The methods employed in B were re were simultaneously introduced into each of peated, one week later. Sand crabs collected the four paper cups and the reaction time were taken to the laboratory and the anten recorded. This was regarded as the interval nae, antennules and contents of the dissected from the instant the tenth drop struck the digestive systems examined microscopically surface of the water, to any visible response for evidence of filter feeding. by the crab. The observed times, both for D. Approximately equal portions of Ulva, control and test animals, was averaged for Velella, shrimp and Physalia were separately five separate trials. After each experiment, ground with sand and sea water, filtered paper cups and contents were discarded. These through number 40 mesh grit gauze and each blinded sand crabs and the controls were also filtrate (approximately 25 Cc.) diluted to 1,000 examined microscopically for evidence of fil cc. of sea water. Each solution was thrown on ter feeding. the thin film of a receding wave and the F. The methods employed in E were re effects, ifany, noted. For each solution, a new peated (with fresh paper cups, sand, and sea location on the beach was chosen. Again, water) except that both the compound eyes sand crabs which responded to any of the and the antennae were removed. solutions were taken to the laboratory where G. The methods employed in E were re antennae, antennules, and the contents of the peated, except that the compound eyes and dissected digestive systems were examined the antennules were removed. microscopically for evidence of filter feeding. H. An attempt was made to test crabs with E. Twenty-five male and 25 female sand compound eyes, antennae and antennules crabs were collected and their compound eyes removed. However; only 24 (of 50) sand crabs were removed leaving the major portion of survived the severe operative shock. Ofthese, the eye stalks intact. These blinded sand crabs over half remained on their backs on the were placed in a'small tank and allowed one surface of the sand, displayed little or no day to recover from operative shock, during action and were dead the following morning. which time they received additional sea water. The colored Physalia solution was introduced Physalia was again ground in sand and sea to the remaining 12 survivors in the small water, filtered through number 40 mesh grit tank and the results noted. These crabs were gauze and the filtrate (25 Cc.) diluted with dead on arrival at the laboratory and I regret approximately 1,000 cc. of sea water. The that the contents of their digestive systems diluted filtrate, colored with 0.5 grams of were not examined microscopically. Male and neutral red was placed in a clean, 1 liter bottle female controls were run as before (E,F,G). fitted with four discharge pipettes. Adjust RESULTS ment of a single Hoffman clamp permitted the simultaneous, drop by drop, delivery of A. Microscopic examination of sand crab the filtrate into four paper cups (300 cc. capac antennae and antennules collected during the ity) each of which contained beach sand, high surf revealed sand grains but no organ sufficient to cover the bottom 2 em., and 75 isms enmeshed in the setae. The dissected cc. of sea water, sufficient to produce a water digestive systems, especially the pyloric caeca, depth of 2 em. Four sand crabs were simul were blue, gorged with nematocysts and the taneously tested: (1) a control male, (2) a shredded remains of Physalia zooids. There control female, (3) a test male, and (4) a test were no microorganisms nor was there any 384 PACIFIC SCIENCE, Vol. IX, October, 1955 evidence, either from nematocysts or zooids, tems revealed nematocysts and the bluish re that Velella had been eaten. mains of Physalia but no microorganisms. B. As the wind subsided and Physalia and E. Of the 50 sand crabs collected for this Velella no longer appeared in the surf, pieces part ~f the experiment, only 18 males and 15 of algae (VIva fasciata; Turbinaria ornata, females survived the operative shock of eye Sargassum obtusifolium, Polyopes c1arionensis) , removal. Many, even when molested, re Velella and shrimp tossed into the thin film mained motionless on their backs; others of receding waves all failed to evoke any righted themselves and, colliding one with visible response from sand crabs but when another scurried around the tank. Finally, Physalia was used, sand crabs emerged. Often without any definite pattern of orientation, 4 or 5 would seize a single colony simul most submerged. Although the Physalia solu taneously and, submerging, all take part in tion was clearly discernible as it left the four pulling it below. Others, often arriving at the pipettes and entered the salt water of the pa scene too late and finding' no prey, would per cups its course became less apparent as quickly submerge as the wave receded. It was it approached the sand so that determination not difficult to collect 25 or more sand crabs of the exact moment of contact of filtrate and by using a single Physalia colony over and crabs was impossible. This condition was over again in this manner. Microscopic ex often further complicated by the spontaneous amination of the antennae and antennules emerging of crabs before the first drop of revealed sand grains but' no organisms en filtrate could possibly have reached them. The meshed in the setae. The dissected digestive addition of 0.5 grams of neutral red to the systems were, as before, gorged with nema solution made it easy to follow the course of tocysts and the blue remains of shredded the filtrate. Sand crabs in all four cups emerged Physalia, but no microorganisms were present. in great confusion during the introduction of C. As in B, when the waves of the calm sea the first six drops of filtrate. It was impossible broke on the beach, no crabs responded to for me to determine whether the control males VIva. However, when Velella, commercial and females or the blinded males and females shrimp, and Physalia were tossed separately responded more quickly. Microscopic exam· onto this region of the beach, crabs emerged. ination of test and control crabs revealed only Microscopic examination of antennae and a few sand grains enmeshed in the setae of antennules again revealed sand grains but no antennae and antennules. The digestive sys organisms enmeshed in the setae. The dis tems contained the remains of Physalia but sected digestive systems were empty save for there was no evidence of microorganisms. sand grains and what appeared to be mucus. F. Of the 50 sand crabs collected for this D. Filtered and diluted portions of-ground' experiment, only 30 survived the operative VIva, Velella and shrimp thrown on receding shock of the removal of both the eyes and waves failed to evoke any visible response antennae. These exhibited about the same from the submerged crabs. However, when activities as those described in E. Again there the Physalia filtrate was used sand crabs was much confusion. Some turned over on emerged, excitedly ran up and down the their backs, whereas others, colliding one with beach but, failing to locate the Physalia, another, hurried around and around the tank quickly submerged. Microscopic examina and finally, without special orientation, sub· tion of the antennae ana antennules of these merged. Compared to crabs having only the collected sand crabs revealed a few sand compound eyes removed the reaction of these grains but no microorganisms enmeshed in crabs to the introduced 6ltrate was dilatory the setae. Again the dissected digestive sys- and weak. Often 3 to 5 minutes elapsed after ·Eeeding Habits of Hippa-MATIHEWS 385

the tenth drop of Physalia solution was in DISCUSSION troduced before the crabs stirred. As in E the Gregarious sand crabs lie submerged in controls emerged before the tenth drop of large "beds" with only eyes and antennules Physalia solution was introduced. Microscopic projecting above the shifting, wave-washed examination of the antennules of operative sand. In Emerita, for example, exceedingly and control crabs revealed only sand grains long antennae, folded behind the maxillipeds enmeshed in the setae and the contents of the (vide Weymouth and Richardson, 1912, pl. dissected digestive systems again revealed un 1, fig. 7), are thrust out anterolaterally as altered nematocysts but no microorganisms. waves recede; thus characteristic V-shaped G. Of the 50 sand crabs collected for this ripples, which mark the "bed's" location, are experiment about the same number (30) sur formed (vide MacGinitie and MacGinitie, vived the operative shock of removal of eyes 1949, figs. 145-146, p. 302). However, Ha and antennules as survived the removal ofeyes waii's only hippid (Edmondson, 1946, p. and antennae. These exhibited about the same 265), has exceedingly short antennae (vide . activities as those described in E. There was Dana, 1855, Atlas: Crustacea, pI. 25, fig. 7a), the same confusion, the same colliding one thus the characteristic V-shaped ripples result with another, the Same running around the from water striking the antennules, rather tank and the same submerging without any than the antennae. special orientation, Again 3 to 5 niinutes Weymouth and Richardson (op. cit. p. 11) elapsed after the tenth drop of Physalia solu conclude that microorganisms found in the tion was introduced before the crabs emerged. stomach of Emerita analoga were strained No difference, either in time or in magnitude from waves and that antennae and mouth of response, was discernible between blinded parts "are remarkably adapted to this form of sand crabs with antennae removed and blinded feeding, and unfitted for any other." In sand crabs with antennules removed. As in Emerita analoga the function of sensory an E the controls emerged before the tenth drop tennal pits in feeding appears negligible as of Physalia solution was introduced. Micro highly diversified materials generally reported scopic examination ofthe antennae oftest and from their stomachs (diatoms, radiolarians, control crabs revealed. only sand grains, and foraminifera, spicules, one-celled algae and the contents of the dissected digestive sys sand) are "about what would be obtained by tems revealed only unaltered nematocysts. unselective straining ofthe water along shore" H. After the tenth drop of the colored (Weymouth and Richardson; op. cit., p. 9). Physalia solution reached the 12 sand crabs In contrast to the unselective antennal that had survived the removal of eyes, an straining of Emerita, feeding by H. pacifica, tennae and antennules 15 to 21 minutes was at least during high surf, is selective as evi required before any visible response occurred. denced by their acceptance of Physalia and As in previous experiments the controls their rejection of algae, shrimp, and Velella. emerged before the tenth drop of Physalia Moreover, this selectivity is attributed, in part solution was introduced. For the most part at least, to some sense other than sight for, the test crabs merely broke the surface of the although all experimental solutions probably sand, appeared to "tread water" momentarily, appeared similar to the crabs, the Physalia and then settled below the surface. As pre solution alone evoked visible responses. Yet, viously mentioned, these crabs were dead on so long as the eyes remained intact, selection arrival at the laboratory and their digestive based on some chemical sense (vide Mon systems were not examined. No microorgan crieff, 1946, p. 152; Ramsay, 1952, p. 85; and isms were present on the controls. Prosser et aI., 1950, p. 453) although a strong 386 PACIFIC SCIENCE, Vol. IX, October, 1955

probability, could not be held entirely re tract is empty and its preferred food scarce. sponsible for this phenomenon. Neither the presence of shredded Physalia However as blinded H. pacifica reacted to in the digestive tracts, nor the absence of Physalia solutions almost immediately, the minute organisms enmeshed in the setae of presence of some selective chemical sense is the antennae and antennules precludes the demonstrated and, in light of similar research possibility of unselective feeding by H. paci among other arthropods, attention was at fica, especially during the weeks ofcalm which once centered on antennae, antennules and often follow periods of heavy surf. Inasmuch mouth parts. as during periods of calm sand grains and The removal of the antennae of blinded mucus but no microorganisms have been H. pacifica and the subsequent increase in found in the digestive tract of H. pacifica the reaction time (3-5 minutes) indicates that the question of unselective feeding remains un chemical sense is in part located on these answered. structures. Likewise, the removal of the an tennules of blinded H. pacifica and the sub REFERENCES sequent increase in reaction time (3-5 min BONNET, DAVID D. 1946. The Portuguese utes) also indicates that the chemical sense man-of-war as a food source for the sand is in part located on these structures. Inas crab (Emerita pacifica). Science 103 (2666): much as the increased reaction time following 148-149. the removal of the antennae so nearly approx DANA, J. D. 1855. Crustacea, Atlas. United imates the increased reaction time following States Exploring Expedition ... under the the removal of the antennules, one can con command of Charles Wilkes, U.S.N. C. clude that these two structures are equally Sherman, Printer, Philadelphia. important in the degree of selectivity of the EDMONDSON, C. H. 1925. Marine zoology of chemical sense. tropical central Pacific, Crustacea. Bernice To determine whether the chemoreceptors P. Bishop Mus., But. 27: 1-26. were limited to the antennae and antennules ---1946. Reef and shore fauna of Hawaii. in blinded H. pacifica, both structures were Bernice P. Bishop Mus., Spec. Pub. 22: 1-388. removed. The increased reaction time (15-21 MACGINITIE, G. E., and N. MACGINITIE. minutes) following the removal of both an 1949. Natural history of marine animals. xii tennae and antennules indicates that probably + 473 p. McGraw-Hill Book Company, there are chemoreceptors present on the still New York. intact mouth parts, although they are much MONCRIEFF, R. W. 1946. The chemical senses. less effective than those of the antennae and vii + 424p.John Wiley &Sons, New York. antennules. PROSSER, C. LADD, et al. 1950. Comparative The chemoreceptors located on antennae animal physiology. ix + 888 p. W. B. Saun and antennules of H. pacifica are therefore, ders Co., Philadelphia. responsible for the detection of Physalia dur RAMSAY, J. A. 1952. A physiological approach ing the high surf; the sense of sight being to the lower animals. ix + 148 p. Cambridge responsible primarily for actual location and Unive~sity Press, London. subsequent seizure. This same chemical sense WEYMOUTH, F. W., and C. H. RICHARDSON. must also be responsible for detection, during 1912. Observations on the habits of the calm seas, of shrimp and Velella which H. crustacean, Emerita analoga. Smithsn. Inst., pacifica quite readily seizes once its digestive Misc. Collect. 59 (7): 1-13.