Food Habits of the Gastropod Mitra litterata lamarck:Relation to Trophic Structure of the Intertidal

Marine Bench Community in Hawaii'

ALAN J. KOHN2

ABSTRACT: The stenoglossan gastropod Mitra litterata Lamarck, common onintertid al solution benches on Oahu, preys on sipunculids occurring in burrows inreef limestone. Phascolosoma scolops Selenka and DeMan is the main component ofthe diet. Phascolosoma sp. cf. P. beronis Edmonds and Aspidosiphon elegansChamisso and Eysenhardt are also eaten. This is the first report of the nature of thefood of a member of the family Mitridae. Mit ra litterata belongs to a distincttrophic subweb from other co-occurring predatory gastropods of similar size andpopulation density-Morula gramtlata (Duclos), which preys mainly on herbivorousgastropods, and Conus spp., which eat herbivorous polychaetes.

RELATIVELY SMALL to moderate-sized predatorygastropod molluscs of the genera Conus, Momla,and Mitra, all belonging to the prosobranchsuborder Stenoglossa, are abundant and promi­nent carnivores in the biotic community of inter­tidal solution benches that comprise almostone-third of the shoreline of Oahu, Hawaii(Wentworth, 1939). Up to eight species ofConus (superfamily Toxoglossa), with meanshell lengths 20-35 mm, occur at densities of0.2-0.6/m2. Most are specialized predators ondifferent species of Eunicidae and Nereidae,families of herbivorous polychaetes (Kohn,1959).

In order to determine the position of theother predatory gastropods in the trophic struc­ture of the community and to relate this to theirtaxonomic position, their feeding activities havebeen examined. Morula granulata (Duclos)(superfamily Muricacea) is smaller than theConus spp. (mean shell length 14 mm) butmore numerous (81m2), and it eats only othermolluscs, mainly small browsing gastropods ofthe genera Ceritbium, Rissoina, Heliacus, andBittium, and the mussel H ormomya (Miller,1968).

Mitra litterata (superfamily Volutacea)(mean shell length 17 mm) is the second most

1 Contribut ion N o. 356, Hawaii Institu te of MarineBiology. Manuscript received February 25, 1970.

2 Department of Zoology, Uni versity of Washing­ton, Seattle, Washington 98105 .

abundant predatory gastropod . Two small quan­titative samples, at Kahuku and Kapoho Point,Oahu, contained 3 individuals in 65 m2 (0 .051m2) and 2 individuals in 9.3 m2 (0.22 /m2),respectively. This note reports a study of thetrophic position of M. litt erata in the solutionbench community. M. litterata probably preysexclusively on sipunculids, thus exploiting aresource not known to be utilized by any otherco-occurring gastropods. Little is known of otherpredators of sipunculids: Fischer (1925) listsanemones (Sagartia and others), crabs (Car­cinus and Eupagurus) , and fishes (plaice andcod), but he gives no further information.

Virtually no information exists on the natureof food and feeding in the Mitridae, althoughmembers of the suborder Stenoglossa are gen­erally assumed to be carnivores. Cernohorsky(1965) noted that Vexillum spp. are attractedby carrion, on which they feed. In a popularbook, Abbott (1962) mentioned worms andclams as food of mitrids but gave no furtherinformation. Risbec (1955) proposed a toxo­glossan feeding mechanism in the Mitridae, butthis is incorrect. The mitrid radula is typicallyrachiglossan and is borne at the end of an ex­tremely long pleurembolic proboscis, which mayextend the length of the shell. In Strigatella ,the subgenus to which Mitra litterata belongs,the radular teeth are laterally elongate. Cerno­horsky (1966) described and figured the radulaof M. litterata. The rachidian teeth of the sped-

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484 PACIFIC SCIENCE, Vol. 24, October 1970

T ABLE 1

G UT AND F ECES CONTEN TS OF 24 Mitra litt erataINDIVIDUALS ON I NTERTIDAL BENCHES AT

KAH UKU , K APO HO POINT, AN D

H AU ULA , OAHU

Food in Natnre

Results of examination of the alimentary tractsand feces of Mitra litterata are summarized inTabl e 1. Of the 24 specimens examined , each of16 contained the remains of one sipunculid. Allspecimens were collected during the morning orlate at night, and it is likely that they fed atnight. Furthermore, attempts to induce feedingin the laboratory succeeded only at night. The

In no case was the "accessory proboscis" visi­ble during the feeding process. This muscular,vermiform structure, considered by Risbec(1955) to release a venom, orig inates in theproboscis behind the radula and was observedto extend and retract through the mouth duringdissection of living specimens. Its function re­mains unknown.

The prob oscis and radula appear to fun ctionexclusively as conveyors, capturing the sipun­culid and moving it, intact and undamaged,into the mid-esophagus and stomach. In oneinstance, Mrs. S. Miller collected a Mitra lit­terata in the act of swallowing a Phascolosomascolops Selenka and DeMan . The pred ator wasdissected five hours later , revealing the preyin the stomach, markedly distending the stomachwall. The trunk of the sipunculid was mostlyintact, the introvert mostly digested ; all damageappeared to have been caused by chemical diges­tion. The sipunculid was removed from thestomach and remained alive until it was fixedtwo hours later.

9

3

3

4

16

No.M . litterataFOOD IT EM F OUND

SipunculidsPhascolosoma scolops

(Selenka and D eMan)Phascolosoma sp. d .

P. beronis EdmondsAspidosiph on elegans

(C hamisso and Eysenhardt )

T otal sipunculids

Minute, unidentifiable,soft fecal material

mens I examined had four, and the lateral teethseven or eight conical denticles of nearly uni­form height.

OB SERVATIONS

Feeding

The feeding process of M . litterata has beenobserved in the laboratory, specimens have beenobserved feeding in nature, and the food ofothers in nature has been identified from theexamination of alimentary tracts of pr eservedspecimens and of fecal material recovered fromspecimens kept alive in the laboratory aftercollection. In every case, the food has been asipunculid worm of species that characteristicallyoccur in burrows in calcareous rock. The papillaeand hooks of sipunculids pass undigestedthrough the gut of Mitra, facilitating identifica­tion of the pr ey. Species identifications weremade primarily from the monograph of Selenka,DeMan, and Bulow (1 883).

Observations on feeding in the laboratorywere made at night, when M . litterata is nor­mally active and when the intro verts of burrow­ing sipunculids are extended. When placed ona piece of reef limestone containing one or morespecimens of Aspidosiphon elegans Chamissoand Eysenhardt, M ytra litterata extends the si­phon along the substrate , waving it from side toside. When the siphon contacts a sipunculid, thehead is extended and also moved from side toside until one of the extended tentacles contactsthe worm . Usually the proboscis extends onlyafter both siphon and tentacle have touched thesipunculid. The padlike proboscis lip, bearingprojecting, nonmotile, and evidently sensorycilia, then contacts the sipunculid and movesover its exposed body surface. The proboscisbegins to engulf the sipuncul id when the lipsare affixed to the introvert, or to the anteriorend of the trunk if the introvert is withdrawn.The rate of ingestion probably varies with thesize of the sipunculid. In laboratory observa­tions, M. litterata did not succeed in extractingA spidosipbon elegans from its burrow; it re­quired several minutes to ingest a small speci­men of A . elegans removed from its burrow andplaced in a dish. The distal end of the proboscisis not expansible and probably limits the diam­eter of prey that can be eaten.

Food H abits of M itra litterata-KoHN

rate of passage of food through the gut is notknown, but, if the mean number of food itemsper gut is taken as the daily feeding rate, asseems reasonable from studies of Conus of sim­ilar size (Kohn, 1959) , this rate is about 0.7sipunculid per M itra litt erata per day.

The sipunculid fauna of Hawai i is incom­pletely known . Edmonds ( in prep aration) listsfour species of Pbascolosoma, but not P. scolops,the commonest prey of M itra litt erata on inter­tidal benches. Except for slight differences inthe platelets of the trunk papillae, the specimensstudied agree in all details with the originaldescrip tion of Phascolosoma scolops. However,the identification should be regarded as tenta­tive because the taxonomy of this group ofsipunculids is not settled . Th e genus A spid osi­ph on has been reported from Hawaii by Ed­mondson (1946) and Edmonds (in prepara­tion) , but neither author identi fied the species.The commonest sipunculids in areas whereMitra lit terata occurs are Phascolosoma scolopsand A spidosiphon elegans, found in burrows ,probably of their own construct ion, in the reeflimestone . The species listed as Phascolosomasp. d. P. beronis Edmonds (Ta ble 1) differsfrom the original description of that species onlyin the pigmentation of the tentacles, the nephri ­dium, and the spots on the trunk. A fourthsipunculid, Phascolosoma pelma Selenka andDeMan, previously reported from Hawaii byEdmondson (1946) and Edmonds (in prepara­tion) , was found in the same habitat but wasnot eaten by any of the M itra litterata examined.Phascolosoma spp . share the same microhabitatas A spidosiphon but may be foun d also in sandbound by algae and animals on the surface ofthe reef rock, and in crevices. Several species ofsipunculids of these genera frequently co-occurin adjacent burrows in the same rock (Rice,1969) .

Four 100-625-cm2 samples from the solut ionbench at Kahuku (Sta. 5 of Kohn, 1959) con­tained 6 to 26 sipunculids each and gave a meandensity of 470j m 2• Phascolosoma scolops wasthe most abundant species. A substrate sampleof 270 em- from the solution bench at KapohoPoint contained 20 sipunculids, of which 19were Aspid osipb on elegans, suggesting a densityof 740 jm2. A crude estimate of the feeding rateof the M itra litt erata population, based on the

485

minimal data presented above, is about 0.08sipunculid eaten per square meter per day. M.litterata would thus consume about 5 percent ofthe standing crop of sipunculids per year, anestimated rate quite similar to the feeding rateof Conus on polychaetes at Kahuku (Kohn,1959) .

DISCUSSION

M itra litterata preys on Phascolosoma andAspidosipbon on intertidal solution benches, ap­parently extracting the sipunculids from theirburrows with the long proboscis and radula andconveying the prey, intact and alive, to thecapacious stomach. It thus exploits a food re­source that is not utilized at all by M omlagramtlata and Conus spp ., co-occurring preda­tory stenoglossan gastropods of roughly similarsize and population density. The diets of thethree genera, belonging to three different super ­families, do not overlap , and the different spe­cies are the primary carnivores in distinct foodsubwebs.

Paine ( 1963) described in detail a subweb ofpredatory gastropods on an intertidal sand barin Florida. On Hawaiian solution benches, how­ever, there is no equivalent of the large fasci­olariid gastropods foun d there, that prey gener­ally on smaller carnivorous gastropo ds. Thisis pr obably because of the hard, rather smoothtopography of solution benches and their ex­posure to desiccation at low tide and to violentwave action at high tide (Kohn, 1967) . Thehabitat does not provide shelter suitable forlarge, motile invertebrates, and selection appea rsto favor small body size (Kohn, 1968 ) . Th eharshness of the environment may thus effec­tively preclude a secondary carnivore level inthe trophi c structure of the benthic community .

ACKNOW LEDGMENTS

This study was supported by National ScienceFoundation grant GB-6134, and in part by GB­5942X. I thank participants in the SummerTraining Program, Hawaii Institute of MarineBiology, for aid with the field work; Dr. MaryE. Rice for aid in ident ifying the sipunculids ;Dr. Vera Fretter and Dr. R. T . Paine for dis­cussion and criticism of the manuscript ; and

486

Dr. 1. G. Eldredge for providing the manu­script of the second edition of Reef and ShoreFauna of Hawaii.

LITERATURE CITED

ABBOTT, R. T . 1962 . Sea shells of the world.New York, Golden Press.

CERNOHORSKY, W. O. 1965. The Mitridae ofFiji. Veliger, vol. 8, pp. 45- 61.

CERNOHORSKY, W. O. 1966. Radulae of fourspecies of Mi trid ae (Mollusca : Gastropoda).Journal of Conchology, vol. 26, pp. 15-18.

EDMONDS, S. J. In preparation. Phylum Sipun­cula and Phylum Echiura. In: 1. G. Eldredge,ed., Reef and shore faun a of Hawaii. 2nd ed.Honolulu, B. P. Bishop Museum.

EDMONDSON, C. H . 1946. Reef and shore faunaof Hawaii. Honolulu, B. P. Bishop Museum.

FISCHER, W . 1925. Echiuridae, Sipunculidae,Priapulidae. Tierwelt der N ord- und Ostsee,Lief. I, Teil VId.

KOHN, A. J. 1959. The ecology of Conus inHawaii. Ecological Monographs, vol. 29,pp. 47- 90.

- - - 1967. Environmental compl exity andspecies diversity in the gastropod genus Conus

PACIFIC SCIENCE, Vol. 24, October 1970

on Indo-West Pacific reef platforms. Ameri­can N aturalist, vol. 101, pp. 251-259.

--- 1968. Microhabitats, abundance andfood of Conus ( Gastropoda) on atoll reefsin the Maldive and Chagos islands. Ecology,vol. 49, pp . 1046-1062.

MILLER, A. C. 1968 . The ecology of M orulauva and M orula granulata on Checker Reef.Unpublished report, Hawaii Institute ofMarine Biology.

PAINE, R. T. 1963. Trophic relationships of 8sympatric predatory gastropods. Ecology, vol.44, pp. 63-73.

RICE, M. E. 1969 . Possible boring structures ofsipunculids. American Zool ogist, vol. 9, PI'803-81 2.

RISBEC, J. 1955. Considerations sur l'anatomiecomparee et la classification des gasteropodesprosobranches. Journal de Conchyliologie, vol.95, pp. 45-82 .

SELENKA, E., G. D EMAN, and C. BULOW.1883-1884. Die Sipunculider. In: C. Semper,ed., Reisen im Archipel der Ph ilippinen,Teil II, Bd. 4, Apt. 1, pp. 1-131.

WENTWORTH, C. K. 1939. Marine bench-form­ing processes. II. Solution benching. Journalof Geomorphology, vol. 2, pp. 3-25.