Endosphaerium funiculatum, gen. nov., sp. nov., a new predaceous fungus mutualistic in the gills of freshwater pelecypods

JOURNAL

OF INVERTEBRATE

PATHOLOGY

30, 41%421(1977)

Endosphaerium funiculatum, gen. nov., sp. nov., a New Predaceous Fungus Mutualistic in the Gills of Freshwater Pelecypods PETER N. D’ELISCU Department

of Biology,

University

of Santa

Clara,

Santa

Clara,

California

95053

Received February 7, 1977 A new predaceous Endosphaerium

fungus mutualistic in the gills of freshwater gen. nov., sp. nov

pelecypods

is described:

funiculatum,

tle cavity community. This involved the feeding of 14C-labeled Chlorella to the clam and observing subsequent labeling of symbionts by autoradiographic methods. In order to clarify further the physical aspects of the symbiosis, fungi were surgically removed from the gills of several live clams, and signs of tissue repair were investigated. Windowing of the shell and mantle of large clams (12-16 mm) allowed experimental observation of the capture and haustorial invasion of vinegar nematodes, Turbatrix sp., and spinose rotifers, Filinia sp. Microdissections and thin sections of late incubation juveniles ofMusculium were examined for signs of Endosphaerium or its zoospores.

INTRODUCTION

During studies of the microenvironment and symbionts of the mantle cavity of small freshwater bivalves, a complex community was observed. The flow of oxygen and food in the water current created by the clams supported many organisms living within the mantle cavity. This generally stable environment supported various rotifers, oligochaetes , hydrophilid beetle larvae, amoebae, ciliates, tardigrades, nematodes, and some unique fungi. One sphaeriid clam, Musculium transversum, lived in apparent mutualistic symbiosis with a new gill surface phycomycete, which is predaceous on other components of the microcommunity. The unique habitat and morphology of this predator led to investigations of its host interactions and subsequent description as Endosphaerium funiculatum. METHODS

Preliminary investigations of Endosphaerium suggested a close physiological association with its molluscan host. Host clams used in further studies were collected in several localities in California, Arizona, and Texas. Thin sectioning and differential staining of host gill tissues were used to establish the morphological characteristics and physical host-symbiont interactions of the fungus. Physiological interactions with the host were established during routine radioisotope applications to the entire man-

RESULTS

AND DISCUSSION

Endosphaerium occupied the lamellar surfaces of the enlarged internal gills of M. transversum. A knot-like mass of hyphae at the base of the thallus served as an anchor wedged in between the gill lamellae, while a rope-like hyphal cord extended out into the mantle cavity (Fig. 1). Thin sectioning and differential staining showed no evidence of host tissue invasion by the fungus. When 14C-labeled Chlorella were fed to the host clam, no subsequent radioactivity was detected in the fungus by autoradiographic techniques. Other nonfungal symbionts did acquire radioactivity possibly through host tissue disruption, invasion, or coprophagous activity. 418

Copyright 0 1977 by Academic Press. Inc. All rights of reproduction in any form reserved.

ISSN 0022-2011

419

FIG.

1. Endosphaeriumfunicultum

No. 71819 B.P.I. from the gills of Musculium

In addition, the host did not show any signs of repair or tissue prol~eration in the gill area occupied by the fungus when the thallus was carefully teased from the anchorage. The fungal thallus was reduced to a tightly coiled hyphal sphere within the reduced gill spaces in dormant aestivating clams. During aestivation, the host may provide the resting fungal stage with the necessary moisture and temperature requirements. Twelve to thirty percent of adult aestivating clams from various Arizona locations supported living fungal hyphae. Aestivating sphaeriids retain moisture through copious secretion of mucus. Since many members of the Pythiaceae and other fungal taxa are amphibious in terms of spore or hyphal dispersal, the alternation of aquatic and semiaquatic life did not appear to be a drastic adaptive problem (Sparrow, 1960). These observa-

transversum; 850 pm.

tions suggested that the fungus was not a parasite of ~~~c~~~~~, but a symbiont at another trophic level. Endosphaerium normally preyed upon rotifers, capturing them on adhesive hyphae. Observations of nematode and rotifer capture through windows in the shell and mantle demonstrated that, after prey were passively entangled in the adhesive hyphae, they were invaded by rapid internal growth of trophic haustoria. Some free-living predatory fungi also capture rotifers and other small animals by trapping them in web-like nets, snaring them in hydraulic ringlets, or sticking them on adhesive pegs (Duddington, 1962; Prowse, 1954; Sparrow, 1960). Specimens from field collections demonstrated that Endasphaerium also preys occasionally on nematodes and trematodes. In particular, the fungus preys on one

420

PETER N. D’ELISCU

nematode, Seinura sp., which may parasitically disrupt gametogenesis in the normally hermaphroditic clam. In Musculium, Seinura invades gonadal tissues and also disrupts genital ducts. Stasis of genital ducts is a common observation in Seinura invasions of the clam. While the presence of the predatory fungus probably does not establish complete protection, it does confer an important advantage to its molluscan host in resisting this nematode parasite. Conadal tissue removed from fungus-bearing clams appeared normal for the respective seasonal conditions of the hosts. This symbiotic association is apparently mutualistic, with Endosphaerium benefiting through physical and indirect physiological support and Musculium receiving some limited protection from a potentially destructive parasite. Since some members of the molluscan family Sphaeriidae serve as intermediate hosts for avian and mammalian trematodes and leptospiras, this bivalve-fungus mutualism may be important in some vertebrate and associated parasite population levels (Kingscote, 1971; McDonald, 1969; Yamaguti, 1958). The reproductive biology of both host and fungus further indicates the complexity and interdependence of the relationship. The Sphaeriidae retain their embryos and early juveniles in brood pouches within the gills. The incubation period is seasonal and often temperature dependent (D’Eliscu, 1972). Although sexual reproduction in Endosphaerium has not yet been observed, the asexual reproductive development of the fungus appeared to be coordinated with the seasonal reproduction of the host clam. Microdissection and thin sectioning demonstrated that 33% of 100 late incubation juveniles of Arizona-collected Musculium examined contained the cyst-like zoospores of Endosphaerium. Rarely, incubation juveniles from clams not supporting any fungi also contained zoospores. This suggested that the transfer of the fungus was both vertical (successive gen-

erations) and horizontal (adult to adult) within the host population. Asexual reproductive stages of the fungus, producing zoospores from the distally septate sporangia, were found only in actively reproductive clams. Unlike some rotifer-catching fungi such as Zoophagus and Sommerstoffia, Endosphaerium produces sporangia from the symbiotic thallus, rather than from modified filamentous haustoria from within the prey (Duddington, 1968; Sparrow, 1958). In addition, reproductively active fungi were usually reoriented in the gill, the hyphal cord and its sporangia extending into the spaces of the host’s brood pouch. Possibly there is some reproductive initiation or release mechanism from host to fungus. The coordination of reproductive periods and transfer of zoospores to the developing juveniles of the host insure a continuing mutualistic symbiosis in successive generations. More behavioral observations and some chemical analyses could perhaps better characterize the biology of this novel fungus, its unique habitat, and the mechanisms of interaction with its molluscan host. Endosphaerium funiculatum D’Eliscu (Fig. 1). Thallus volutus, predatorius, nonramosus, et funiculatus, ad summa externa branchiae a&us, minimus (200950 pm). Musculium transversum partus et adultus incolens. Zoosporae renoideae, salebrosus, solitus immobilis, 7-22 pm. Retinaculum suo more nodosus et ancorus.

Unbranched, rope-like thallus attached to the branchial lamellae of Sphaeriidae (Bivalvia); thallus 200-950 pm, varying with host mantle cavity volume; holdfast knotlike; hyphae with contiguous or alternating bands of adhesive surface; predaceous on rotifers and nematodes; finely branched haustoria intracellular and intercellular in prey; eucarpic with zoosporangia filamentous; seasonal development coordinated with host reproduction: encapsulated zoospores reniform (7-22 pm), laterally biflagellate, but apparently nonmotile and

ENDOSPHAERIUM

FUNICULATUM

deposited within the mantle cavity of host juveniles; mutualistic symbiont with Musculium transversum Say; Pythiaceae: Peronosporales. Holotype: Slide No. 71819 B.P.I., U.S. National Collection, Agricultural Research Center, Maryland (Slide X-9-71a of author). Prepared from Musculium transversum collected October 9, 1971, from a small naturally formed oxbow pond (Noon Ranch) near Arivaca, Arizona (T.21s-R.9e, Section 28, NE Quarter, State of Arizona). Collections: Type locality and adjacent ponds; Allen Lake, Arizona; White Water Canyon, California; East and West Yegua Creeks, near Lincoln, Texas. ACKNOWLEDGMENT I wish to thank Mr. William N. Palmer enthusiastic assistance in Texas field localities.

for

gen.

nav.,

sp. nov.

421

REFERENCES P. N. 1972. Are sphaeriids placental animals? Proc. West. Sot. Malacol. 5, 26-27. DUDDINGTON, C. L. 1962. Predaceous fungi and the control of eelworms. Viewpoints Biol. 1, 151-200. DUDDINGTON, C. L. 1968. Fungal parasites of invertebrates: Predaceous fungi. In “The Fungi III” (G. C. Ainsworth and A. S. Sussman, eds.), pp. 239-251. Academic Press, New York. KINGSCOTE, B. F. 1971. Leptospires in finger-nail clams. J. Wildl. Dis., 7, 178-185. MCDONALD, M. E. 1969. Catalogue of helminths reported from water fowl (Anatidae). Bur. Sport Fish. Wildl. Spec. Rep., 126, l-692. PROWSE, G. A. 1954. SommerstofJia spinosa and Zoophagus insidians predaceous on rotifers, and Rozellopsis inflata , the endoparasite of Zoophagus . Brif. Mycol. Sot. Trans., 37, 134-150. SPARROW, F. K. 1960. “Aquatic Phycomycetes,” 2nd ed. University of Michigan Press. YAMAGUTI, S. 1958. “Systema Helminthum: Trematoda I. Interscience, New York. D’ELISCU,