Pacific Science (1974), Vol. 28, No.4, p. 361-373 Printed in Great Britain Biology of the Polyclad Prosthiostomum (Prosthiostomum) sp., a New Coral Parasite from Hawaii I PAUL L. JOKIEL2 AND SIDNEY J. TOWNSLEy3 ABSTRACT: Prosthiostomum (Prosthiostomum) sp., a species of polyclad flatworm yet to be described, is an obligate ectoparasitic symbiont of the hermatypic coral Montipora. Field and laboratory studies have demonstrated an intimate parasite/host association involving the utilization of host corals as food and sub­ strate by the parasite. Development of larvae is within the immediate host en­ vironment; consequently, infections are produced through direct infection. Various aspects of the biology, such as the developmental history, feeding habits, and parasite/host response to thermal environment, are reported. It is concluded that all aspects ofthe life history ofthis species show adaptations toward host specificity. This represents a rare example oftrue coral parasitism since most animals known to feed on coral tissues are considered to be facultative predators. The optimal thermal environment for the parasite appears to coincide with that of the coral host, a phenomenon which may tend to produce a seasonally stable parasite/host inter­ action. The parasite appears to become a serious coral pest only in disrupted systems such as artificial laboratory situations or in the polluted sections of Kaneohe Bay, Oahu. UNTIL THE LAST DECADE the Scleractinia and identified by Jean Poulter as Prosthiostomum their relatives were believed to be nearly im­ (Prosthiostomum) sp. This discovery led us to mune to predation and parasitism (Wells 1957). invesdgate the host specificity, the method However, records ofanimals known to feed on and frequency of infection, and various other living coral tissues and coral mucus have been aspects of its biology. The results of con­ increasing. It is now recognized that gastro­ trolled laboratory experiments with various pods, polychaetes, crustaceans, and asteroids, species ofcorals and otherpotentialfood sources as well as bony and cartilaginous fishes, all and substrates indicate that this polyclad is an possess representatives that regularly utilize obligate ectoparasitic symbiont on its host corals as food sources (reviewed by Robertson Montipora. To our knowledge this is the first 1970). In laboratory experiments on the growth report ofa polyclad utilizing coelenterate tissue and metabolism on a variety of Hawaiian as a food source. corals we found that specimens of the per­ In this paper we are using the currently ac­ forate hermatypic coral described by Vaughan cepted definitions for symbiosis and parasitism (1907) as Montipora verrucosa (Lamarck) were as employed by Henry (1966) and Cheng (1964, readily destroyed by a polyclad that has been 1970). The original definition of symbiosis em­ ployed by DeBary (1879) as the "living to­ I Hawaii Institute of Marine Biology contribution gether" of two heterospecific organisms, with no. 442. Coral Reef Management and Research contri­ no type of mutual or unilateral dependency bution no. 13. This work partially supported by Environ­ mental Protection Agency grants 18050 DDN and being implied, is retained. Parasitism is accepted R800906. Manuscript received 27 September 1973. as one category of symbiosis and is defined as 2 University of Hawaii: Hawaii Institute of Marine an intimate and obligatory relationship between Biology, P.O. Box 1346, Kaneohe, Hawaii 96744; and two heterospecific organisms. In these associa­ Department of Oceanography, Honolulu, Hawaii tions the parasite (usually the smaller ofthe two 96822. 3 University of Hawaii, Department of Zoology, partners) is metabolically dependent on the Honolulu, Hawaii 96822. host. The relationship may be permanent or 361 362 PACIFIC SCIENCE, Volume 28, October 1974 which feed on coral mucus have been described as obligate ectoparasites of the coral Pocillopora (Knudsen 1967). Outright host specificity among known coral parasites and predators is uncommon. Most of the animals known to feed on coral tissue and coral mucus have been classified as facultative predators (Robertson 1970). The obligate habits observed in Prosthiostomum (P.) sp. might be far more widespread than has been suspected, because there are already several indications that other members of this group may feed on coral tissue. Poulter (in press) lists four species of polyclads known to have been collected from corals. Various undetermined species ofplanarians have been found living on Montipora, Lobophyllia, Srylophora, and Hydro­ plana (Kawaguti 1944). Occasionally these animals were observed to form thick assem­ blages that covered the coral, causing it to become whitish as a result. The planarians re­ portedly contained zooxanthellae in fairly large quantities. Although he did not so state in his paper, Kawaguti (personal communication) believes that some of these species were feeding on the coral tissue, and he is presently investigating an acoel that appears to feed on Acropora tissue. Our able co-workers Eric B. Guinther and Gerald S. Key provided valuable advice on various aspects of this paper. We are deeply indebted to J. L. Poulter for her identification and description ofthe animal and for her advice throughout the study. Eric B. Guinther drew the adult flatworm shown as Fig. 1. FIG. 1. Drawing of Prosthiostomum (Prosthiostomum) sp. from living adult animal measuring 12 mm in length. GENERAL DESCRIPTION AND BEHAVIOR temporary. Finally, in parasitism the association The material on which Poulter (in press) is obligatory because the parasite cannot sur­ based her description of Prosthiostomum (Pros­ vive if it is prevented from making contact thiostomum) montiporae was obtained from our with its host. In this paper we show that experiments. This polyclad is a typical pros­ Prosthiostomum (Prosthiostomum) sp. is parasitic thiostomid(see Fig. 1). The body is small, much on its host, and meets the established criteria longer than broad. Field specimens were of the classic definition as well as being generally from 4 to 8 mm in length, but repre­ consistent with contemporary usage. For sentatives up to 12 mm long have been col­ example, Bosch (1965) used the term parasitic lected. Animals grown under optimal condi­ to describe wentletraps (Epitonium ulu) that feed tions in the laboratory are frequently from 12 and reproduce on the coral Fungia scutaria. Like­ to 14 mm long, and rarely may reach 18 mm. wise the xanthid crabs Trapezia and Tetralia Living specimens are translucent, but assume ItliJ•.JI£Wams: d Mii 'J::illWW ; # @##It"#Bk.g# OlttM Biology of a New Coral Parasite from Hawaii-]OKIEL AND TOWNSLEY 363 .,.• • ••.t I­..­• • I I .-. •'. .t· I • ."•..• .-..,• .... •',.1. ".,: • ......' .'. 4mm 6mm 12mm FIG. 2. Cerebral eyespot groups traced from photomicrographs of Prosthiostomttm (Prosthiostomttm) montiporae specimens measuring 4 mm, 6 mm, and 12 mm in length. the brownish to terra cotta color ofthe ingested least-squares linear regression was performed coral tissue within their gut. The gut contents on these data (see Fig. 3). The F-ratio of the offeeding animals are concentrated in the lateral regression is significant (P < 0.001), and the digestive diverticula with small amounts in the coefficient of determination indicates that 75 central intestinal lumen. The living animals thus percent of the variance in the number of eye­ appear to possess a light brownish medial spots is explained by variation in length. stripe (intestinal lumen) flanked by dark brown During the winter (water temperature ap­ mottling from the lateral digestive diverticula. proximately 23° C) it took several months to The mottled pattern grades into the transparent grow specimens of the 12-mm size class as outer periphery of the animal. The ventral opposed to several weeks at 26°_27° C. These surface in living specimens is opaque white. animals had as many as 64 eyespots in the There is a group of cerebral eyes at the an­ cerebral group, whereas the 12-mm animals terior end forming a deltoid pattern, as well as grown at 26°_27° C had approximately 37 eye­ a pair of ventral eyes. The marginal eyes form spots. Presumably then, the number ofeyespots an irregular pattern along the frontal border; is related more directly to age than to length. each being approximately one-halfthe diameter Growth in this species is indeterminate, with of a cerebral eye. The eyes are distributed be­ size of flatworm and number of cerebral and hind the anterior margin in the form of an arc marginal eyespots continuing to increase with with a deltoid cluster of cerebral eyes (Fig. 1). time, without a definite maximum. The larger flatworms possess more eyespots It is common to find this Prosthiostomum in than do smaller individuals. Examples of cere­ associations of many individuals. Like most bral eyespots are shown in Fig. 2. We measured polyclads they are cryptic, and are found in the flatworm length and number ofcerebral eyes for deep recesses of the coral head, wedged into a group of specimens taken from a population tiny crevices and cracks, or on the underside of grown in a tank and held for 1 month at 26°_ the coral. The parasite may tend to enhance the 27° C. The flatworms were relaxed in a solution branching growth form of Montipora since it of magnesium chloride before being examined prefers to live and feed in the cryptic and lower under a dissecting scope with ocular microm­ portions of the coral head, and avoids the more eter. The smaller individuals could not be exposed projections. handled without damage, and so were relaxed Laboratory and field observations indicate and measured while still on the corals. Length that Prosthiostomum (P.) sp. is negatively photo­ of relaxed specimen and number of cerebral tactic. In contrast, those animals in which eyes were recorded for each individual, and a the zooxanthellae are truly symbiotic exhibit 364 PACIFIC SCIENCE, Volume 28, October 1974 • y=8.67+1.93x • • V') 30 w >-w .... • • « •• ~ CO •• • W ~ W U • u.. 0 • ~ w CO ~ :::) • Z • • °O~-~--~4--...L6--....L8--....Ll0---L12---1 LENGTH (m m) FIG.