"BULLETIN OF MARINE SCIENCE, 27(4): 658-{j67,1977

FEEDING BEHAVIOR, CERAS STRUCTURE, AND NEMATOCYST STORAGE IN THE AEOLID , NEAPOLITANA ()

Edwin J. Conklin and Richard N. Mariscal

ABSTRACT The predation of on the sea anemones A nemonia sargassensis, Antllopleura krebsi, and Lebrunia danae was observed. Feeding behavior by Spurilla involved four distinct phases: approach, initial contact and withdrawal, re-approach, and feeding. Spurilla appeared to be stung by the anemones only during the first 1 to 3 min of feeding, suggesting a possible acclimation of the nudibranch to the anemOne prey. It was found that the nematocyst complement of Spurilla could be replaced in only 6 days. Phase contrast, light, and scanning electron microscopy confirmed the existence of small bundles of sensory cilia, as well as larger, motile cilia presumably involved in movement of the mucous sheet. The bundles of cilia appear to be contributed by three or more cells. The cell surface of the ceras is covered with closely packed, interconnecting microvilli which may be sites for chemoreception and/or mucus secretion.

A number of studies over the years have this system has not been generally used by reported the predation by nudibranchs on those working with nudibranchs. Marsical various types of coelenterates and the storage (1974c) has recently updated Weill's (1934) of the latter's nematocysts (Grosvenor, 1903; system and attempted to make it more use- Glaser, 1910; Kepner, 1943; Miller, 1961; ful to a wider scientific audience. We have Edmunds, 1966; Thompson and Bennett, used this system in order to identify and 1969). In the best known of these preda- follow throughout the feeding cycle specific tors, the aeolid nudibranchs, the nematocysts types of nematocysts and to determine their are ingested and then migrate to the tips of eventual fate. the long dorsal where they are stored. There are also a number of accounts in Here the nematocysts are incorporated in the literature concerning the general feeding cavities at the tips () where they of nudibranchs on various coelenterates remain functional. (Russell, 1942; Tardy, 1964; Edmunds, Very little is known, however, about the 1966; Rosin, 1969; Wobber, 1970; Harris, specific types of nematocysts stored by nudi- 1971, 1973; Waters, 1973). We have been branchs and the turnover time of these ne- able to follow the predation of a single spe- matocysts between feedings. One study cies of nudibranch on three different spe- (Glaser, 1910) has reported that a change cies of sea anemones and present for the in the diet of a nudibranch eventually results first time a photographic record of such a in a change in the nematocyst complement, feeding, illustrating the key points in the but only after a period of 1 month. As will normal feeding behavior of the nudibranch be seen in the present study, the turnover as well as the behavior of the anemones in time can be much less than this. response to predation by a nudibranch. One apparent reason for the lack of Very little is known about the surface knowledge in this area has been the diffi- structure of the cerata of aeolid nudibranchs culty in identifying and following the spe- or the possible sensory structures which cific nematocyst types from the prey to the might be involved in the release of the stored predator. Weill (1934) has established a nematocysts. In the present study, we have useful system of identifying nematocysts, but been able to examine for the first time the 658 CONKLIN AND MARISCAL: NUDIBRANCH FEEDING BEHAVIOR 659

Figures 1-6. Aquarium photographs of approach and feeding behavior of the aeolid nudibranch Spurilla neapolitana on the Anemonia sargassensis. The Spurilla is 3 cm long. (l) Spur- ilia approaches Anemonia; (2) Initial contact is made by Spurilla with the anemone's tentacles and edge of oral disc; (3) Immediately after contact, nudibranch erects and flares cerata and withdraws from anemone; (4) Nudibranch re-approaches anemone and begins to feed on edge of oral disc; (5) As feeding continues, nudibranch moves further on to oral disc of anemone. Note lack of response on part of the anemone. (6) Upon the conclusion of feeding, TIlldibranch moves away and remains quies- cent until next feeding. ultrastructure of the tip and surface of the (Delle Chiaje, 1823) and the sea anemones cerata of an aeolid nudibranch by means of Anemonia sargassensis Hargitt, 1908; An- scanning electron microscopy and to relate thopleura krebsi (Duchassaing and Michel- these findings to previous reports in the otti, 1860); Bunodeopsis antilliensis Duer- literature. den, 1897; and Lebrunia danae Duchassaing and Michelotti, 1860. METHODS AND MATERIALS Spurilla and Bunodeopsis were collected The organisms used in this study were together from Sargassum drifting in the open the aeolid nudibranch Spurilla neapolitana ocean near French's Reef off Key Largo, 660 BULLETIN OF MARINE SCIENCE, VOL. 27, NO.4, ]977

Florida. Anemonia was collected at a depth After the initial contact(s), Spurilla then of 1 to 4 m on rock jetties near Panama City, moved in to begin feeding, generally prefer- Florida. The specimens of Anthopleura ring the tentacles or oral disc (Figs. 4, 5) were coUected on rock groins at low tide at in the case of Anemonia and Anthopleura Coquina Beach, Anna Maria Island, Florida. (Table 1). After feeding had begun on Lebrunia was conected on coral heads at a these two , Spurilla often crawled depth of 5 m off Key Largo, Florida. over the anemone at will, apparently with- The nudibranchs were maintained in cov- out eliciting nematocyst or spirocyst dis- ered 250 ml glass jars in which water was charge (Fig. 5). A single feeding generally changed every second day, and the anemones lasted about 1 h with a single nudibranch in re-circulating, glass aquaria, both with consuming 25 to 100% of the anemone, de- controlled feedings, pH, salinity, and tem- pending on size. perature. The nudibranchs were starved for At the cessation of feeding (Fig. 6), the 2 days between experiments. nudibranchs generally became inactive and Nematocysts were observed with a Reichert often remained in one place in the bowl. Zetopan negative and positive contrast mi- However, the day following feeding was croscopeand identified using the illustrated characterized by active crawling in what key in Mariscal (197 4c) . could be interpreted as search behavior. For scanning electron microscopy, cerata were excised and fixed in Parducz's (1967) Behavior of the Anemones during fixative for 1 h and then prepared for criti- Feeding by SpurilLa cal point drying foUowing the procedures of Initial contact with Spurilla prior to feed- Mariscal (1974 a, b). The tissue was then ing apparently triggered spirocyst and/or mounted, rotary-coated with gold-palladium nematocyst discharge by the anemones, since in a vacuum evaporator and examined with the nudibranch could be seen to adhere a Cambridge 8tereoscan 84-10 scanning strongly to the tentacles of the latter. How- electron microscope. ever, these initial contacts did not appear to elicit a typical coelentcrate feeding re- RESULTS sponse (Lenhoff, 1974). Following a Nudibranch Feeding Behavior period of from 1 to 3 min, the adhesion of the tentacles appeared to decrease and fi- A characteristic pattern of approach and nally cease entirely. In addition, the nudi- feeding by Spurilla could be broken down branch no longer responded as if it was be- into four distinct phases: approach, initial ing stung after this time period. Contact of contact (s) and withdrawal, re-approach, the anemone's tentacles by a wooden probe and feeding. at this time, however, still elicited adhesion, Within 1 to 2 min after placing an anem- suggesting that no change had occurred in one in a SOO-ml bowl with Spurilla, the the anemone's capability to discharge its nudibranch began to move toward the anem- nematocysts and spirocysts. one (Fig. 1). Initial contact resulted in a sudden, momentary withdrawal by Spurilla Another common response by Anemonia as if it had been stung (Figs. 2, 3). There and Anthopleura to contact with Spurilla appeared to be no consistent portion of the was a slight to moderate contraction of the body by which Spurilla first contacted the tentacles and, in the case of the latter anem- anemone. The head tentacles, , one, a strong contraction of its column to and cerata all were used when feeding on less than a third its total height. This latter Anemonia, but only the rhinophores and response appeared to make it difficult for head tentacles when contacting Lebrunia Spurilla to find a place on the column to be- and Anthopleura (Table 1). gin feeding and thus it commonly began CONKLIN AND MARISCAL: NUDIBRANCH FEEDING BEHAVIOR 661

Table 1. Summary of observations of the predatory behavior of Spurilla neapolifana on sea anemones

Percent Total Observations Part of Spurilla First Portion of Anemone Initially Contacting Anemone Attacked by Spurilla Number of Total Anemone Anemones Number of .Rhino- Head Oral Presented Observations phore Tentacle Cerata Column Disc Tentacle Frond -.------Anemonia 5 12 25.0 41.7 33.0 25.0 8.3 66.7 NA* Lebrul/ia 4 6 83.3 16.7 0 0 0 0 100 AI/fhopleura 1 2 50.0 50.0 0 0 100 0 NA*

• NA == not applicable. Allemoll;a and AlltJropleura do not have fronds. feeding first on the oral disc of Anthopleura microbasic amastigophores identical to those (Table 1). found in this anemone. The morphology of Lebrunia is quite un- After return to the laboratory and 6 days usual compared with a "typical" sea anem- of feeding on only Anemonia sargassensis, one. It possesses a series of six to eight the cerata cnidosacs contained only basitri- dichotomously branched, alga-like fronds chous isorhizas, the main nematocyst in this extending out from the upper part of the species of anemone (Table 2). No Bunode- column. The fronds bear hemispheric vesi- opsis nematocysts remained, having appar- cles which are packed with extremely large ently been released or ingested. macrobasic amastigophore nematocysts. In Similar results were obtained in feeding spite of this, and in spite of the fact that Lebrunia to Spurilla. Prior to this series of Lebrunia is capable of severely stinging experiments, the nudibranchs had been feed- humans, Spurilla always began feeding on ing exclusively on Anemonia for 9 days. On the fronds initially. In response to this only the third day of feeding on Lebrunia predation, Lebrunia often actively attempted danae, the macrobasic and microbasic amas- to creep away from the nudibranch, fre- tigophores characteristic of this species had quently autotomizing the frond being eaten begun to appear in the cnidosacs and, by by thc nudibranch and leaving it behind. day 6, had replaced the Anemonia nemato- On one occasion, after crawling away to the cysts. A control nudibranch which had been top of the dish, the Lebrunia lost its grip on fed on Anemonia, but starved during the the substrate and fell upside down on a course of the above experiment showed only Spurilla. In spite of the fact that the nudi- Anemonia nematocysts in its cnidosacs (Ta- branch had been feeding on Lebrunia previ- ble 2). ously with no apparent ill effects, it was im- mediately stung and in the process of being Morphology and Ultrastructure of ingested when it was removed from the the Cerata anemone with forceps. Although the nudi- branch was quickly separated from the Le- The cerata of Spurilla, like those of other brunia, it died a short time later. aeolids, contain diverticula of the digestive gland which apparently serve as passageways by which ingested nematocysts reach the Nematocyst Storage in Spurilla cnidosacs in the tips (Fig. 7). A sphincter When collected from floating Sargassum muscle separates the from the re- weed, Spurilla was found to be feeding on mainder of the ceras (Fig. 7). Once the the sea anemone, Bunodeopsis antilliensis. nematocysts reach the cnidosac, they have Examination of the cerata cnidosacs re- somehow been formed into cnidocysts (small vealed that they contained two types of packets of nematocysts) perhaps by a phago- 662 BULLETIN OF MARINE SCIENCE, VOL. 27, NO.4, 1977

Table 2. Turnover in types of nematocysts observed in Splirilla neapolitana cnidosacs after feeding on a different species of sea anemones

.. ~_._------Percent Nematoeyst Types Present Feeding Nudibraneh Regime of Nematocysts Number or Before 1 Day 2 Days 3 Days 6 Days Nudibranehs Types Present Observations Feeding Later Later Later" Later ------Fed on a) Before feeding: 3 100 100 100 75 a Anemonia Amastigophores of Blinodeopsis b) After feeding: Basitrichs of Anemonia 3 a a a 25 100 Fed on a) Before feeding: Lebrllnia Basitrichs of Anemonia 100 100 100 75 10 b) After feeding: Amastigophores of Lebrllnia a a a 25 90 Unfed control a) Before experiment: Basitrichs of Anemonia 100 lOa 100 100 100 b) After experiment: Basitrichs of Anemonia laO lOa 100 lOa 100 ------• Spurilla were fed again 3 days after initial feeding. cytic cell, and are then stored in the cnido- Covering the surface of the cerata are sac until use (Figs. 8, 9). large bundles of motile cilia (Fig. 13). Interestingly enough, most, if not all, of These seemed to be more abundant towards the nematocysts are incorporated in the the base of the cerata and were capable of cnidocyst, and the cnidocysts in turn incor- actively moving small particles, and perhaps porated in the cnidosac so that the functional a mucous sheet, over the surface. Smaller ends of the nematocysts are pointed out- non-motile bundles of cilia, called sensory wards, similar to their orientation in the bundles by Edmunds ( 1966) were found anemone tentacles from which they came most commonly towards the ceras tip (Figs. (Fig. 10). Under the light microscope, in- 7, 11, 12, 13). The ciliary bundles were dividual nematocysts, as well as those con- found at the junction of three or more cells tained in the cnidocysts, can be observed to and may be formed as a result of contribu- begin discharging immediately upon contact tions from several cells (Fig. 13). with the surrounding medium following ex- Under high magnification, it could be trusion from the cnidosac (Figs. 9, 10). seen that the surfaces of the cells composing The nematocysts are extruded through the cerata are covered with a dense layer of the ceras tip by way of the cnidopore, a small short microvilli (Fig. 14). Fine connections opening apparently controlled by a sphincter could be seen between the microvilli and muscle (Fig. 11) . Near the cnidopore are some seemed to be oriented in small rosettes, located the openings of the defensive glands, somewhat reminiscent of the presumed sen- described by Edmunds (1966) and thought sory structures in coelenterates (Mariscal, to be involved in defense against predators 1974 a, b). (Fig. 12). In some cases, extruded fired nemato- CONKLIN AND MARISCAL: NUD.lBRANCH FEEDING BEHAVIOR 663

Figures 7-10. (7) Phase contrast micrograph of tip of ceras from Spurilla neapolitana showing sphinc- teres) separating digestive diverticulum (dg) from cnidosac (cs). Note sensory bundles of cilia (sb) ncar ceras tip. Bar is 50 JLm long. (8) Phase contract micrograph of packets of nematocysts (cnido- cysts, cc) being extruded from the cnidopore at the tip of the ceras following stimulation. Bar is 50 JLm long. (9) Phase contrast micrograph showing discharge of nematocysts (basitrichous isorhizas of A nemonia) following extrusion of the cnidocyst (cc) from cnidopore of the ceras after stimulation. Bar is 50 JLm long. (10) Phase contrast micrograph of several extruded cnidocysts (cc) showing orien- tation of contained nematocysts (basitrichous isorhizas of Anemonia) in a single direction. Bar is 50 I'm long. 664 BULLETIN OF MARINE SCIENCE, VOL. 27, NO.4, 1977

Figures 11-14. (11) Scanning electron micrograph of tip of ceras from Spurilla showing cnido- pore (cp) through which nema10cysts extruded and the many bundles of sensory and motile cilia. Wrinkled nature of ceras surface is fixation artifact. Bar is 20 .um long. (12) Scanning electron micro- graph showing cnidopore (cp) surrounded by openings to defensive glands (dg). Two discharged nc- matocysts (n) are lying on the surface of ceras following extrusion from cnidopore. The nematocysts are microbasic amastigophores from Bunodeopsis. Bar is 20 .urn long. (13) Scanning electron micro- graph taken near tip of ceras of Spurilla showing sensory bundles (sb) and motile bundles (mb) of cilia. Note that ciliary bundles are located at the junction of three or more cells. Bar is 20 p'm long. (14) High power scanning electron micrograph of ceras surface of Spuril/a showing many intercon- necting microvilli. Bar is 0.5 .urn long.

cysts were found lying near the cnidopore on Some feel that the secretion of copious the ceras tip (Fig. 12). This again confirms amounts of possibly toxic mucus may serve the idea that coelenterate nematocysts are as a protective device (Glaser, 1910; Rus- fully capable of normal discharge after pass- sell, 1942; Harris, 1971, 1973; Waters, ing through the digestive tract of a nudi- 1973) while others have raised the possi- branch. bility that the epidermal vesicles found on some aeolids might provide protection by DISCUSSION blocking the penetration of nematocysts (Graham, 1938; Edmunds, 1966). Evi- The question of how a nudibranch avoids dence from the present study suggests that being stung during predation on coelen- another mechanism might be involved. terates has intrigued a number of workers. In the present study, it was noted that the CONKLIN AND MARISCAL: NUDIBRANCH FEEDING BEHAVIOR 665 nudibranch elicited nematocyst and/or spiro- by Herrnkind et a1. (1976) in the field. cyst discharge only during the initial stages This may be a common defensive reaction of feeding (first 1 to 3 min), and that after on the part of this particular species in re- this time the nudibranch was able to crawl sponse to a variety of environmental dis- over the oral disc and tentacles with im- turbances. punity. This is suggestive of some form of Another new result of the present study acclimation, perhaps similar to that under- was the relatively short time required for gone by anemone fishes when first attempt- turnover of the nematocyst complement in ing to move into the tentacles of their sea the cerata. According to Glaser (1910), 30 anemone host (Mariscal, 1971). It is pos- days were required, but we found that only sible that some substance produced on the 6 days were required to replace one nemato- surface of the nudibranch, or perhaps ac- cyst type with another in the cerata. In ad- quired from the anemone during this brief dition, Spurilla selectively retained only the period could mask the nudibranch's normal largest nematocysts from its prey. stimuli for nematocyst or spirocyst discharge. The surface ultrastructure of an aeolid However, the relatively short length of time nudibranch was examined for the first time required for this process in the case of the in the present study. We were able to con- nudibranch suggests that it may be different firm Edmunds (1966) observation that the than the anemone fish situation. sensory bundles of non-motile cilia were It was surprising to find that Lebrunia more abundant near the tips of the cerata. danae could be preyed upon as readily as Edmunds (1966) has shown that these sen- the other anemones. Lebrunia possesses ex- sory bundles are directly connected with tremely venomous nematocysts which are nerve cells in the cerata. He has suggested capable of raising large, painful welts on that the sensory cilia may be involved in the human skin and can completely immobilize extrusion of nematocysts from the cnidosac small crustaceans and fishes on contact. and perhaps in triggering the secretions from As long as Spurilla approached Lebrunia the defensive glands which surround the gradually from the side, as it did the other cnidopore. anemones, it was not harmed. However, In addition to the ciliary bundles, the epi- sudden massive contact with the fronds of dermal cell surfaces of the cerata are cov- Lebrunia proved to be fatal for Spurilla in ered with a dense layer of interconnecting the one case observed. microvilli. These could be involved in the In other cases, when nudibranchs which secretion of the mucous coat of the nudi- normally feed on hydroids are brought into branch or possibly have a sensory function, contact with sea anemone tentacles, they are or both. For example, microvilli have been often stung (Grosvenor, 1903; Edmunds, implicated as the site of chemoreception in 1966). The reasons for the above observa- other species (Graziadei, 1969; Murray and tions remain unclear, but they are again sug- Murray, 1970; Beidler and Gross, 1971). gestive that some form of gradual accli- However, there is no information concern- mation process may be necessary before ing either possibility in aeolid nudibranchs. nudibranchs can successfully prey on certain The function of nematocyst storage in species of coelenterates. aeolids has been assumed to be defensive The autotomy of the nematocyst-bearing and some experimental and field evidence is fronds and escape response of Lebrunia was available to support this idea (Edmunds, also of interest since the other anemones 1966; Mariscal, 1974c). We have found examined in the present study did not exhibit that the nematocysts obtained from the coel- this behavior. Autotomy of Lebrunia fronds enterate prey of aeolid nudibranchs are fully due to rough handling has also been observed functional and capable of being discharged 666 BULLETIN OF MARINE SCIENCE, VOL. 27, NO.4, 1977 immediately upon release from the cerata. Kepner, W. A. 1943. The manipulation of the The storage of nematocysts by aeolid nudi- nematocysts of Pellllaria tiarella by Aeolis pi/ala. J. Morpho!. 73: 297-311. branchs in general and the specific retention Lenhoff, H. M. 1974. On the mechanism of of the largest and/or most venomous ne- action and evolution of receptors associated matocyst types (Thompson and Bennett, with feeding and digestion. Pages 211-243 ill L. Muscatine and H. M. Lenhoff, eds. 1969) suggest that this phenomenon has Coelenterate biology: Reviews and new per- probably played an important role in the spectives. Academic Press, New York. success of this group of organisms. Mariscal, R. N. 1971. Experimental studies on the protection of anemone fishes from sea anemones. Pages 283-315 ill T. C. Cheng, ACKNOWLEDGMENTS ed. Aspects of the biology of symbiosis. University Park Press, Baltimore. This research was aided by a Grant-in-Aid of 1974a. Scanning electron microscopy Research from Sigma Xi, and NSF Grant #GB of the sensory surface of the tentacles of sea 40547. The authors would like to thank Mr. Gregg anemones and corals. Z. Zellforsch. 147: Stanton and Ms. Claudia Mills for collecting the 149-156. Spllrilla used in the study. 1974b. Scanning electron microscopy This is contribution No. 41 of the Tallahassee, of the sensory epithelia and nematocysts of Sopchoppy and Gulf Coast Marine Biological As- sociation. corals and a corallimorpharian sea anemone. Pages 519-532 in Proceedings of the second international coral reef symposium J. Great LITERATURE CITED Barrier Reef Committee, Brisbane. 1974c. Nematocysts. Pages 129-178 Beidler, L. M., and G. W. Gross. 1971. The ill L. 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Current topics Tardy, J. 1964. Comportement predateur de in comparative pathobiology. Academic Eolidiella alderi (Mollusque, Nudibranche) Press, New York. C. R. Acad. Sci. 255: 3250-3252. Herrnkind, W. F., G. Stanton, and E. Conklin. Thompson, T. E., and Bennett, I. 1969. Phy- 1976. Initial characterization of the com- salia nematocysts: Utilized by mollusks for mensal complex associated with the sea anem- defense. Science 166: 1532-1533. one, Lebrllllia dallae, at Grand Bahama. Waters, V. L. 1973. Food-preference of the Bull. Mar. Sci. 26: 65-71. nudibranch Aeolidia papillosa, and the effect CONKLIN AND MARISCAL: NUDIBRANCH FEEDING BEHAVIOR 667

of the defenses of the prey on predation. thus sp. from Monterey Bay, California. Veliger ]5: ]74-]92. Veliger 12: 383-387. Weill, R. 1934. Contribution a l'etude des cni- daires et de leurs nematocystes. Trav. Sta. DATE ACCEPTED: September 3, 1976. Zoo\. Wimereux 10-11: 1-701. Wobber, D. R. 1970. A report on the feeding ADDRESS: Department of Biological Science, Flor- of Dendronotus iris on the anthozoan Cerian- ida State University, Tallahassee, Florida 32306.