BULLETIN OF MARINE SCIENCE, 41(2): 351-360,1987

BEHAVIOR OF JUVENILE NEROCILA ACUMINATA (ISOPODA, CYMOTHOIDAE) DURING ATTACK, ATTACHMENT AND FEEDING ON FISH PREY

Earl Segal

ABSTRACT Juvenile Nerocila acuminata are blood and tissue feeders which, under laboratory condi- tions, show no prey specificity attaching to and feeding upon Menidia beryllina (silversides), Cyprinodon variegatus (sheepshead minnow), Mugil cephalus (mullet), Fundulus similis (kil- lifish), Trachynotus carolinus (pompano), Lagodon rhomboides (pinfish), and Anchoa mitchilli (anchovy) from the Gulf of Mexico. Attacks occurred either in mid-water or from the bottom. In mid-water, isopods would swim in from the rear, on any compass setting, adjusting their dorso-ventral swimming orientation to land with outstretched pereopods, Speed adjustments were precise since, in over 50 attacks, overshooting did not occur. From the bottom, isopods which were lying on their backs with the first three pairs of per eopods extended, either hooked a slowly passing fish or flipped their pleopods and propelled themselves onto a fish. Thus, juvenile N. acuminata are behaviorally equipped to attack either pelagic or demersal fishes. Undamaged fish did not respond overtly either to the sight of or transient contact with the isopods; damaged fish took evasive action. If attachment occurred on the body rather than on the fins, the fish responded with a variety of dislodgment tactics the vigor of which diminished with additional attachments.

In the isopod family Cymothoidae all known species appear to be pelagic while young (Williams, 1984) and either parasitic or commensal as adults when they take up residence on host fish by one of three modes: by attaching to the surface; by entering the gill chamber and/or buccal cavity; or by burrowing under the epidermis. Recently, Brusca (1981) has monographed the Cymothoidae of the eastern Pacific while offering extensive insights into the developmental and life histories, host parasite relations and ecological ramifications of host specificity of various species. Almost all the available information concems the adults; juveniles are poorly known and their relationships to their hosts or prey are little understood. A fortuitous collection of juvenile Nerocifa acuminata Schioedte and Meinert 1881 from the Gulf of Mexico offered an opportunity for a detailed descriptive study of various aspects of their behavior while swimming, feeding and interacting with their prey. In a more limited way it presented a situation for observing the responses of prey to small predators, a subject Barlow (pers. comm.) feels has been sadly neglected. Since juvenile N. acuminata will be shown to be temporary blood feeders I consider them to be micropredators rather than parasites making the same dis- tinction Marshall (1981) does when comparing Diptera that land on their hosts to take a blood meal then flyaway with those that spend their lives on their hosts whether they feed or not. Thus fish upon whom juvenile N. acuminata feed are their prey.

MATERIALS AND METHODS

From 10 December 1961 through 19 May 1962, 8 I juvenile Nerocila acuminata Shioedte and Meinert, 1881, were collected in seine hauls from an isolated shallow lagoon to the west side of the Houston ship channel jetty on Galveston Island, Texas. Collections were made on 10 December (-60 specimens), 17 December (3), 25 January (15 collected, 10 recovered), 8 May (I) and 19 May (2). The last collection on 19 May was from a smaller back beach lagoon which received some fresh water 351 352 BULLETIN OF MARINE SCIENCE. VOL. 41. NO.2. 1987 over and through the ship channel wall. Systematic attempts to obtain additional N. acuminata over the following1.5years and isolated attempts over the next 15years were unsuccessful.In the aftermath of hurricane Carla, which hit the Texas coast during the fall of 1961,a buildup of sand occurred along the beaches and in the lagoons on Galveston Island progressively blocking entrances and cutting off water exchange.With the warming of the shallow lagoon water during the spring and summer of 1962 (33°Con 19 May in the isolated lagoon)Sargassum sp., which had accumulated during the hurricane, died and decayed, effectivelykilling the lagoons. When the animals were captured the majority had a bright red gut. Numerous fish (see Results) were seined with the isopods but on no occasion were the isopods found on the fishin the seine. Both isopods and fishwere returned alive to the laboratory at Rice University in Houston and housed apart in aquaria and plastic boxes. All fish collected with the juvenile N. acuminata were young and adults of small species,e.g., Cyprinodon variegatus, Fundulus similis and Menidia beryllina or youngoflarger species, e.g., MugU cephalus and Trachynotus carolinus. Becauseof limitations in laboratory holding space, no attempt was made to procure larger fish from the lagoon. Both isopods and fishes were maintained and tested at 15°Cand 280/00sal. in natural lagoon water (conditions under which the animals were collected).All observations of and arranged interactions between isopods and fish were carried out with one primary objective-to record the actions of livingjuvenile cymothoid isopods. Responses of the prey were noted.

RESULTS External Features. -Figure IA illustrates the dimensions and shape of a repre- sentative juvenile Nerocila acuminata. The eyes are large, oval and curved around the sides of the head. This is characteristic offree swimming, juvenile cymothoid isopods. In every individual there are a minimum of two rows of ommatidial facets on the ventral surface of the head. The uropods, pleopods and pleotelson all possess plumose marginal setae. All seven pairs of pereopods are prehensile, terminating in long, narrow, curved dactyli. Pigmentation is extensive, consisting of myriad aggregations of starburst shaped chromatophores. Size. - Of the 13 juveniles subsequently available for measurement, the largest was 18.6 mm (L) by 5.1 mm (W) with a body index (L-W) of 3.65 while the smallest was 12.8 mm (L) by 3.9 mm (W) with a body index of 3.28. However, body indices ranged from 3.28 to 3.95 with an X of3.58 and no discernible trend of body index with body size. Non-prey Oriented Behavior. -SWIMMING. As swimmers the animals were re- markably graceful and adroit. They swam rapidly in a straight line interspersing loops, half immelmans and turns. They normally swam ventral side down but occasionally switched to dorsal side down for short distances when they reversed direction after bouncing off an obstacle, e.g., a fish or the sides of the aquarium (Fig. 2A). On no occasion did the isopods avoid an obstacle by changing course before contact. Upon contact they either looped upward or downward and headed in the opposite direction. Or, the animals turned to either side, on the horizontal or the oblique, and continued on whatever angle their turns took them. Much of the swimming appeared to be random and non-prey oriented. NON-SWIMMING. Periodically the animals would glide to a halt on the bottom, ventral side down. After a few seconds to a few minutes they would resume swimming with the same dorso-ventral orientation. Occasionally isopods settled on the bottom, ventral side up. Since this invariably led to an attack it will be presented below. Molting, with cessation of swimming and feeding, also took place on the bottom. Nine individuals attached by the first three pairs of pereopods to any available bottom projection. Three remained in the original attachment location; two for 3 and one for 4 days. The latter completed the molt and resumed swimming. All others either died or disappeared and were presumably eaten. SEGAL: BEHAVIOR OF JUVENILE NEROC/LA ACUM/NATA PREYING ON FISH 353

Figure IA. Representative, free swimmingjuvenile Nerocila acuminata with characteristic large eyes, streamlined shape and pereopods ending in long, curved dactyli. Figure lB. Menidia beryl/ina (silverside) hosting 10juvenile N. acuminata (three individuals on right side). All isopods arc oriented in the same direction. Dark gut of isopods indicates feeding.

Prey Oriented Behavior. - Isopod attacks took place either while both parties were swimming or when a fish swam slowly over an isopod which was lying on the bottom, ventral side up. In the swimming mode, isopods always approached from the rear, maintaining in the dorsal position, or assuming in all other positions, an appropriate attachment orientation. Thus, an isopod coming in from above would, depending upon its approach speed, either glide in or spurt in on the dorsal or dorsolateral surface; an isopod approaching from below would reorient 1800 on its axis and come in ventrally (Fig. 2B). From any other compass orientation 354 BULLETIN OF MARINE SCIENCE, VOL. 41, NO.2, 1987

Figure 2A. (Left) Non-feeding contact behavior of N. acuminata and potential prey fish. Figure 2B. (Right) Mid-water attack behavior of juvenile N. acuminata. Attacks were always from the rear either above, below or to the sides of the fish. an isopod would tilt accordingly and swim in on the flank. Speed adjustments appeared to be precise since of the more than 50 observed swimming attacks not one resulted in an overshoot. Whether an attack resulted in an attachment de- pended, in good part, on the response of the prey (see below). In the bottom attack the isopod played a relatively passive role. Lying on its back with limbs extended it could hook, with its first three pairs of pereopods, any accessible region of a slowly passing fish. Occasionally, when contact had not been made, an isopod would flip its pleopods and propel itself onto the fish. Invariably, a bottom attack resulted in attachment to an anal fin (Fig. 2C). If an attack was successful an isopod did not necessarily remain at the site of attachment. Isopods were capable of crawling over the surface to the degree that no area of a fish was inviolate. Position adjustments invariably resulted in the isopods assuming the same antero-posterior orientation as their prey (Fig. IB). Attachment Time and Feeding. - When a single isopod was paired with a single fish (N = 10) attachment lasted from 0.5 to approximately 24 h. Without sacrificing the isopods two criteria may be used to determine whether an attached isopod is feeding or has fed: (I) if the gut of the isopod reddens while attached or (2) if a fish, alive or dead, shows a wound where an isopod had been attached. However, time on the prey is not a reliable indicator of feeding time: isopods, both fed and unfed, may attach but not feed; the response of the prey may act as a time constraint (see below) thus detachment may be involuntary; death of the prey may not be followed by detachment (N = 3); attachment may occur on a recently deceased fish with feeding (N = 3) or nonfeeding (N = 2); and isopods may remain on a fish (Fundulus similis) which has shallowly buried itself under sand (N = 2) (Fig. 2D). Specificity. -No prey specificity was evident. Juvenile N. acuminata attacked and fed upon all seven species of fish collected with them in seine hauls. Species serving as prey were Menidia beryllina (silverside), Cyprinodon variegatus (sheeps- head minnow), Mugil cephalus (mullet), Fundulus similis (killifish), Trachynotus SEGAL: BEHAVIOR OF JUVENILE NEROC1LA ACUM/NATA PREYING ON FISH 355

20 2C

Figure 2C. (Left) Bottom attack behavior of juvenile N. acuminata. Attachment was usually to an anal fin or, if the isopod used its pleopods, to the ventral surface. Figure 2D. (Right) Retention of attachment position by juvenile N. acuminata on Fundulus similis (killifish) which buried itself under the sand. Orientation of isopod altered. carolinus (pompano), Lagodon rhomboides (pinfish) and Anchoa mitchilli (an- chovy). In 14 pairings (two for each species) between a single isopod (gut opaque- a minimum of 3 days of isolation) and a single undamaged fish, all 14 were attacked and fed upon within 3 h. In short term experiments (terminated when feeding was evident) juvenile N. acuminata, offered a choice between individuals of two or more species, attached and fed nonselectively (four independent multiple encounters). However, in each of three long term encounters (5 or more hours) between 5 to 10 N. acuminata and two (1) or three (2) Menidia beryllina, there were indications that attachment and feeding by one or more isopods may pref- erentially lead to attachment and feeding by others. Fate of Isopods. -Of all the arranged or fortuitous interactions between isopods and fish, I observed only one incident in which a MugU cephalus (at 12.7 cm the largest individual seined and used in the study) caught an N. acuminata in mid- water but the isopod was immediately expelled and resumed swimming. However, since a number of isopods which had been left overnight with various fish dis- appeared, I must assume they were eaten. Response of Fish to Isopods. -In arranged interactions with individuals of all seven species the sight offree swimming isopods did not evoke any overt response (evasive movements) among those that had not previously been fed upon (N = 35). Neither did transient contact ofisopods with fish, either head-on or glancing, evoke a response (N = 9). However, both the sight of and transient contact with isopods of fish that had previously been attacked and fed upon did result in the fish taking evasive action (N = 13). When an isopod attached to a fish the response appeared to be site related. If on a fin, the fish usually did not respond (N = 15). If on the body, particularly on the eyes, at the gill aperture, along the lateral line, or near the anal opening the fish invariably gave an initial response which was often quite violent- breaking surface, rapid swimming, twisting of the body, rubbing against objects (sides of the aquarium, rocks on the bottom, the bottom itself) or diving under the sand when present (F. similis)- before resuming its normal activities. Such dislodge- ment tactics were occasionally successful (N = 14). There appeared to be a dim- inution in vigor and variety of dislodgement tactics with subsequent isopod at- tachments. A striking example of this occurred with the M. beryllina which hosted ION. acuminata (Fig. 1B); by the fifth attachment the fish no longer responded. Fate of the Prey.-All fish that had been attacked and fed upon showed varying 356 BULLETIN OF MARINE SCIENCE, VOL. 41, NO.2, L987

severity of wounds at the feeding site. The minimum time to death was 5 min for a small (3.07 cm) M. beryllina fed upon by one isopod. In general, the larger the fish the longer it lived and the greater number of feeding isopods the more quickly it died.

DISCUSSION From the few developmental and life history studies conducted on cymothoid isopods (Brusca, 1981) the picture that has emerged is one in which, following development in the female marsupium, free swimming young with six pairs of pereopods and yolk (manca stage) are released into the water column. A subsequent molt or molts results in an animal with seven pairs of pereopods and an absence of yolk Uuvenile stage). Additional molts result in a young male which may still function in the water column or may be on the definitive host but in either case still capable of swimming (transition stage). With maturity the young male changes on the host into a functional male. Since all cymothoid isopods are believed to be protandrous hermaphrodites the functional male may then change into a female depending upon the male-female situation on the definitive host. On morphological grounds Brusca (1981) has synonymized Nerocila callfornica Schioedte and Meinert, 1881 from the east Pacific and Hawaii with N. acuminata Schioedte and Meinert, 1881 from the west Atlantic. Thus, all references to N. callfornica will be as N. acuminata with a qualifying geographic designation. All known species of Nerocila, as adults, reside on the surface of their hosts (Brusca, 1981; Morton, 1974; Trilles, 1964). There is little available information on the free swimming juveniles of Nerocila because, as Trilles (1964) comments for Mediterranean species, juveniles are rare because they are such active swim- mers. I have shown that free swimming juvenile N. acuminata will attack and feed on seven fish species in five families: Menidia beryllina (Atherinidae); Cy- prinodon variegatus and Fundulus similis (Cyprinodontidae); Mugil cephalus (Mu- gilidae); Trachynotus carolinus (Carangidae); Lagodon rhomboides (Sparidae); and Anchoa mitchilli (Engraulidae). From a variety of sources and his own detailed studies Brusca (1981) has compiled a record of 40 different host species (some questionable) of eastern Pacific adult Nerocila acuminata. The most common hosts are members of the families Embiotocidae, Mugilidae, Serranidae, Atherinidae and Engraulidae. Western Atlantic adult host records are not as reliable-many are simply noted without specifying either the numbers of fishes or isopods or whether host tissue damage (as used by Brusca) was evident. Richardson (1905), Comeaux (1942), Hutton (1964) and Hastings (1972) collectively report nine host species from nine different families with minor familial overlap between the two geographic prov- inces. Adult Nerocila acuminata thus tend to show a broad host preference covering a mix of pelagic schooling, pelagic but living close to the bottom and strictly demersal fishes. According to Brusca (1978a; 1981) this broad preference is based more on ecological availability than on taxonomic specificity. Juveniles, at least from the Gulf of Mexico, similarly show broad prey preference with, remarkably, no duplication of species hosting adults. However, the duality of prey attack modes (from the water column and from the bottom-see below) predisposes the last juvenile instar or the young male, if still swimming, to settle on ecological equiv- alents of the prey species. Although Hastings (1972) reports finding a gravid female Nerocila acuminata with 112 juveniles (prehatch stage of Brusca, 1978a) averaging 3-4 mm long, the SEGAL: BEHAVIOR OF JUVENILE NEROCILA ACUMINATA PREYING ON FISH 357

only known measurements of juveniles of this species are those of Brusca (1978a) from a single population in the Northern Gulf of California and those of mine from a single population in the Gulf of Mexico. Clearly, the Gulf of Mexico juveniles are longer relative to body width and thus have larger indices; Gulf of Mexico mean body index of 3.58, Gulf of California mean body index of 3.06. Brusca (I 978a) proposes that at least three juvenile instars are represented in the Gulf of California sample with the last instar measuring between 12.2 to 13.6 mm in length prior to molting into a functional adult male on the definitive host. However, juvenile N. acuminata from the Gulf of Mexico are still swimming and feeding at sizes up to 18 mm long which suggests to me that either (1) there are additional instars which were not collected from the Gulf of California, (2) Gulf of Mexico juveniles simply grow bigger before metamorphosing into males or (3) juvenile N. acuminata can delay metamorphosis and continue molting until a suitable definitive host is available. There is, at this time, simply no way to differentiate between alternatives. Among investigators who have witnessed juvenile or young male cymothoids attacking fishes, only Brusca (1978b) and I describe the attack modes. Although adult Lironeca vulgaris, with which Brusca worked, reside in the gill chamber (male) and buccal cavity (female), and adult Nerocila acuminata resides on the host's surface, juveniles of both show a similar duality of attack orientations. Lironeca vulgaris either strike downward onto the potential host, attaching sub- dorsally near the gill aperture, or from the bottom strike upward at a passing fish, attaching subventrally near the gill aperture. Within a few hours to a few days the juveniles crawl into the gill chamber. Nerocila acuminata juveniles demon- strate a greater repertoire of attack orientations since, while in the water column, they attack not only from above but also from either side and below with, inci- dentally, precise speed adjustment; there were no overshoots in more than 50 attacks. They also attack from the bottom orientation and usually hook a fin. I am not proposing that Nerocila acuminata juveniles are necessarily more versatile than Lironeca vulgaris juveniles. Clearly, L. vulgaris juveniles, at least the instars that were observed, are seeking a home while the N. acuminata juveniles are seeking a meal. The possible full range of attack orientations in L. vulgaris may not have been observed or the goal may have restricted their movements. Certainly attachment near the gill aperture would facilitate entry into the gill chamber. For surface dwelling species (genera Nerocila, Anilocra, Renocila) in which the adults reside in a variety of locations (Bowman and Mariscal, 1968; Brusca, 1978a; Morton, 1974; Trilles, 1965; Williams and Williams, 1980; 1981) attachment sites may be unrelated to final sites; feeding juveniles, as demonstrated in this study, crawl where they wish. There is no reason to assume that last instar juveniles or young males, whether still in the water column or on the host, lose this capability. The role of free swimming cymothoids has been much debated; do they feed or do they not. Unfortunately, much of the debate is not based upon observations of living animals and where it has, the answers are equivocal. Juvenile as well as adult cymothoids are morphologically equipped to gouge and suck (Gunther, 1931) and an esophageal gland blood anticoagulant has been uncovered in adults of M einertia oestroides (buccal/gill dweller) and Anilocra physodes (surface dweller) (Romestand and Trilles, 1976). Because of the contained yolk supply, newly released cymothoids (mancae) probably do not feed as suggested by Menzies et al. (1955) for Livoneca (=Liro- neca) convexa (buccal/gill dweller) and Nerocila acuminata (Brusca, 1978a). Among investigators who have witnessed the release of man cae neither Kroger and Guth- 358 BULLETIN OF MARINE SCIENCE, VOL. 41, NO.2, 1987

rie (1972) with Olencira praegustator (buccal/gill dweller) nor Williams (1984) with Ani/oera ehromis (surface dweller) mention whether feeding occurs. Only Sandifer and Kerby (1983) report that while mancae released from gravid females of Lironeea ovalis (buccal/gill dweller) readily attach to juvenile striped bass and white perch they did not feed during the first day. Unfortunately, the trials were terminated either to check the isopods that had not been eaten or to check the fish when all the isopods disappeared and were presumed eaten. Juveniles and/or young males, at least among those species where some time appears to be spent in the water column and regardless of whether they are buccal/ gill or surface dwellers, do feed. Neroeila aeuminata fed on all seven species of fish offered to them but feeding following attachment was not mandatory. In trials conducted by Lindsay and Moran (1976) free swimming males of Lironeca ovalis and Olencira praegustator (all free swimming stages were referred to as males since the developmental history of neither species is known) attached to all nine species of fish offered, with Lironeca feeding on five and Olencira on three. Since the mancae of Lironeea ovalis used by Sandifer and Kerby (1983) were 3 to 5 mm in length and the males used by Lindsey and Moran (1976) were 3.2 to 14.1 mm in length the latter authors were obviously dealing with mancae, juveniles and young males without distinguishing which were feeding. Significantly, juveniles of all three isopod species may feed on the same (Menidia beryllina) or different fish which are not reported to be hosts of the adult isopods (see above and Lindsey and Moran, 1976). While all three isopods have a broad geographical distribution, one is a surface dweller with broad host preference (N. aeuminata), one is a buccal/gill dweller with broad host preference (L. ovalis) and the third is a buccal/gill dweller with narrow host preference (0. praegustator, only reported from Brevoortia sp.-the Atlantic menhaden). Lindsey and Moran (1976) among others refer to free swimming and feeding juvenile and young male isopods as facultative parasites on intermediate hosts. I suggest we are dealing with micro predators attacking and feeding on their prey regardless of the life styles of the adults. Juvenile N. acuminata behave much like adult Roeinela bellieeps pugetensis, in the isopod family Aegidae, which attack, take blood and kill young pink salmon Onehorynchus gorbuseha (Novotny and Mahnken, 1971). They suggest that the term parasitism is misapplied; Rocinela is a specialized predator. I conclude that the same reasoning may apply to juvenile Neroeila aeuminata and all juvenile cymothoids that feed and do not remain on their prey. Following the juvenile's final molt into a functional male, which ostensibly takes place on its definitive host, it no longer actively or only feebly swims and may now truly be referred to as a parasite. Incidentally, the similarity in feeding behavior of juvenile N. aeuminata and adult Roeinela parallels the similarity in mouth ap- pendages of cymothoids and aegiids and the close evolutionary affinities of the two families as proposed by Brusca (1981). Molting in free swimming mancae, juveniles and young male cymothoids has not previously been reported. Juvenile Neroei/a aeuminata leave the water column and with the first three pairs of pereopods attach to the bottom prior to initiation of the molting process. The molting stimulus is unknown. It appears not to be the absence of prey since a number began the process while potential prey were still available. Once attached the isopods remain in place and in typical isopod fashion carry out the molt in two stages-first the anterior portion of the body, then the posterior. Only one individual of nine completed the molt, within 4 days, and resumed swimming and feeding. The remainder either died or disappeared SEGAL: BEHAVIOR OF JUVENILE NEROCILA ACUMINATA PREYING ON FISH 359 and, I presume, were eaten. Thus, during this highly vulnerable period, the isopods themselves are subject to predation. They may also be preyed upon while in the water column. Tham (1973) reports that first and second larval instars (probably mancae) of Nerocila phaeopleura are eaten by their definitive host Chanda gymnocephalus prior to settling and crawling through the gill chamber into the buccal cavity (the isopod identification is un- doubtedly incorrect since all known species of Nerocila are surface dwellers). Mancae of Lironeca ovalis were also observed being eaten by young Morone saxatilis (striped bass) while others disappeared in tanks with M. saxati/is and M. americana (white perch) (Sandifer and Kerby, 1983). In this study, a number of non-molting juvenile N. acuminata disappeared when left overnight with their prey. Whether the isopods were alive or dead is unknown. I observed only one isopod caught while swimming by a Mugil cephalus; however, it was immediately expelled and continued swimming. Isopods as well as other Crustacea have been well documented as food for planktivorous fishes (Stepien and Brusca, 1985). While the behavior of fishes in response to large predators is well documented (Keenleyside, 1979), that to small predators has been neglected. Small predators, by virtue of their size, present special problems. If a fish is to take evasive action prior to an attack it must recognize the predator as such. There are indications, from this study, that naive fishes do not recognize small predators since none took evasive action either at the sight of swimming Nerocila acuminata or to transient contact with them. But fishes that had been attacked and fed upon did. However, since all directed mid-water attacks by isopods took place from the rear either above, below or to the sides, and none were unsuccessful, the question of the efficacy of evasive tactics, regardless of the prior experiences of the fish, may be moot. Following attachment, and apparently depending upon the degree of irritation (i.e., depending upon the site), a fish will utilize a second defensive option. It may dislodge the isopods by initiating any or all of the following: breaking the surface, swimming rapidly, thrashing the body, rubbing against the bottom or against objects on the bottom or dividing under sand. Both Tham (1973) and Sandifer and Kerby (1983) have witnessed some dislodgement tactics particularly when mancae attached on or near an eye. The tactic often worked. All seven prey species responded to juvenile Nerocila acuminata with a variety of such behaviors also with some success. Since there was no way to determine how securely any single isopod was attached to a fish, dislodgement predictability cannot be ascertained. Moreover, multiple attachments led to a decline in the vigor of dislodgement responses due either to fatigue or the debilitating effects of the wounds or the loss of blood. Thus, once attacked and fed upon, there is increased probability of further successful isopod attachments and feeding. Since the observations in this study were made on contrived interactions be- tween isopods and constrained fish, the numbers of the former may have been unnatural and the latter had no opportunity to exercise additional escape options, i.e., swimming away and/or hiding. In the natural environment the threat posed by micro predators may be considerably reduced. However, small fish, whether young or adult, are at greater risk since it may take only one feeding to kill.

ACKNOWLEDGMENTS

I wish to thank T. Bowman of the Smithsonian Institution and R. Brusca of the Los Angeles County Museum of Natural History for corroborating identification of the juvenile Nerocila acuminala. I further wish to thank R. Brusca for many helpful conversations and correspondence. He showed 360 BULLETINOFMARINESCIENCE,VOL.41, NO.2, 1987 splendid good grace with my persistent telephone calls. Both he and my colleague L. Allen read portions of the manuscript.

LITERATURE CITED

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DATEACCEPTED: January 2, 1987.

ADDRESS: Department oj Biology, California State University, Northridge, 18111 Nordhoff Street, Northridge, California, 91330.