An Inducer of Molluscan Metamorphosis Transforms Activity Patterns in a Larval Nervous System.
MICHAEL G. HADFIELD 
Abstract. Larvae of the nudibranch mollusc Phestilla sibogae metamorphose in response to a small organic compound released into seawater by their adult prey, the scleractinian coral Porites compressa. The transformations that occur during metamorphosis, including loss of the ciliated velum (swimming organ), evacuation of the shell, and bodily elongation, are thought to be controlled by a combination of neuronal and neuroendocrine activities. Activation of peripheral chemosensory neurons by the metamorphosis-inducing compound should therefore elicit changes within the central nervous system. We used extracellular recording techniques in an attempt to detect responses of neurons within the larval central ganglia to seawater conditioned by P. compressa, to seawater conditioned by the weakly inductive coral Pocillopora damicornis, and to non-inductive seawater controls. The activity patterns within the nervous systems of semi-intact larvae changed in response to both types of coral exudates. Changes took place in two size classes of action potentials, one of which is known to be associated with velar ciliary arrests.
For a number of molluscan larvae, specific chemical compounds from the juvenile environment can act as chemosensory cues and trigger metamorphosis. For example, inductive compounds may be given off by the adult prey (Hadfield and Karlson, 1969; Hadfield, 1977, 1978; Chia and Koss, 1978, 1988; Lambert and Todd, 1994; Avila et al., 1996; Lambert et al., 1997), by adult conspecifics (Pechenik, 1980; McGee and Targett, 1989; Pechenik and Gee, 1993), by bacteria associated with adult conspecifics (Fitt et al., 1990; Tamburri et al., 1992), and by the algal food of the juveniles (Scheltema, 1961; Kriegstein et al., 1974; Switzer-Dunlap and Hadfield, 1977; Morse et al., 1979; Levantine and Bonar, 1986; Morse, 1990; Boettcher and Targett, 1996; Leise et al., 1996). In gastropods, sensory neurons that may mediate the induction of settlement and metamorphosis occur on the head, between the ciliated velar lobes (Bonar, 1978; Chia and Koss, 1982, 1984; Wodicka and Morse, 1991; Baxter and Morse, 1992; Uthe, 1995; Marois and Carew, 1997; Kempf et al., 1997), and on the foot (Chin and Koss, 1989). Our understanding of how these neurons function is still limited. Observations of Morse and colleagues (Trapido-Rosenthal and Morse, 1985; Baxter and Morse, 1987, 1992; Morse, 1990; Wodicka and Morse, 1991) strongly imply that receptors for lysine, an amino acid that modifies inducer reception, lie on chemosensory cilia in the apical sensory organ of larval abalone. If pre-competent nudibranch and abalone larvae are exposed to an inducer substance, they display habituation--that is, decreased rates of metamorphosis--when they reach competency (Hadfield, 1980; Hadfield and Scheuer, 1985; Trapido-Rosenthal and Morse, 1986; Avila et al., 1996). Habituation is thus a phenomenon associated with the morphogenetic pathway that directly initiates metamorphosis.
More recent studies are beginning to elucidate further internal mechanisms that are downstream from the chemosensory processes. These include changes in gene expression (Degnan and Morse, 1993, 1995; Degnan et al., 1997), protein synthesis, and second messenger levels (Inestrosa et al., 1993). Although the cellular circuitry that actually drives metamorphosis is still unknown, recent pharmacological studies have revealed some attributes of this pathway. Serotonin, which occurs widely in larval molluscan nervous systems (Goldberg and Kater, 1989; Marois and Carew, 1997; Kempf et al., 1997), apparently acts as a neurotransmitter or neuromodulator that promotes metamorphosis in the mud snail Ilyanassa obsoleta (Couper and Leise, 1996). The neurotransmitter dopamine appears to be necessary for metamorphorphosis in the nudibranch Phestilla sibogae and the slipper limpet Crepidula fornicata, whereas norepinephrine may endogenously inhibit this process in Crepidula (Pires et al., 1996, 2000). Nitric oxide appears t o be yet another endogenous inhibitor of metamorphosis, as shown by studies on Ilyanassa (Froggett and Leise, 1999). Yet, even with these recent advances, we still have much to learn about the integrative mechanisms that follow the reception of chemosensory information to produce, ultimately, a juvenile organism.
Electrophysiological studies conducted on a variety of molluscan veligers have also provided some insight into their neural activities. Rapid and coordinated velum-wide ciliary arrests are driven by action potentials in the ciliated cells of the preoral band (Mackie et al., 1976; Arkett et al., 1987), and ramp depolarizations can slow ciliary beating on a more localized level (Arkett et al., 1987). Thus, metachronal beating appears to be controlled by the relative depolarization of the ciliated cells and is modulated by excitatory neuronal input, presumably from the brain ganglia (Carter, 1926; Mackie et al., 1976; Arkett et al., 1987). These mechanisms are likely to be involved in the cessation of ciliary beating that accompanies larval settlement and crawling, behaviors that often precede metamorphosis. Barlow (1990) demonstrated that the ciliated velar cells in abalone larvae change their spiking activity only as an indirect response to the presence of the inducer substance. They do not act as sensory rec eptor cells. Arkett et al. (1989) recorded depolarizing receptor potentials from sensory neurons in nudibranch larvae in response to a settlement-inducing substance, although the use of cobalt anesthetic in their experiments limits the conclusions that can be drawn from their electrophysiological traces. Larvae of several molluscan species can be induced to metamorphose by an increase in external potassium ion concentration (Baloun and Morse, 1984; Yool et al., 1986; Pechenik and Heyman, 1987; Todd et al., 1991; Inestrosa et al., 1992; Pechenik and Gee, 1993), a classical method for depolarizing nerve cells (Nicholls et al., 1992), which again suggests that the peripheral nervous system, the larval central nervous system (CNS), or both are active during the initial phases of metamorphosis. If so, changes in the activity of central neurons, as well as in peripheral sensory receptors, should be detectable as they respond to a natural inducing substance.
The full range of metamorphic phenomena will most likely be controlled by neuroendocrine products as well as by classical synaptic interactions (Scheltema, 1974; Schacher et al., 1979), but molluscan metamorphosis includes at least two relatively rapid events that may be under direct neuronal control. These are loss of the velum, a process common to all molluscan veliger larvae, and shell dehiscence, which occurs in many opisthobranchs (Bonar and Hadfield, 1974; Hadfield, 1978). These events, in addition to the chemosensory initiation of metamorphosis, could involve neuronal networks within the CNS that drive appropriate effector organs. Indeed, Hadfield (1978) summarized data in support of the hypothesis that the nervous system was the most likely and sufficient regulatory system underlying all facets of metamorphosis in molluscs.
To learn more about the role played by the nervous system during the metamorphosis of marine invertebrates, we used larvae of a nudibranch mollusc, Phestilla sibogae, to study the response of the CNS to a natural metamorphosis-inducing compound. The scleractinian coral Porites compressa is the major prey for adult P. sibogae in Hawaii. A small organic compound that is a natural exudate from live P. compressa induces metamorphosis in developmentally competent larvae (Hadfield and Karlson, 1969; Hadfield, 1977; Hadfield and Pennington, 1990). Our extracellular recordings from the exposed dorsal surface of the brain ganglia provide evidence that activity patterns in the CNS change in the presence of the coral extract. We propose that the electrical changes we observed are associated with the initiation of metamorphosis, and that some of them are specific responses to larval exposure to P. compressa.
Materials and Methods
Veliger larvae of the nudibranch Phestilla sibogae Bergh were cultured in the laboratory in 0.2-[micro]m-filtered natural seawater (FSW) using previously described methods (Miller and Hadfield, 1986; Pires and Hadfield, 1991). During initial experiments, insufficient electrical activity was recorded from the epidermal surfaces of intact larvae, so we used an in vitro reduced preparation to maximize our ability to record spiking activity. To facilitate access to the larval brain, larvae without shells were used in all electrophysiological experiments. Deshelled larvae settle and metamorphose normally, although they do not undergo shell dehiscence (Pennington and Hadfield, 1989). Larval shells were decalcified by culturing about 100 larvae in a stender dish in 30 ml of artificial seawater (ASW) (Cavanaugh, 1956) lacking the usual 2.14 X [10.sup.-3] M sodium bicarbonate and buffered instead with 0.01 M Tris to pH 7.0 (Pires and Hadfield, 1993). Nine-day-old larvae were kept in ASW overnight so that metamorphica lly competent, shell-less, 10-day-old larvae were available as experimental subjects. About 70% of larvae cultured in this fashion had no shells 14 h after immersion. Deshelled larvae were rinsed in six changes of FSW over the following 2 h to reacclimate them to normal seawater (pH 8.3) before experimentation began.
Isolated larval heads (Fig. 1) were produced by chilling 20-25 individuals in FSW in a small petri dish in an ice water bath. Larvae became immobile as the FSW temperature approached 0[degrees]C. Small knives made from broken razor blades (Pires and Hadfield, 1993) were used to remove the visceral mass and foot from these cold, anesthetized larvae. This cut (line A in Fig. 1A) exposed the dorsal surface of the brain for extracellular recording, although it may have also eliminated part of the pedal ganglia. The eyes and statocysts remained in this isolated head preparation.
We also conducted experiments on animals from which only the visceral mass was removed (head-foot preparations). Results were similar, but we have chosen to leave those data unreported because fewer controls were conducted. Initial activity patterns in all experiments were somewhat varied (Fig. 3A, C, E, G), so data from different dissected veliger heads were not pooled.
Immediately after being cut, the chilled, isolated heads were transferred to fresh FSW at room temperature, whereupon they recovered normal metachronal beating of the velar cilia. Electrical recordings were made with a fire-polished glass micropipette suction electrode with an inner tip diameter of 40 to 50 [micro]m. The suction electrode was appressed to the exposed dorsal surface of the brain and gentle suction was applied to maintain contact between the electrode and the larval tissue.
Larvae were exposed to one of three experimental solutions: FSW, FSW containing the natural metamorphosis-inducing compound produced by Porites compressa Dana (ISW) or a similar exudate from the relatively non-inductive coral Pocillopora damicornis (PSW). PSW induces less than 30% metamorphosis compared to 90% induced by ISW (Hadfield, 1977). Adult P. sibogae do not use Pocillopora as prey (Hadfield, 1977). ISW and PSW were prepared by placing about 22 g of living coral into 250 ml of aerated seawater in a covered beaker. Coral tips were used to maximize the ratio of living tissue to skeleton. The coral was removed after 48 h and the resulting conditioned seawater passed through a 1.2-[micro]m filter. ISW and PSW were stored in the refrigerator and used within 48 h of production. Freshly made ISW normally induces more than 92% of 10-day-old intact larvae to metamorphose within 24 h. If the coral showed signs of ill health during preparation of ISW or PSW, the coral and solutions were discarded. Assays for th e metamorphosis-inducing capabilities of ISW and PSW were compared to FSW controls and conducted with intact larvae as previously described (Pennington and Hadfield, 1989). Assays were examined at 24 and 48 h and scored for number of larvae, juveniles, and empty shells. We also tested 34 isolated heads for their ability to metamorphose. These heads were cultured under sterile conditions for 48 h as previously described (Pires and Hadfield, 1993), then examined for loss of ciliated velar cells.
Electrophysiological data were recorded for 5-10 min before and after the addition of experimental solutions. The decision to expose each head to control or experimental solutions was made before recordings were initiated. Experiments were conducted in 35 x 10 min plastic petri dishes in about 6 ml of FSW. Changes to bath solutions were made manually; 4 ml of the bath solution were exchanged four times over the course of 1-3 min, during which time recording continued. Solution changes sometimes introduced mechanical artifacts, so results are reported for spiking activity occurring after solution changes were complete. Changes in spiking activity typically began 2-3 min after solutions first contacted the larval head. Data were collected from a new isolated head for each experiment, amplified through a differential AC amplifier (A-M Systems, Inc.), and recorded in digital format on videocassette tape through an Instrutech VR-100 PCM (pulse code modulation) device. This device has a manually operated event mar ker, or "cue" switch. When depressed, a positive 2.5-V deflection from ground is recorded on a separate channel on the videotape. Data were played back directly onto a Western Graphtek thermal chart recorder or, alternatively, collected on a 486 Insight computer and analyzed with the Enhanced Graphics Acquisition and Analysis (EGAA) software programs, ver. 3.50.02 (RC Electronics, Goleta, CA). Action potentials of different magnitudes were identified and counted using the EGAA Waveshape Recognition program, which stores start and stop times in digital data files. As necessary, files were converted to standard ASCII text format and analyzed further with Microsoft Excel 97 (Microsoft Corp.). Traces with relatively few spikes were analyzed directly from chart recorder records or the EGAA display screens. Two-sample analyses (two-tailed t tests) were conducted with Statgraphics Plus ver. 7.1 (Manugistics, Inc., Rockville, MD) or GB-STAT 6.0 (Dynamic Microsystems, Silver Spring, MD). Results were graphed with Delt aGraph 4.0 (SPSS, San Francisco, CA).
Extracellular recordings from the dorsal surfaces of brains in isolated heads of competent veliger larvae displayed two general sizes of spiking units in FSW (Fig. 2). Continuous recordings were made while the preparations were exposed to the various experimental solutions. The largest spikes, between 200 and 500 [micro]V, were associated with partial or velum-wide ciliary arrests that occurred spontaneously in all preparations (Figs. 2, 3; Mackie et al., 1976; Arkett et al., 1987). No stimulation was needed to elicit this activity. Initial patterns of activity in FSW were varied, but we recorded spontaneous ciliary arrest spikes (CASs) in all preparations (Fig. 3A, C, E, G). CAS activity typically occurred tonically, as relatively regular trains of single action potentials at 1 Hz or less. Spikes from smaller units (20-100 [micro]V) also occurred spontaneously, but with less regularity (Fig. 3A, C, E, G).
Ciliary arrest was often accompanied by a contraction of the entire velar lobe; during prolonged arrest periods the cilia and velar tissue were held in an upright position. At CAS frequencies below 1 Hz, velar cilia resumed beating between arrest spikes (Figs. 2; 3A, B). During spiking activity at frequencies above 1 Hz, cilia remained relatively motionless (Fig. 3F).
We compared firing rates of CASs and the smaller units (SUs) before and after addition of experimental and control solutions to 13 isolated heads. In one experiment, addition of FSW elicited statistically significant changes in firing frequencies of both CASs and small spikes (Figs. 4A, 5A). In the remaining two experiments, as expected, no statistically significant differences were seen in spiking activity after the addition of FSW (Figs. 3A, B; 4A; 5A).
In contrast to larval heads that were exposed to FSW, those exposed to ISW exhibited some type of statistically significant change in firing pattern, in either CASs, SUs, or both, in 6 of 7 experiments (Figs. 4B, 5B). In only one experiment, #90-61 (Fig. 3E, F), did we fail to observe any statistically significant differences in spiking activity in response to ISW. However, in this experiment, after the addition of ISW, CASs tended to occur in short bursts of 2-4 spikes (Fig. 3F). Short bursts of spikes elicited longer periods of ciliary arrest than did single CASs, and were often accompanied by contractions of the velar lobes. We observed similar results from preparations with an intact foot on several occasions (data not shown). In 4 of the 7 experiments, addition of ISW elicited a significant decrease in the frequency of CASs (Fig. 4B) and a change in the spiking activity of SUs (Fig. SB).
The addition of PSW to isolated heads elicited no statistically significant changes in firing rates (Figs. 4C, SC), but in all cases, PSW elicited a qualitative change in CAS activity. With PSW, the firing pattern of the CASs became irregular (Fig. 3H), which accounted for the significant increase in variance that occurred in all experiments (Fig. 4C). No such increase in variance was detected for the firing rates of small spikes.
Finally, we tested 34 isolated heads for their ability to metamorphose. The results were equivocal: four (12%) lost velar cilia, suggesting that isolated heads may be able to detect and respond to ISW, depending, perhaps, upon the amount of intact central nervous tissue. Because a large proportion (56%) died within 48 h, we cannot make a definitive conclusion about the metamorphic capabilities of isolated heads.
Metamorphosis in the nudibranch Phestilla sibogae is triggered by a chemosensory event, namely, the perception by a competent larva of a small organic compound given off by its adult prey, the coral Porites compressa (Hadfield and Scheuer, 1985; Hadfield and Pennington, 1990). In 6 of 7 experiments, we recorded statistically significant changes in electrical activity from in vitro heads of larval P. sibogae shortly after the addition of a metamorphic inducer. In 3 of the 4 experiments in which spiking activity in small units changed, activity increased. In 4 of the 7 experiments with ISW, firing rates of velar ciliary arrest spikes decreased. Although we did not record consistent responses from all preparations, it is clear that long-lasting changes in electrical activity are initiated within minutes of initial exposure to the coral inducer.
Competent larvae of P. sibogae display a rapid behavioral response to ISW that can be reliably observed under laboratory conditions (Koehl and Hadfield, unpubl. obs.). These larvae, which are negatively buoyant, stop swimming and rapidly sink when encountering ISW (Hadfield, unpubl. data). In the field, such a response would increase the chances of a larva contacting its adult food source. External signs of metamorphosis occur only 18-20 h after larvae have been exposed to an inducer substance for at least 4-6 h (Hadfield, 1977; Hadfield and Pennington, 1990). During this delay period, crucial physiological transformations and biochemical pathways must be activated as a prelude to the more obvious morphological transformations of metamorphosis.
The reduced preparation that we used may have produced neural activity different from that which occurs in an intact organism. The isolated heads retained most of the brain ganglia as well as intact velar lobes, eyespots, and statocysts. However, central circuits may have been damaged by a loss of ganglionic tissue, resulting in decreased connectivity and insufficient afferent information. This in turn may have led to unusual patterns of activity. Because we are reporting results from a relatively small number of experiments with a limited number of controls, we cannot fully explain the variability in endogenous activity, nor the variability in our results. The responses to Porites compressa that we recorded in four experiments would lead to an increase in larval sinking, but not to a complete cessation of ciliary beating, as seen in the behavioral responses mentioned above. This suggests that the isolated heads are not responding in a completely normal fashion.
Larval Phestilla can apparently differentiate between their adult prey and at least one other coral species in their reef habitat. In addition to positive metamorphic responses, negative responses to unfavorable or even potentially lethal juvenile environments have been reported for other invertebrates, including several polychaete species (Woodin, 1986, 1991; Woodin et al., 1993; Walters et al., 1996), bryozoan larvae (Walters et al., 1996), and veligers of the gastropod Ilyanassa obsoleta (Leise et al., 1996). The ability of Phestilla larvae to respond differentially to species of Porites and Pocillopora is thus not without precedent. How many coral species these small larvae can distinguish remains to be investigated.
Beat frequency of the velar cilia is modulated by excitatory neural input in veliger larvae of the snails Mangelia nebula (Mackie et al., 1976) and Calliostoma ligatum (Arkett et al., 1987) and the abalone Haliotis rufescens (Barlow, 1990). Velum-wide ciliary arrests are caused by an action potential that propagates throughout the velar ciliated cells. The large action potentials we recorded were always associated with ciliary arrests and were smaller than, but similar to, the signals recorded from the velum of Mangelia and Calliostoma (Mackie et al., 1976; Arkett et al., 1987). The exact origin of the large spikes in Phestilla is unclear; they may be the propagated action potentials of the ciliated cells, or a combination of these spikes plus the summed output of central activity that drives ciliary arrests. In her work with larval abalone, Barlow (1990) found that exposure to an inducer substance increased the likelihood and duration of ciliary arrests. In our experiments, we mostly observed a decrease in firing frequency of the CASs, which would lead to fewer, not more, ciliary arrests. Only the qualitative change to short bursts of CASs, as seen in some experiments (e.g., #90-61) would lead to longer ciliary arrests.
The behavioral relevance of the spiking activity in the smaller-sized units is unknown. We do not know if their activity arises from circuits that detect environmental odorants or drive motor activities, such as crawling or changes in swimming speed or direction. As elicited by ISW, the bursts of smaller action potentials are irregular, unlike bursts from any of the well-known molluscan motor systems (e.g., Getting and Dekin, 1985) or recently described olfactory circuits (Gelperin and Tank, 1990; Gelperin et al., 1993, 1996; Laurent and Davidowitz, 1994; Laurent et al., 1996; Delaney et al., 1994). Activity in the smaller larval units was also quite variable, with firing rates ranging from a few spikes per minute to hundreds per minute. We have no explanation for such variability, beyond suggesting that the amount of SU activity may reflect the amount of tissue lost during dissection. We also have no explanation for the increase in SU activity seen in one control experiment (Fig. 5A). Extracellular recordin gs from distal stumps of either the rhinophoral or oral-tentacle nerves of adult P. sibogae display changes in firing activity of small units in response to Porites compressa that are similar to the changes we record from SUs in response to ISW (Boudko and Hadfield, unpubl. data). We can only speculate that the SUs recorded from larval P. sibogae might indicate olfactory activity.
The high mortality rate that occurred in experiments on the metamorphic capabilities of isolated heads does not allow us to make a definitive statement about their ability to metamorphose. Isolated velar lobes do not metamorphose-- that is, they retain their ciliated velar cells in the presence of ISW--but such lobes lack the neural apparatus that can respond to a metamorphic inducer (Pires and Hadfleld, 1993). Although our results support the idea that larval perception of an inducer substance depends upon peripheral chemosensory neurons and central processing circuitry, an additional caveat is warranted. Suction electrodes do not provide a tight seal against passage of fluid between the bathing medium and the core of the electrode. Thus, in our experiments, ISW in the bath seawater could have been interacting directly with neurons of the CNS as well as with epidermal sensory neurons. Thus, the neural activity we recorded in response to ISW may or may not duplicate neural activity occurring within intact la rvae at the initiation of metamorphosis. Still, the responses we recorded suggest that the beginning of this process in Phestilla sibogae is accompanied by lasting changes in central neural activity.
We would like to acknowledge the late Dr. Robert Kane for many constructive discussions. We also thank Dr. Anthony Pires for advice, important technical suggestions, and help in conducting the in vitro induction experiments, Dr. Stephen Kempf for his critique of an earlier version of this manuscript, Dr. Louise Page for neuroanatomical insights, and Bryan Turner for technical assistance. This work was supported by ONR grant N00014-9 l-J- 1533 and NSF grants DCB-8903800 to M.G.H. and IBN-9604516 to E.M.L. We are also grateful to UNCG for providing equipment and a Research Assignment award to E.M.L.
(*.) To whom correspondence should be addressed. Present address: Department of Biology, University of North Carolina Greensboro, Greensboro, NC 27402-6174. E-mail: Esther_Leise@uncg.edu
Abbreviations: ASW, artificial seawater; CAS, ciliary arrest spike; CNS, central nervous system; FSW, 0.2-[micro]m-filtered natural seawater; ISW, Porites-conditioned seawater; PSW, Pocillopora-conditioned seawater; SU, smaller units.
(1.) Kewalo Marine Laboratory, Pacific Biomedical Research Center, University of Hawaii, 41 Ahui Street, Honolulu, Hawaii 96813; and (2.) Department of Biology, University of North Carolina Greensboro, Greensboro, North Carolina 27402-6174
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|Author:||LEISE, ESTHER M.; HADFIELD, MICHAEL G.|
|Publication:||The Biological Bulletin|
|Date:||Dec 1, 2000|
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