Marine Biology (2004) 145: 265–276 DOI 10.1007/s00227-004-1327-5

RESEARCH ARTICLE

R. Morgan Æ M. Jangoux Juvenile–adult relationship in the gregarious ophiuroid Ophiothrix fragilis (Echinodermata): a behavioral and morphological study

Received: 21 October 2003 / Accepted: 4 February 2004 / Published online: 17 March 2004 Springer-Verlag 2004

Abstract Ophiothrix fragilis forms dense beds in the Juveniles and adults are closely associated with one North Sea–English Channel region, where juveniles are another, and both the morphology and behavior of exclusively found on adults. The aim of this study was to juveniles play an important role in that relationship. see how the behavior and morphology of juveniles could help elucidate the close juvenile–adult relationship found in this species. Juveniles are found on the disk, arms and in the bursae of adult conspecifics, the ones on the disks Introduction being significantly larger. No clear advantage seems to be gained by the juveniles being in the bursae, and their Aggregations in echinoderms are not uncommon. Rep- presence there is most likely due to juvenile movement resentatives from all five classes can form dense aggre- on adults and to and from adults. Hooked spines serve gates from a few hundred to a few thousand individuals as anchory organs during the early life of the juvenile, per square meter (Salsman and Tolbert 1965; Brun 1969; but growth of the arms enhances its anchory capabilities Dana et al. 1972). Such aggregations are either a re- and the hooked spines become secondary in that respect. sponse to environmental stimuli, social behavior or both They regress as ophiuroids become older, to the and can be short- or long-term events. A localized food advantage of the other spines used in suspension feeding. source or the period of reproduction can make a variety Juveniles are attracted to conspecifics, and true gregar- of species aggregate, but these will usually disperse once ious behavior has been observed. The tip of each arm, the food source has been depleted or reproduction has the terminal tentacle, plays a major role in distance taken place (Beach et al. 1975; Vadas et al. 1986; Sloan chemoattraction, with juveniles needing at least one in- and Northway 1982; Young et al. 1992). Long-term tact terminal tentacle to be able to initiate a response. aggregations can, however, be found, with some of the Electron microscope observations of the terminal ten- more impressive ones formed by ophiuroids (Brun 1969; tacle permitted us to recognize two different potential O’Connor et al. 1983; Fujita and Ohta 1989). receptor structures designated sta¨ bchens. The first pos- The brittle-star Ophiothrix fragilis forms dense beds, sesses one long projecting cilia and is mostly present reaching densities of up to 2,000 individuals m)2.Itisa around the base of the terminal tentacle, while the other suspension-feeding species, the individuals of which live has one to five short projecting cilia and is mostly found in hydrodynamically active areas rich in suspended on the tip. No receptors are found on the shaft. material. They would receive a high delivery rate of food Receptors are not associated with secretory structures. that, as Warner (1979) pointed out, is necessary to maintain the high densities at which they live. Social behavior would also be an important factor necessary Communicated by O. Kinne, Oldendorf/Luhe for such crowded populations to exist. Broom (1975) R. Morgan (&) Æ M. Jangoux showed that, in the field, adults are able to recognize Laboratoire de Biologie Marine (CP 160/15), conspecifics once they are in contact with them. Universite´ Libre de Bruxelles, 50 Avenue Franklin Roosevelt, In the North Sea–English Channel region, where the 1050 Brussels, Belgium main recruitment event occurs in September (Davoult E-mail: [email protected] Tel.: +32-2-6502970 et al. 1990), a large number of juveniles are found on the Fax: +32-2-6502796 adults. Juveniles are also found all year round on adults, M. Jangoux although in somewhat lower numbers than during the Laboratoire de Biologie Marine, Universite´ de Mons-Hainaut, main recruitment period (Smith 1940; Warner 1969, 20 Place du Parc, 7000 Mons, Belgium 1971). 266

In the majority of benthic invertebrates, juvenile niles were measured under a dissecting microscope using a micro- mortality is high and the postsettlement/juvenile period meter eyepiece, while calipers were used for the adult measurements. We used the method of Guille (1964) to measure the disk diameter. is an important one, regulating the whole population Results were analyzed by ANOVA at the P<0.05 level. Nonlinear (Gosselin and Qian 1997; Hunt and Scheibling 1997). regression analysis was done using the SYSTAT program. Juveniles were found only on adults in O. fragilis beds, the former being localized either on the arms, disks and/ or in bursae of the latter (Warner 1971; Davoult et al. Juvenile behavior 1990). This last observation has led some authors to believe that young juveniles move into the bursae after To test for juvenile movements within the adult population, five adult individuals deprived of juveniles were placed in an aerated metamorphosis to find shelter and an ample supply of aquarium (17·11·12 cm). Ten juveniles (1–1.5 mm disk diameter) oxygenated seawater (Smith 1940). This would imply a were subsequently deposited on the arms of the adults (two juveniles close relationship between adults and juveniles and, per adult). Every 2 days, for 10 days, the adults were examined supposedly, some adaptive particularities that help under a dissecting microscope, the aquarium was carefully checked and the locations of juveniles were noted. The adults could easily be maintain such a relationship. discriminated by the high polychromacy found in this species, and The aim of this study was to investigate how the the arms, by the presence of regenerating or broken ones. behavior and morphology of juvenile O. fragilis helps To test for any olfactory response in juveniles, a Davenport’s maintain the close juvenile–adult relationship found in Y-system was used (Davenport 1950). Figure 1 illustrates the experimental set-up. Twenty assays (each assay needing one juve- this species. nile with a disk diameter of 1–1.5 mm) were made per test, each assay lasting until the juvenile either made a choice or did not move for 20 min (for the different stimuli used see Table 3). The seawa- ter, sand, oysters and conspecific ophiuroids used for the tests came Materials and methods from the site of the adult population. Ophiocomina nigra came from a mixed Ophiothrix–Ophiocomina population situated in Brittany Specimens of Ophiothrix fragilis (Abildgaard, 1789) were collected (France). During the experiment the flow rate was maintained at ) monthly from January to December 2002 by SCUBA diving at approximately 50 ml.min 1. Wemeldinge (Netherlands), from a population situated on an sandy To evaluate whether the arm tip plays any role in chemorecep- bottom/oyster bed at a depth of 25 m. These were brought back to tion, tests using a Davenport’s Y-system were made with 20 juve- the marine biology laboratory of the Universite´ Libre de Bruxelles niles having intact arm tips or having two, four or five amputated and kept in a closed-circuit aquarium until further manipulations arm tips. Experiments with intact and amputated individuals were were carried out. performed using a single set of 20 juveniles (each juvenile with a disk diameter of 1–1.5 mm). After each amputation, individuals recov- ered for at least 3 h before the next experiment was carried out. Results of the Davenport Y-system tests were analyzed using a Location and biometry of juveniles Yate’s Chi-squared test at the P<0.05 level. After each collection trip, 150 adult individuals were carefully examined under a dissecting microscope for any juveniles present, and the location of the latter noted (either ‘‘apparent’’ on arms or Juvenile morphology disks or ‘‘cryptic’’ in bursae). To check for any juveniles present in the bursae, these were carefully opened up with the help of a scalpel. Juveniles were fixed in Bouin’s fluid in seawater, dehydrated in a The length of the longest arm and the disk diameter of the juveniles graded series of ethanol, critical point dried, mounted on stubs, and of the adults on which they were found were also noted. Juve- coated with gold and observed on a Jeol JSM-6100 scanning

Fig. 1 Davenport’s Y-system: aquaria in which different stimuli were placed (A, B) and receptacle in which water was collected (C ). Region 1 was where the juvenile Ophiothrix fragilis were placed at the start of the experiment. Choice between A and B was considered made once the disk of the juvenile was in region 2 (left or right) 267

Table 1 Ophiothrix fragilis. Location and mean disk diameter of The relative lengths of the arms and disks of juveniles juveniles from conspecific adults [s.d. standard deviation; * signif- are shown in Fig. 3. By following a single cohort of icantly different (ANOVA, P<0.05)] juveniles over time (Fig. 3; t1–t3), we could infer that Location on Percent Mean disc growth of the disk relative to the arms was reduced in adult juveniles diameter (s.d.) the early stages of juvenile development. However, be- cause of the low number of individuals in the cohort, ‘‘Apparent’’ juveniles Disc 32 2.78* (1.00) nonlinear regression analysis was carried out using the (n=152) Arm 63 1.91 (1.04) data from all juveniles. Nonlinear regression analysis ‘‘Cryptic’’ juveniles Bursa 5 0.96 (0.25) 2 (n=8) revealed highly significant fitting (R =0.445; F=1,090; P<0.001) of the model Y=r{1)exp[)k(X)xi)]}, with the following estimated parameters: the asymptotic ratio electron microscope. To observe the skeletal plates of juveniles, the arm length/disk diameter r (5.15±0.28, 95% confidence epidermis was partly digested by fixing specimens in Bouin’s liquid in distilled water for 7 days. These were then dehydrated in a interval), the growth rate constant k (1.70±0.62) and graded series of ethanol, critical point dried, mounted on stubs, the initial disk diameter when the lengths of the arms coated with gold and observed on a Jeol JSM-6100 scanning tend towards 0 xi (0.235±0.18). Under such conditions, electron microscope. the reduced growth of the disk relative to the arms oc- curs up to a disk diameter of approximately 1.75 mm, after which growth becomes isometric (the ratio arm Results length/disk diameter stabilizes around 5).

Location and biometry Behavior From January to December 2002, a total of 160 juveniles were counted on 1,800 adults, all months combined. Movements of juveniles on the adults and to and from Their locations are provided in Table 1. The majority of adults was apparent (Table 2). Since no juveniles were juveniles (95%) were ‘‘apparent’’ and found either on ever found in the aquarium, when the full count of ten the arms or disks of adults (see Fig. 4A, B), the juveniles could not be reached, it was supposed that the remaining 5% was ‘‘cryptic’’ and found in the bursae. unaccounted for juveniles were cryptic (inside one of the No seasonal differences were noted in the location of the adult bursae). Rate of movement was low, but the juveniles on the adults (Chi-squared, P>0.5). majority of juveniles actually moved during the 10 days The mean disk diameters of the juveniles from the of observation. A bursa was occupied by a juvenile only three locations (i.e. the disks, arms and bursae of adults) once, and the juvenile only remained cryptic for a short are given in Table 1. Juveniles on the arms and in the while (maximum of 2 days). Movements from one adult bursae were significantly smaller than the ones found on to another occurred three times. the disks (ANOVA, P<0.05). The largest cryptic juve- The results of the Davenport Y-system tests are nile had a disk diameter of 1.26 mm. Of apparent represented in Table 3. The first series of experiments juveniles, 26% were smaller than this. Juveniles, no was intended to test whether an olfactory response oc- matter their size, were found in the whole range of adult curred in juveniles in contact with the original substrata disk diameters, as illustrated in Fig. 2. (sand and oysters), i.e. that on which the population lived in nature (Table 3A). When water (natural seawater from the area), oysters, or sand was presented

Fig. 3 Ophiothrix fragilis. Growth of arms and disks in juveniles (n=160) (t1, t2 and t3 single cohort of juveniles followed over Fig. 2 Ophiothrix fragilis. Range of adult disk diameters on which time). A juvenile from the cohort is represented by a cross. The juveniles were found fitted curve is Y=5.15{1)exp[)1.7(X)0.235)]} 268

Table 2 Ophiothrix fragilis. Movement of juveniles on conspecific adults (two juveniles placed on each investigated adult) (D0 start of experiment; D2–D10 days of observation; solid box apparent juvenile)

to juveniles, no response was observed. The second individuals of the sympatric species Ophiocomina nigra. series was intended to determine whether juvenile Though the latter did not induce any response when O. fragilis are attracted by adult conspecifics, or by presented against seawater to O. fragilis juveniles, these

Table 3 Ophiothrix fragilis. Results of Davenport’s Y-system tests columns are the results of Yate’s Chi-squared statistical tests on: attractability of intact juveniles against non-ophiuroids, at- (1 comparisons between the number of times the juvenile starts tractability of intact juveniles against conspecific and non-conspe- moving when presented with a stimulus and the number of times it cific adults and attractability of intact and dissected juveniles starts moving without a stimulus; 2 comparison between the against conspecific adults (NIA number of arms with intact frequency of choices between A and B and the frequency 50%) terminal tentacle in tested juveniles; SW seawater). The last two

NIA Stimulus Assay Results Yate’s Chi2

Aquarium A Aquarium B A B Null 1 2

A. Attractability of intact juveniles against non-ophiuroids 5 SWSW202018 5 Sand SW 20 3 0 17 1.0000 0.0833 5 Oysters SW 20 1 2 17 1.0000 0.5637 B. Attractability of intact juveniles against conspecific and non-conspecific adults 5 O.nigraa SW 20 1 3 16 0.6579 0.3173 5 O.fragilisa SW 20 10 2 8 0.0029 0.0209 5 O.fragilisa O.nigraa 20 12 1 7 0.0011 0.0023 5 O.fragilisa O.fragilisb 20 4 13 3 0.0000 0.0291 C. Attractability of intact and dissected juveniles against conspecific adults 5 O.fragilisb SW 20 18 2 0 0.0000 0.0003 3 O.fragilisb SW 20 17 0 3 0.0000 0.0000 1 O.fragilisb SW 20 11 0 9 0.0069 0.0009 0 O.fragilisb SW 20 5 2 13 0.1299 0.2568 aTested adult individuals (n=10) bTested adult individuals (n=50) 269 reacted to conspecific adults whether they were used the base, the shaft and the tip (Figs. 5A, 6). The terminal against seawater, or against individuals of O. nigra. tentacle has two types of receptors (Fig. 5B). The first Furthermore, the juveniles could discriminate between a have a single long projecting cilia (ca. 6 lm) emerging large and small group of conspecifics, with a clear from a cuticular projection; these are commonly found preference for the former, indicating aggregating around the base, but can also occur on the tip of the behavior (Table 3B). tentacle (Fig. 5B, C, E). The second type consists of one In view of these results a third series of experiment to five short cilia (ca. 1 lm) arising from smaller cutic- was done; it was designed to locate the area responsible ular projection; these are quite common on the tentacle for the juvenile olfactory response. Most sensory tip, but can also be found around its base (Fig. 5B, E). receptors in echinoderms occur on ambulacral append- The shaft is mostly devoid of receptors (Fig. 5A, D). ages (e.g. Cobb and Moore 1986; Flammang et al. 1997). Figure 6 reconstructs the side and upper views of an We hypothesize consequently that olfactory receptors elongated and retracted terminal tentacle. When re- also would be located on these appendages, particularly tracted, only the base and tip of the tentacle are visible on those of the arm tips. Twenty juveniles were tested, (Figs. 5B, 6C). None of the receptors found seem to be one by one, against seawater versus a group of 50 con- associated to gland openings, and no such openings have specific adults, and all experiments of this series were been observed along the terminal tentacle by SEM done using the same set of juveniles and of adults. observations. Juveniles were first tested with their arm tips intact. Then, two of the five arm tips were amputated, and the juveniles were tested again. Similar experiments, using Discussion the same set of juveniles, were repeated after the indi- viduals had had four arm tips amputated, then all (five) Juvenile processes are known to be important in regu- of their arm tips. The results are presented in Table 3C. lating populations (Gosselin and Qian 1997; Hunt and The results indicate that at least one intact arm tip ap- Scheibling 1997; Fraschetti et al. 2003). The fact that in pears to be necessary to induce an olfactory response in the gregarious species Ophiothrix fragilis juveniles are juveniles. No significant differences were noted (Yate’s always found on adults (Warner 1971; Davoult et al. Chi-squared, P>0.05) between control experiments 1990) indicates an intimate relationship between the two (stimuli: water against water; Table 3A) and those age groups, which could have an impact on the survival experiments involving juveniles with all arm tips ampu- of the juveniles and, ultimately, on the future of the tated (stimuli: water against 50 conspecific adults; population. Table 3C). O. fragilis lives in hydrodynamically active areas that would require the young juvenile to be able to hold onto its conspecific host. The smallest juveniles have two Morphology: SEM observations comparatively large hooked spines on the penultimate article of each arm. These spines, reported by MacBride Figure 4C illustrates a partly digested and recently (1907), are used as organs of attachment, the young metamorphosed juvenile to show its skeletal organiza- juvenile being able to hook itself to the comparatively tion; this early juvenile has a disk diameter of 150 lm, larger spines of the adults. Hooked spines are found in and each of the five arms consists of a terminal article and other ophiuroids. Guille (1964) observed them on the a single adjacent ‘‘regular’’ article. As a juvenile grows, congeneric species Ophiothrix quinquemaculata, which more articles are added, and each new article arises in also lives gregariously and whose juveniles are found on between the terminal and the penultimate article before conspecific adults and on sponges. Juvenile Ophiomastix becoming itself the penultimate (Fig. 4D). The terminal annulosa that live in symbiosis with Ophiocoma scolo- article has an unpaired soft structure, the so-called ter- pendrina also possess hooked spines on their distal-most minal tentacle, while other articles have paired tube-feet articles (Hendler et al. 1999). As these authors suggested, (Fig. 4D, E, F). The terminal tentacle always appears hooked spines seem to be ideal anchory organs, at least thick and smooth at low SEM magnification, while tube- during the first part of the juvenile’s life. feet are more slender and develop papillae, the more Growth of the arms will enhance the anchorage proximal tube-feet having the more conspicuous papil- capabilities of the juvenile. Results have shown that arm lae. In early juveniles, the penultimate arm article bears a growth relative to disk growth is more important during pair of large, actinally located hooked spines, with a ta- the early post-settlement phase of the juvenile’s life. pered tip. In older juveniles the hooked spines of the Longer arms would help the juveniles to better cling to more proximate articles lose their characteristic shape the adults, making the use of the hooked spines sec- and become less developed than the other spines, which ondary. Indeed in developing juveniles, hooked spines grow in size and number (compare Fig. 4E and F). near the disk are less developed than the more distal SEM observations of the terminal tentacles at a high ones. In late juveniles hooked spines are still present magnification allowed the determination of different (Koehler 1969; present study), though reduced in size potential receptor structures, the locations of which compared to the neighboring spines, and they would no helped us divide the tentacle into three distinct regions: longer serve as anchorage organs. This is also seen in 270

O. annulosa, whose spines lose their hooked shape and c become straight once the juveniles have reached a size of Fig. 4 Ophiothrix fragilis. A, C–F SEM photographs; B light 3.8–4.5 mm in disk diameter, the size when they leave micrograph. A Juvenile on arm of conspecific adult (scale bar: 500 lm). B Juvenile on disk of conspecific adult (scale bar: 5 mm). their host (Hendler et al. 1999). C Recently metamorphosed juvenile with partly digested epidermis Suspension-feeding O. fragilis use their arm tube-feet (scale bar: 100 lm) (inset: hooked spine). D Older juvenile (scale and spines to capture particulate material (Warner and bar: 500 lm). E Distal articles of an arm of a juvenile (scale bar: Woodley 1975). Longer arm length would therefore 100 lm). F Proximal articles of an arm of a juvenile (scale bar: enhance their suspension-feeding capabilities, and pri- 100 lm) (a article; aa adult arm; ad adult; hs hooked spine; j juvenile; s spine; ta terminal article; tf tube-feet; tt terminal ority growth of the arms compared to the disk could be a tentacle) means of developing these feeding organs first. However, because arm growth is favored during the early juvenile’s stage, the arm length and the disk diameter. In Amphiura life (up to 1.75 mm disk diameter) and because juveniles filiformis, a suspension-feeder with a long life-span and are found on the adults until they reach 4 mm, this slow growth, the growth rate of the arms is higher than hypothesis is unlikely. that of the disk for a long period of time (Skold et al. Juveniles are probably opportunist-feeders on the 2001). This species, which lives buried in the sediment, adults, as has already been suggested by Warner (1971). would not need anchory organs, but would have to de- Adults can use their spines to collect suspended material, velop its arms to be able to feed in the water column. It the former eventually being cleaned off by their tube-feet would also be interesting to compare the juvenile growth (Warner and Woodley 1975). Yet, as most juveniles are in O. fragilis living solitarily in the Mediterranean and in found between the spines, this would enable them to clean O. fragilis living gregariously in the North Sea–English off the adult spines with their own tube-feet. In the Channel. Mediterranean, adult O. fragilis live solitarily under Arm growth will also enhance juvenile mobility, as rocks, but juveniles recruit massively and aggregate on shown in the results of experiments on juvenile move- sponges, while being mostly absent from the adjacent al- ment in an adult population (Table 2). Although gal turf (Turon et al. 2000); this suggests that juveniles are movement is reduced, juveniles have been found to move commensal of sponges. The fact that juveniles have not on the adults to and from each location (arm, bursae, been found anywhere other than on adults or on other disk) and to and from adults. The fact that juveniles suspension-feeding/filtrating organisms suggests that the found on the disk are significantly larger than those young are not able to feed by themselves and would need found in other locations might be an indication that the help of such a host. This hypothesis is supported by the movement towards the disk is preferred once juveniles fact that, in the case of the solitary O. fragilis, the juveniles have attained a size that would make it easier for them leave the sponges when they are still only 1 mm, but stay in to cling to the adult disk using their arms. This type of contact with them with their arms until they reach 4 mm, clinging posture has been observed in other juvenile an indication that they might still need the sponge for ophiuroids (McClintock et al. 1993; Hendler et al. 1999). feeding until that stage. Other species also recruit on Only juvenile mobility would explain their occurrence in adults or on filtering/suspension-feeders (Guille 1964; the bursae, as nothing can be said of the advantages they Patent 1970; Hendler 1984; Hendler et al. 1999). The might find there. Smith (1940) suggested that juveniles abundance of food provided by the host would not only might find refuge and a higher oxygen concentration in enhance survival and growth, but would make the initial the bursae, but if this were true we would expect to find growth of the juvenile arms unimportant as suspension- many more individuals there than were actually found feeding organs compared to organs of attachment. (only 5% of the investigated juvenile population was in It would, however, be interesting to compare the bursae). This hypothesis is justified by the fact that a growth of the arms and disk in juveniles of a variety of higher percentage of similar-sized juveniles were found species with contrasting modes of feeding and/or that outside the bursae than inside them. The largest cryptic live in contrasting habitats. Larval plasticity is known to juvenile measured 1.26 mm in disk diameter, but 26% of occur in echinoderms; during their development, echin- the apparent juveniles were smaller than this. oplutei vary the length of the arms they use in feeding , Movement of juveniles could become disadvantageous to adjust to the quantity of food available and to help as they may become more prone to being taken away by reduce the time spent in the plankton until metamor- the current. The fact that this species lives in high densities ) phosis (Pedrotti and Fenaux 1993; Fenaux et al. 1994). that can reach 2,000 individuals m 2 would reduce the The same could be true during the postlarval/juvenile risk of the juvenile being isolated from the group. How- period in ophiuroids, and a compromise between arm ever, if this should happen, they are capable of finding growth (feeding and locomotory organs) and disk their way back to the group, as juveniles are strongly growth (gonad development) might be observed. Time attracted to conspecifics, while not being attracted by to reproduction could be shortened or lengthened individuals of sympatric species (see Table 3). Working depending on the evolutionary direction of this com- on adult individuals, Broom (1975) demonstrated social promise. Unfortunately no true comparisons can be behavior through contact. He removed an individual made with other brittle-stars, as very few studies on from a patchy population and placed it downstream from growth rates have simultaneously examined the juvenile the nearest conspecific group. The individual did not 271

move upstream, but rather crossed the current and stop- with age. Thus, in adult O. fragilis, contact chemorecep- ped once it had, by chance, reached another conspecific. tion appears to be stronger than distance chemoreception, The fact that it did not move upstream suggests that dis- as has also been observed in some asteroids (Sloan and tance chemoreception in adults either no longer exists or Northway 1982). Another possibility is that chemore- that the threshold response for such chemoreception rises ception might vary depending on the internal and external 272

Fig. 5A–E Ophiothrix fragilis. A Profile view of the terminal tentacle of a juvenile (scale bar: 10 lm). B Receptors on the base of the terminal tentacle (scale bar:1lm). C Apical view of the terminal tentacle of a juvenile (scale bar:10lm). D Side view of the tip and shaft of the terminal tentacle of a juvenile (scale bar:1lm). E Apical view of the tip of the terminal tentacle of a juvenile (scale bar:5lm) (b base of the terminal tentacle; s shaft of the terminal tentacle; sl sta¨ bchen wih a long projecting cilia; ss sta¨ bchen with short projecting cilia; t tip of the terminal tentacle)

factors affecting the ophiuroid. It has been shown that of the initial migration and that once the ophiuroid attains intermediate-sized O. fragilis (3–8 mm disk diameter) a competitive size it would move back to the adult pop- migrate from the adult population to adjacent rocky ulation. We could therefore imagine a compromise be- outcrops and epifaunal clumps before moving back to the tween strong aggregative behavior and intraspecific adult population (Warner 1971). Warner (1971) suggested competition, whereby, depending on the state of the that intraspecific competition for food could be the cause ophiuroid, one might take precedence over the other. In 273

receptors found either on the spines or the tube-feet of the brittle-stars. Based on this focus, most authors consider receptors to play a role in feeding, by regulating the secretion of the mucus glands with which they are often associated. When they are not associated with glands, receptors are considered to play a role in either contact or distance chemoreception, but these are only suppositions based on their locations. Radial symmetry clearly offers an advantage in responding to a chemical gradient, and the presence of sensory organs at the extremities of the arms would maximize the reception of this gradient, independent of the orientation of the animal (Castilla and Crisp 1970). No previous study exists on receptors found on the tips of the arms in ophiuroids (Table 4). Our experiments demonstrated the major role of the arm tips of juvenile O. fragilis in detecting aggregates of conspecific adults. These experiments involved four tests using the same set of 20 juveniles. The low number of juveniles and the fact that we wanted to use a homogenous lot of individuals (1–1.5 mm disk diameter) compelled us to use the same set. However, we reduced stress by providing at least a 3- Fig. 6A–C Ophiothrix fragilis. Schematic representation of the h recovery period between each successive test. During terminal tentacle of a juvenile individual. A Side view when elongated. B Side view when retracted. C Apical view [sl sta¨ bchen this time, juveniles did not seem to change their behav- with long projecting cilia; ss sta¨ bchen with short projecting cilia (1– ior. Furthermore, dissections and handling did not have 5 cilia)] a long-term effect on the tested juveniles (all of them were still alive a week after the experiments). Lastly, the Mediterranean, O. fragilis also displays such migra- movement of amputated juveniles was not effected. The tions with age (Turon et al. 2000). The juveniles that results have shown that at least one intact arm tip is aggregate on sponges leave them once they have attained a needed to detect adult individuals and to stimulate size of 1 mm, but stay in contact with them with their movement toward them. SEM studies indicated that arms. At around 4 mm they then make a second migration each arm tip consists of a single distal ossicle associated to the adult habitat, under rocks. In this case, Turon et al. with an unpaired and thickened terminal tentacle. The (2000) suggested that migration from the sponge could be latter harbors potential ciliary receptors. These resemble due to more intense predation on them as they grow and the sta¨ bchens found on other ophiuroids, which have the need to find refuge habitat. Whether the aggregation been considered to be sensory structures (Table 4) of juveniles on sponges is due to intraspecific recognition (Reichenspurger 1908; Whitfield and Emson 1983). still needs to be established; however, we are again faced Sta¨ bchens are short, cone-like, cuticular projections, with a case in which external factors (predation) might usually having a single cilium emerging from the apex, modify the behavior of the ophiuroid. The need for and are often associated with secretory cells (Table 4) juveniles to find refuge habitats has been shown in a (Byrne 1994). However, in O. fragilis none of the sta¨ b- number of species that recruit in such environments and chens of the terminal tentacle are associated with then migrate from them to the adult habitat as they grow secretory cells. Yet, two types of receptors were seen, i.e. (Hendler and Littman 1986). In the North Sea–English sta¨ bchens with one to five short projecting cilia and Channel region the dense beds of adult ophiuroids could sta¨ bchens with a single long projecting cilium. The first be considered a refuge habitat for juveniles. Studies on are mostly found on the tip of the terminal tentacle, aggregative behavior and chemoreception under different while the others occur around its base. Furthermore, the external and internal conditions should be considered. single long projecting cilia are the longest (6 lm) that However, regardless of the type of chemoreception (con- have, until now, been described (Table 4). tact or distance) both imply the occurrence of specialized The receptors with short projecting cilia resemble the sensory receptors. sta¨ bchens previously described for Amphipholis squa- An overview of the major works on sensory receptors mata (Table 4) (Whitfield and Emson 1983), which, as in ophiuroids can be found in Table 4. The majority of for O. fragilis, are not associated with secretory cells. As these studies are based on either histological, scanning the receptors were observed on the tip of the spines, and/or transmission electron microscopy observations. these authors considered them to play a role in distance The putative role of the receptors are founded on the or contact chemoreception. A similar conclusion can be location of these on the animals and/or their association made for O. fragilis. The short ciliated sta¨ bchens are with other structures such as gland openings. Further- more commonly found at the tip of the terminal tentacle. more, most of these studies have revolved around The tip and the shaft can be retracted and are, therefore, 274

Table 4 Sensory receptors and their characteristics (? no information available; TEM transmission electron microscopy study; SEM scanning electron microscopy study; H histological study)

Species Type of receptor Cilia Length above Location Associated with Presumed Observation Reference cuticle secretory cells role

Acrocnida brachiata Sta¨ bchen ? ? Spines Yes, in patches Glandular H Buchanan (1963) Amphipholis squamata Sta¨ bchen 1 2.5 lm Arm spines Yes, 3–18 sta¨ bchen Regulating mucus TEM; SEM Whitfield and (common)–6 (except oral pair) near fibrillar gland secretion for Emson (1983) opening feeding Sta¨ bchen 2 2.5 lm Arm spine tips No, 1–3 grouped Chemical and contact (common)–6 (except oral pair) sta¨ bchen receptors reception Sta¨ bchen ? ? Spines Yes Glandular H Buchanan (1963) Amphiura chiajei Sta¨ bchen ? ? Spines Yes, in patches Glandular H Buchanan (1963) Amphiura filiformis Sta¨ bchen ? ? Spines Yes Glandular H Buchanan (1963) Hemipholis elongata Sensory-secretory 1 ? Tip of tube-feet Yes, 2 secretory Regulating mucus TEM; SEM Hadjuk (1992) complex cells for secretion for 1 sensory cell locomotion and feeding Monamphiura aster Sta¨ bchen ? ? Distal third of Yes ? H Pentreath (1970) the spines Ophiactis balli Sta¨ bchen ? ? Spines Yes Glandular H Buchanan (1963) Ophiactis resiliens Sta¨ bchen ? ? Spines Yes Glandular H Pentreath (1970) Ophiocomina nigra Hook-like 2 1.1 lm Proximal third No Monitoring depth of TEM; SEM Ball and structures of the spines; mucus on spines; Jangoux (1990) base of tentacle scales initiating cleaning response in podia Sta¨ bchen ? ? Tube-feet papillae Yes Glandular H Smith (1937) and knob Ophiopholis aculeata Sta¨ bchen ? ? Spines Yes Glandular H Buchanan (1963) Ophiothrix fragilis Sta¨ bchen ? ? Tube-feet papillae Yes Glandular H Smith (1937) Sensitive-glandular 1 ? Tube-feet Yes Regulating mucus H; TEM Martinez (1977) receptor secretion Cilia Groups of ? Tube-feet No ? 2–4 Sta¨ bchen 1 6 lm Terminal tentacle No Distance SEM Present study chemoreception Sta¨ bchen 1–5 1 lm Terminal tentacle No Contact chemoreception Ophiura ophiura Sta¨ bchen 1 3–4 lm Spines and tube-feet ? ? TEM; SEM Cobb and (sparse distribution) Moore (1986) Sta¨ bchen-like receptors 0–1 3–4 lm Spines and tube-feet ? ? Non-projecting 1 0 Tube-feet Yes, pair of cilia ? receptor with pair of secretory cells Non-projecting cilia 1 0 Arm surface No Photic receptor (especially tip) Sta¨ bchen ? ? Tube-feet Yes Glandular H Smith (1937) 275 highly motile, which is ideal for contact reception, as Fenaux L, Strathmann MF, Strathmann RR (1994) Five tests of they would be able to prod and test the substrata. On the food limited growth of larvae in coastal waters by comparisons of rates of development and forms of echinoplutei. Limnol contrary, the base cannot move independently of the Oceanogr 39:84–98 arm, but possesses receptors with long cilia enhancing Flammang P, Gosselin P, Jangoux M (1997) The podia, organs of the surface of the sensory structure. Their role in distant adhesion and sensory perception in larvae and post-metamor- chemoreception is presumed. phic stages of the echinoid Paracentrotus lividus (Echinoder- mata). Biofouling 12:161–171 No or very few receptors are found on the shaft. This Fraschetti S, Giangrande A, Terlizzi A, Boero F (2003) Pre- and could be due to the fact that for a structure to play a role in post-settlement events in benthic community dynamics. Oce- chemoreception it has to be in direct contact with the anol Acta 25:285–295 environment (Laverack 1974). This idea is supported in Fujita T, Ohta S (1989) Spatial structure within a dense bed of the that the only regions still visible when the terminal tentacle brittle star Ophiura sarsi (Ophiuroidea: Echinodermata) in the bathyal zone off Otushi, northeastern Japan. J Oceanogr Soc is retracted are the base and the tip, but not the shaft. 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