Setal morphology and cirral setation of thoracican barnacle cirri: adaptations and implications for thoracican evolution

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1 Journal of Zoology Setal morphology and cirral setation of thoracican barnacle cirri: adaptations and implications for thoracican evolution B. K. K. Chan 1, A. Garm 2 & J. T. Høeg 3 Journal of Zoology. Print ISSN Research Center for Biodiversity, Academia Sinica, Taipei, Taiwan 2 Department of Cell and Organism Biology, Lund University, Lund, Sweden 3 Department of Cell Biology and Comparative Zoology, Institute of Biology, University of Copenhagen, Copenhagen, Denmark Keywords barnacles; cirri; setae; scanning electron microscopy; functional morphology; phylogeny; adaptations. Correspondence Benny K. K. Chan, Research Center for Biodiversity, Academia Sinica, 128 section 2, Academia road, Taipei 115, Taiwan. chankk@gate.sinica.edu.tw Editor: Philip Rainbow Received 8 November 2007; revised 3 March 2008; accepted 5 March 2008 doi: /j x Abstract Thoracic cirripedes are sessile crustaceans that use six pairs of thoracic appendages (the cirri) to catch and handle food. We used scanning electron microscopy to examine the cirri, which include one to three pairs of maxillipeds in six species of thoracican barnacles, in search of correlations between cirral setation and feeding mode. The species studied comprise both pedunculate and sessile forms and represent a wide range of marine habitats as well as morphologies, viz., Ibla cumingi, Octolasmis warwickii, Capitulum mitella, Pollicipes polymerus, Tetraclita japonica japonica and Megabalanus volcano. Of the pedunculates, I. cumingi has the least complex setation pattern consisting of only serrulate types. This is consistent with its very simplified feeding mode and an apparent inability to discriminate between food items. Octolasmis warwickii is slightly more modified, while both P. polymerus and C. mitella have a more diversified setation. The balanomorphan species exhibit by far the most complex cirral setation. This is consistent with the several types of suspension feeding seen in these species, their ability to identify and sort captured food items and even to perform microfiltration in the mantle cavity using the setae on their three pairs of maxillipeds. Our results indicate that in thoracican barnacles, adaptations in feeding behaviour are associated with changes in the setation pattern of the cirri. In addition, the setal types and their distribution on the cirri are potential new characters in future morphology-based analyses of the phylogeny of the Cirripedia Thoracica. Introduction The pedunculate and sessile barnacles (Cirripedia Thoracica) differ from all other Crustacea in having permanently attached adults that use their six pairs of biramous thoracopods (called cirri) for suspension feeding (Anderson, 1994). Up to three of the anterior pairs of cirri can be modified to generally shorter maxillipeds that are specialized to transfer the food to the mouth. The long and slender, posterior cirri form a fan for capturing food from the water column (Darwin, 1852, 1854; Anderson, 1994; Video Clip in Online Supplementary Material). The rami are supported by a twosegmented protopod (pedicle), whose segments are presumed homologues with the coxa and basis of other crustaceans. Mapping cirral morphology onto current hypotheses of barnacle phylogeny (Perez-Losada et al., 2008) shows that it has changed through the evolution of the taxon. Pedunculate species have only a single pair of maxillipeds. They often feed by prolonged extension of the remaining cirri into the water column. Food captured in the cirral fan is transferred to the mouth parts using the basal setae on the second and third cirri, while the first cirri (maxillipeds) prevent the food from escaping (Anderson, 1980; Anderson & Southward, 1987; Anderson, 1994). Balanomorphan barnacles have the first two or three pairs of cirri modified into maxillipeds and they also exhibit a larger repertoire of cirral activities, including both prolonged extension, fast beating and the so-called pumping beat and strong pumping beat (see Anderson, 1994). Captured food is transferred to the mouth by a reciprocal rubbing action of cirri II and III (acting as maxillipeds), while cirrus I (closest to the mouth) holds the food and rejects unwanted food items (Anderson, 1981). In addition, the maxillipeds of balanomorphan barnacles can perform microfiltration using their setae to collect small-sized food particles within the mantle cavity (Anderson, 1981). Almost any function of crustacean appendages depends on various types of setae, which can serve a multitude of both mechanical and sensory functions (Felgenhauer, Watling & Thistle, 1989; Watling, 1989; Garm & Høeg, 2000, 2001; Garm, 2004a). Scanning electron microscopy (SEM) has previously revealed interesting differences in the setation of thoracican mouth appendages (Høeg, Karnick & Frølander, 1994). Surprisingly, the numerous studies of feeding behaviour in thoracican barnacles have never been supplemented by detailed SEM studies of cirral morphology 294

2 B. K. K. Chan, A. Garm and J. T. Høeg Setal morphology and cirral setation Table 1 Feeding mode, habitat and setal types on the cirri in the barnacle species investigated Taxonomic levels Species Cirral activity (see Anderson, 1994) Prolonged extension Extension beat Pumping beat Strong pumping Habitat Pendunculata Iblomorpha Ibla cumingi X Intertidal rock crevices Lepadomorpha Oclomasmis warwickii Scalpellomorpha Pollicipedidae Capitulum mitella Pollicipes polymerus Sessilia Balanomorpha Tetraclitidae Tetraclita japonica Balanidae Megabalanus volcano X X Epizoic on crab carapace X(1) X(1) Exposed rocky shores Exposed rocky shores X(1) X Exposed rocky shores X(1) X Exposed rocky shores Setal types see abbreviations Sr Sr Sr, MC Pa, Sr, MC Pl, Pa, Sr Se, Si, MC Pl, Pa, Sr, Se, Cu Diet (reflected from gut contents) Diatoms and tiny crustaceans (Klepal, 1985) Small particulate organic matter (Chan unpublished) Amphipods and copepods (Chan unpublished) Diatoms, detritus, large crustacea, copepods, shrimps, molluscs, Lewis (1981) Copepods, amphiods, diatoms (Hunt & Alexander, 1991) NIL Sr, serrulate setae; Pl, plumose setae; Se, serrate setae; Si, simple setae; Cu, cuspidate; MC, multicuspidate; (1), waves required; NIL, no information available. and setation. Our hypothesis is that the number, types and distribution of setae on the cirri will reflect adaptations to the various feeding modes that have emerged throughout barnacle evolution. To test this hypothesis, we used the phylogeny in Perez-Losada et al. (2004, 2008) to select and compare a limited number of species, which differ in the phylogenetic position, number of maxillipeds and modes of feeding (Table 1). Materials and methods SEM A total of seven species of barnacles were chosen to cover the major orders and families, including the Iblomorpha, Lepadomorpha, Scalpellomorpha and Balanomorpha (Table 1). The cirri of the barnacles were dissected and fixed in 2.5% glutaraldehyde (seawater based) for 2 h. The samples were then rinsed in distilled water for 30 s and dehydrated in a series of ethanol (30, 50, 75, 90, 95, and 100%), transferred to acetone for 30 min and critically point dried. The specimens were platinum coated and the cirri were observed under a scanning electron microscope (JEOL 840, Tokyo, Japan) fitted with a Semaphore s system (JEOL, Tokyo, Japan) for digital imaging. Video Specimens of the balanomorphan barnacle Balanus improvisus were collected in the Isefjord, Denmark, attached to stones. Their active feeding behaviour (without stimulation by water flow; see Trager, Hwang & Strickler, 1990) was recorded on videotape in the laboratory under fibre light. This species was not part of the SEM investigation, but the active feeding behaviour is similar to the one observed by eye in the balanomorphans studied here. Terminology The descriptions of the setal characteristics and terminology follow Garm (2004a,b), who revised and defined major types of crustacean setae. We are aware that many more 295

3 Setal morphology and cirral setation B. K. K. Chan, A. Garm and J. T. Høeg Figure 1 Representative types of setae in cirripedes. Scale bars in mm. subdivisions are possible in individual studies, but we doubt that further refinement of classificatory systems will serve a practical purpose here. We use the term setules for any ornament on a seta that has an articulated base, irrespective of its morphology. But setules can also sit directly on the cuticle of an appendage or the general body surface (for details, see Garm, 2004a). Pappose setae often have long and slender shafts with setules scattered along the entire length and they always lack a terminal pore (Fig. 1). Plumose setae have long setules along the entire shaft, arranged in two rows on opposite sides, and they also lack a terminal pore (Fig. 1). Serrulate setae are slim with a naked proximal part but with small setules distally, which can be arranged in rows (normally three) or scattered randomly along the shaft; their tips can be sharply pointed or with a terminal or a subterminal pore (Fig. 1). Serrate setae have two rows of densely packed denticles, which become smaller towards the tip; the distal half may also carry setules on the side of the shaft opposite to the denticles and the tip can have a subterminally or a terminally sited pore (Fig. 1). Simple setae have a long, slender and smooth shaft without any outgrowths, and they can have pointed tips or terminate with a pore (Fig. 1). Papposerrate setae are long and slender, with randomly scattered setules on the proximal half of the shaft and, as in some serrate setae, two rows of denticles on the distal half of the shaft. Cuspidate setae are robust with a length to width ratio below 8 (Fig. 1). Multicuspidate setae are robust with a series of very large denticles. The base is broad and the tip of the shaft can be pointed or end with pore (Fig. 1). The cirri were numbered I VI irrespective of whether they are modified into maxillipeds or not. The biramous thoracopods (cirri) of adult thoracicans differ from most other biramous crustacean appendages because the axis between the two rami (exopod and endopod) is rotated so that the exopod lies directly in front of the endopod. In this paper, the anterior side of a cirrus refers to the side facing the oral cone if the appendage is not curled up but is pointing straight away from the body, while posterior indicates the side then facing away from the oral cone. Thus, our anterior is comparable to the side of greater curvature and our posterior to the side of lesser curvature as used in some cirripede literature. Neither set of terms has much comparative value because in other Crustacea, or even in a barnacle cypris larva, our anterior is comparable to lateral and our posterior to medial as a result of the already-mentioned rotation of the cirral appendages. We use the term rami to comprise the endopod and the exopod of a cirrus. Results Ibla cumingi (Iblomorpha) All setae on the cirri are of the serrulate type. Cirrus I (maxilliped) The setae sitting on the posterior faces of the rami have two rows of distinct setules and sharply pointed tips (Fig. 2a and b). There are also comb-shaped denticles on the distal margins of thesegments(fig.2cirrusiincandcirrusiiine).thesetaeon the basipod have small and short setules (Fig. 2d). Cirri II VI The posterior sides of the rami have, at the junction of the segments, setae with two rows of fine setules on the shaft and also comb-shaped setules sitting directly on the distal margins of the ramal cuticle (Fig. 2e and h). The setae on the anterior side of the rami and on the basipod are long and slender with two rows of setules (Fig. 2f, g and i). Octolasmis warwickii (Lepadomorpha) All setae on the cirri are of the serrulate type. 296

4 B. K. K. Chan, A. Garm and J. T. Høeg Setal morphology and cirral setation Figure 2 Ibla cumingi. Setal morphology on cirri I and II. (a, b) Serrulate setae on the endopod and exopod of cirrus I. (c) Fan-shaped setules. (d) Serrulate setae on the basipod of cirrus I. (e g) Serrulate setae on cirrus II. (h) Serrulate setae on the posterior side of cirrus II. (i) Serrulate setae on the basipod of cirrus II. Ex, exopod; en, endopod; ba, basipod; pos, posterior aspects; ant, anterior aspects. Scale bars in mm. 297

5 Setal morphology and cirral setation B. K. K. Chan, A. Garm and J. T. Høeg Figure 3 Octolasmis warwickii. (a c) Serrulate setae on the exopod and endopod of cirrus I. (d) Short setules on the posterior surface of cirrus I. (e, f) Serrulate setae on cirrus II. Ex, exopod; en, endopod; ba, basipod; pos, posterior aspects; ant, anterior aspects. Scale bars in mm. 298

6 B. K. K. Chan, A. Garm and J. T. Høeg Setal morphology and cirral setation Figure 4 Capitulum mitella. (a, b) Serrulate setae on cirrus I. (c, d) Multicuspidate setae on the anterior side of cirrus II. (e, f) Serrulate setae on cirrus II. (g i) Serrulate setae on the posterior surface of cirrus II; arrows indicate the fine gap at the tip. Ex, exopod; en, endopod; ba, basipod; pos, posterior aspects; ant, anterior aspects. Scale bars in mm. 299

7 Setal morphology and cirral setation B. K. K. Chan, A. Garm and J. T. Høeg Figure 5 Pollicipes polymerus. (a c) Serrulate setae on cirrus I. (d) Serrulate setae on cirrus II. (e) Serrulate setae on the posterior side of cirrus II. (f) Multicuspidate setae on cirrus II. (g, h) Pappose setae on cirrus II. (i) Serrulate setae on the posterior side of cirrus II. (j) Serrulate setae on the basipod of cirrus II. Ex, exopod; en, endopod; ba, basipod; pos, posterior aspects; ant, anterior aspects. Scale bars in mm. 300

8 B. K. K. Chan, A. Garm and J. T. Høeg Setal morphology and cirral setation Figure 6 Tetraclita japonica japonica. (a) Serrulate setae on cirri I and II. (b) Plumose setae on the basipods of cirri I III. (c) Simple setae on cirrus II. (d) Serrulate setae on cirri II and III with open-ended tips. (e g) Serrulate setae with two rows of setules on cirrus III and open-ended tips. (h) Serrate setae. (i) Multicuspidate setae on cirrus III. (j) Fan-shaped denticles on the surfaces of exopod and endopod. (k m) Serrulate setae on the junctions between segments on cirrus IV. (n) Cuspidate setae on cirrus IV. (o) Serrulate setae on cirrus IV. Ex, exopod; en, endopod; ba, basipod; pos, posterior aspects; ant, anterior aspects. Scale bars in mm. 301

9 Setal morphology and cirral setation B. K. K. Chan, A. Garm and J. T. Høeg Figure 7 Megabalanus volcano. (a) Feathery plumose setae on the basipod of cirri I, II and III. (b, c) Serrulate setae on the exopod and endopod of cirri I, II and III. (d) Fan-shaped denticles on the surface of exopod and endopod on cirri I III. (e) Short setules on the segment junctions of the exopod and endopod of cirri I III. (f) Serrulate setae on cirrus III; these have terminal pores (G). (h) Serrulate setae on cirrus III. (i) Serrulate setae on cirrus IV. (j) plumose setae on cirrus IV. (k, l) Short flattened setae on the segments junction of cirrus IV. (m) Cuspidate setae on cirrus IV. Ex, exopod; en, endopod; ba, basipod; pos, posterior aspects; ant, anterior aspects. Scale bars in mm. 302

10 B. K. K. Chan, A. Garm and J. T. Høeg Setal morphology and cirral setation Cirrus I (maxilliped) The setae on the posterior edges of the rami have fine setules, loosely arranged in two rows (Fig. 3b). The setae on the anterior edges of the rami have two rows of densely arranged setules (Fig. 3a and c). All setae have a pointed tip (Fig. 3). Cirri II VI On cirrus II, the posterior edges of the rami have, at the junctions between the segments, setae with two rows of setules that are arranged in bundles (Fig. 3e). The posterior surfaces of the rami carry short (10 mm) simple denticles (Fig. 3d). The anterior edges of cirri II VI carry slender setae of various lengths (Fig. 3f). Capitulum mitella (Scalpellomorpha) The cirri carry both serrulate and multicuspidate setae. Cirrus I (maxilliped) The posterior edges of the rami have, at the junctions between the segments, bundles of serrulate setae with two rows of setules (Fig. 4b). Similar setae are present on the surfaces of the basipod (Fig. 4a). The anterior edges of the rami carry both serrulate (Fig. 4b) and multicuspidate setae (Fig. 4c and d) with two rows of large denticles (Fig. 4c and d). Cirri II VI Cirri II VI are similar in morphology, except that the exopods of cirrus II carry additional multicuspidate setae with typical double rows of denticles (Fig. 4c and d) and serrulate setae (Fig. 4e). The posterior edges of the rami have, at the junctions between the segments, two types of serrulate setae, both mm long (Fig. 4g i). One type has dense double rows of setules, but is naked posteriorly, and the pointed tip opens with a narrow gap (Fig. 4i). The other type of serrulate setae has a slightly swollen tip (Fig. 4h). There are also fanshaped denticles sitting directly on the distal margins of the segments (Fig. 4g). On the anterior face of cirri II VI, there are serrulate setae with three rows of sparse setules (Fig. 4f). Pollicipes polymerus (Scalpellomorpha) The cirri carry pappose, serrulate and multicuspidate setae. Cirrus I Cirrus I (maxilliped) carries serrulate setae only. The setae on the posterior sides of the rami are short (Fig. 5a and c). Both these setae and those on the basipod have two rows of small, sparsely spaced setules (Fig. 5a). The setae on the anterior edges have a single row of setules along the shaft (Fig. 5b). The surfaces of the rami are covered with fine fanshaped denticles on distal margins of the segments (Fig. 5c). Cirri II VI Cirrus II carries serrulate, pappose and multicuspidate setae (Fig. 5f). The posterior face of the rami have, at the junctions of the segments, short bundles (100 mm) of curved serrulate setae (Fig. 5e and i). The anterior edges of the rami carry serrulate setae, each with three rows of setules (Fig. 5d). In the middle region, the rami have pappose setae on the anterior edges (Fig. 5g and h). There are also multicuspidate setae with a large single row of denticles (Fig. 5f) and equipped with a terminal pore. The basipod carries serrulate setae only (Fig. 5j). Tetraclita japonica japonica (Balanomorpha) The cirri carry plumose, pappose, serrulate, simple, serrate and multicuspidate setae. Cirri I III (maxillipeds) On the basipod, these cirri carry plumose setae with fine setules on opposite sides, providing a feather-like appearance (Fig. 6b). On cirrus I, the posterior edges of the rami carry slender serrulate setae with two rows of setules and sharply pointed tips (Fig. 6a). There are also three to four rows of fan-shaped denticles (Fig. 6j) at the distal margins of the segments. The anterior edges of the cirrus I rami carry simple setae with a terminal pore (Fig. 6c and d). On cirrus II, the posterior edges of the rami have serrulate setae with double rows of setules (Fig. 6a and c). The edges of each annulus also carry two to three rows of fan-shaped setules (Fig. 6j). Cirrus III has a single row of fan-shaped denticles at the junctions between the segments. The setae on the anterior edges of the rami are serrulate with two distinct rows of sharp setules (Fig. 6e) and a terminal pore (Fig. 6f and g). The setae on the posterior edges are serrulate with three rows of setules (Fig. 6a) and robust, multicuspidate setae with large denticles (Fig. 6i). The multicuspidate setae are more abundant in the proximal regions of the rami. The middle and distal regions have robust, serrate setae with three rows of setules (Fig. 6h) and robust, multicuspidate setae with large denticles (Fig. 6i). Cirri IV VI Cirrus IV has bundles of serrulate setae at the junctions between the segments (Fig. 6k m). In the posterior region of the rami, these bundles consist of short (40 mm), robust and conically shaped cuspidate setae (Fig. 6n). This type has a few, small setules at the distal region of the shaft and a terminal pore (Fig. 6n). On the distal region of the rami, these bundles contain an additional type of slender setae that have small setules and flattened tips with a terminal pore (Fig. 6o). The anterior edges of the rami carry serrulate setae with two rows of setules (Fig. 6n). The morphology and distribution of setae oncirrusvandviisasoncirrusiv. 303

11 Setal morphology and cirral setation B. K. K. Chan, A. Garm and J. T. Høeg Megabalanus volcano (Balanomorpha) The cirri carry plumose, pappose, serrate, serrulate and cuspidate setae. Cirri I III (maxillipeds) On cirrus I, the basipod carries plumose setae with fine setules (Fig. 7a). The anterior and posterior edges of the rami carry long serrulate setae with fine setules and a pointed tip (Fig. 7b and c). There are also fan-shaped denticles at the distal margins of the segments (Fig. 7d). On cirrus II, the basipod has a patch of plumose setae on the posterior edge (Fig. 7a) and serrulate setae on the anterior edge (Fig. 7b and c). On the rami there are, at the junction of the segments, serrulate setae (Fig. 7c), fan-shaped denticles (Fig. 7d) and short flattened setules (Fig. 7e). The basipod of cirrus III carries plumose setae on the distal edge (Fig. 7a). At the base of the rami, there is a patch of serrulate and plumose setae. The anterior edges of the rami carry robust and thick serrulate setae with two rows of setules (Fig. 7f) and long and slender serrulate setae (Fig. 7h). The robust variety has a terminal pore (Fig. 7g). Cirri IV VI The basipod of cirrus IV carries plumose setae and serrulate setae (Fig. 7i and j). On the posterior edges of the rami, at the distal region of the segments, there are bundles of cuspidate setae with terminal pores (Fig. 7m). Towards the posterior region, there are additional long, serrulate setae with a flattened anterior region (Fig. 7k and l). The anterior region of the rami carries serrulate setae only (Fig. 7i). Discussion Among the species studied here, I. cumingi (Iblomorpha) and O. warwickii (Lepadomorpha) have the lowest setal diversity, carrying only serrulate setae on the cirri. Both Pollicipes pollicipes and C. mitella (Scalpellomorpha) have serrulate and multicuspidate setae, while the balanomorphan species have by far the highest diversity of setae, with simple, pappose, plumose, serrulate, serrate, multicuspidate and cuspidate setae in complex patterns of distribution. These observations suggest that, in thoracican barnacles, more diversified feeding habits (e.g. more diverse cirral activities of Tetraclita and Megabalanus) are associated with a more complex setation of the cirri. Setae can serve purely mechanical functions but can also be mechano-, thermo-, hygro- or chemoreceptors (Watling, 1989; Garm, 2004b). (Watling, 1989; Garm, 2004b). In a study on decapod setae, Garm (2004b) established seven setal types (serrulate, serrate, plumose, pappose, cuspidate, simple and papposerrate setae), which perform separate mechano-effectory functions (Garm, Hallberg & Høeg, 2003; Garm, 2004b; Garm, Derby & Høeg, 2004). In decapods, serrulate setae are used for the gentle prey handling (Garm, 2004b). Serrate setae are used for rough collecting and handling of prey. Plumose setae are mainly used for the generation of water currents and act as setal barriers. Pappose setae can be used for water current generation and filter-feeding purposes. Simple, cuspidate, multicuspidate and papposerrate setae are used for rough prey handling. Functions of setae have been little studied outside the decapods, but here we assume that the roles established by Garm (2004b) and Garm et al. (2003, 2004) are also valid for cirripedes. Ibla cumingi and O. warwickii are small barnacles. Ibla lives in small intertidal crevices, while Octolasmis lives within the branchial chambers of crabs (Table 1). The simple pattern with only serrulate setae suggests that food particles taken from the cirral fan are generally small-sized (Klepal, 1985) and that these barnacles do not handle large or highly mobile prey. The midgut caeca of Ibla are simple when compared with balanomorphan cirripedes, which suggests that the diet of Ibla is less complex than that of balanomorphan barnacles (Klepal, 1985). Compared with I. cumingi and O. warwickii, the scalpellomorphan and balanomorphan species studied here are large, intertidal species living on exposed rocky shores, where they rely on strong wave action to bring food items to their cirral net. Analysis of gut contents in C. mitella and Tetraclita indicates that amphipods and copepods comprise the main diet (Hunt & Alexander, 1991), and the presence of serrate and multicuspidate setae in these species dovetails with such an ability to feed on large-sized or mobile food items. Multi-cuspidate setae are also present in some small-sized notochamalid barnacles and Chthamalus malayensis (Foster & Newman, 1987; Southward & Newman, 2003), which suggests that these species are also capable of rough prey handling. Balanomorphan barnacles can, in addition to the other feeding methods, also perform microfiltration by using the first and second cirri (maxillipeds) to feed on phytoplankton and small-sized zooplankton trapped within the mantle cavity. The plumose setae that we describe on the basipod of cirri I III in Tetraclita and Megabalanus may well serve in such microfiltration but could also act as barriers preventing food escape (Anderson, 1994). Balanomorphan barnacles engage in active feeding in still water and passive feeding under strong water currents. With their extended surface, the plumose setae could also serve as mechanoreceptors detecting the strength of the water flow (see Trager et al., 1990). Chemosensitive setae often have terminal and sub-terminal pores (Garm, 2004b; Garm et al., 2005). Allison & Dorsett (1977) revealed that amino acids are important chemical stimuli in the feeding activities of barnacles. In our study, the balanomorphan species have several types of setae with terminal pores (serrate, serrulate, cuspidate), suggesting that they are involved in chemical evaluation of the prey before ingestion. Both Tetraclita and Megabalanus perform active pumping beat and prolonged extension of the cirri during feeding, and chemosensitive setae can assist in locating the position of the prey items caught on the cirri. Ibla and Octolasmis appear to lack setae with terminal pores on the feeding appendages. They often perform prolonged 304

12 B. K. K. Chan, A. Garm and J. T. Høeg Setal morphology and cirral setation extension of the cirri as a passive way to capture food, and the response to the prey items trapped in the cirral fan is probably mainly mechanosensitive. The Iblomorpha is the sister group to all other Thoracica and display many putative plesiomorphic traits (Klepal, 1985, Høeg et al., 1999; Perez-Losada et al., 2004, 2008; Buckeridge & Newman, 2006). The simplified setation found on the cirri of I. cumingi and the apparent inability to discriminate between various food items may therefore well illustrate the ancestral feeding mode in thoracican barnacles. Octolasmis is, in this respect, similar to Ibla. Pollicipes and Capitulum are both more closely related to the balanomorphan barnacles, and, like these, they also possess serrate and pappose setae (Pollicipes). This indicates that they have a more diverse diet in their highly exposed habitat in the rocky intertidal (Lewis, 1981), but they also retain the plesiomorphic pattern with the first pair of cirri and one of the rami of the second cirri serving as maxillipeds, which must limit the complexity of their food manipulation behaviour. The balanomorphan species have the most complex feeding apparatus. They possess two or three pairs of maxillipeds and a highly complex setation, enabling them to feed on a diversity of food items in terms of both type and size. In agreement with our observations from the cirri, Høeg et al. (1994) found that the mouth parts of balanomorphan species had a higher diversity of setae than seen in pedunculate forms, including Pollicipes. The complex apparatus for suspension feeding found in the Balanomorpha represents a very successful system and helps to explain the high number of species in this taxon, but thoracican barnacles can also obtain their food by a diversity of other putatively advanced feeding modes. Species of the Lepadidae, pedunculate barnacles in the neuston, are often large, and Lepas ansifera can prey on small fish (Jones, 1968). Other specialized forms seem to feed in a semi-parasitic fashion on tissues rasped off from their anthozoan or echinoderm hosts (Ross & Newman, 1969, Grygier & Newman, 1991, Yusa & Yamato, 1999). The feeding habits of the Scalpellidae, which inhabit the subtidal to deep sea, remain virtually unknown and the same is true for the Verrucomorpha, or asymmetric barnacles, which is the sister group to the Balanomorpha. Finally, except for the fine study by Achituv (1998), we are almost in the dark concerning the feeding biology of the highly specialized barnacles that settle on whales. For all these barnacle species, it would be interesting to study the setation pattern of the feeding apparatus and evaluate the degree to which it reflects the feeding biology. We conclude that there is a clear correlation between the complexity of cirral setation and diet in thoracican barnacles. Future investigations of feeding behaviour, coupled with detailed morphological studies of the cirri and the mouth parts, may well show that adaptations to specialized feeding have taken many different pathways in the Cirripedia Thoracica. Acknowledgements We would like to thank Dr Jane Walley (University of Wales Bangor) for constructive comments on the paper and Bjarne Bisballe (University of Copenhagen) for excellent assistance with the scanning electron microscope operation. 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