Cnidosac morphology in dendronotacean and aeolidacean nudibranch molluscs: from expulsion of nematocysts to use in defense?

Nudibranchs (Mollusca, Gastropoda) feeding on tentacles and polyps of Cnidaria thereby ingest both latent and discharged nematocysts (NCs) along with the food mass. In eolid nudibranchs (Opisthobranchia, Aeolidacea), many of the undischarged NCs are transported to terminal cnidosacs in their body appendages (cerata) and incorporated as kleptocnidae for defense. In the present report, the occurrence and fate of NCs in the digestive tracts of eolids is compared with hydrozoan-feeding dendronotacean nudibranchs (Opisthobranchia, Dendronotacea), which may show more basic stages in the evolution of cnidosacs. Tomographic reconstructions of the distal tips of cerata were composed from series of semithin light microscopic sections, utilizing 3D-surface rendering software. Doto acuta (Dendronotacea, Dotidae) does not have cnidosacs; transmission electron micrographs show that the NCs are digested in lysosomes of digestive gland cells. In contrast, species of the genus Hancockia (Dendronotacea, Hancockiidae) have several small cnidosacs in each ceras; they accumulate NCs in the digestive cells, as well as in the cnidosacs. Many of the cnidosacs were found open to the exterior with NCs in the process of expulsion. These and other structural details suggest assigning a function of expelling the NCs to the Hancockia spp. cnidosacs. It is proposed that cnidosacs similar to those of Hancockia spp. provide a clue to understanding how the defensive function of eolid cnidosacs may have evolved. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Immature nematocyst incorporation by the aeolid nudibranch Spurilla neapolitana

Ultrastructural observations and histochemical analyses show the presence of immature nematocysts within the cnidosacs of the aeolid nudibranch Spurilla neapolitana. Quantitative and ultrastructural analyses indicate that the immature nematocysts mature within the cnidosacs. The incorporation of unfired immature nematocysts may be the basic mechanism that permits nematocyst uptake, storage and utilization by nudibranchs.     

Protection of human skin against jellyfish (Cyanea capillata) stings

In tropical and subtropical regions of the world, jellyfish stings cause fatalities by means of venom injecting nematocysts. For nematocyst discharge an adequate combination of chemical and mechanical stimulation is required. In order to test whether skin care products can protect against nematocyst discharge, we tested two sunscreens and one lotion applied to pieces of live human skin and exposed them to Cyanea capillata tentacles. (Test specimens were collected in 1990 along the shore of Rømø, Denmark and in the Flensburger Förde.) The fine structure analysis of the cnidom of C. capillata showed a high grade of variation in shape and size. The basic distinctive characteristic for stomocnides and astomocnides, the terminal opening at the tubule tip, could not be found. The identification of spines at the basal tubule of atrichous isorhizas suggested that these should be characterized as basitrichous isorhizas. An association between nematocyst morphology and a special function such as penetration or entanglement was not observed. All nematocyst types penetrated unprotected skin. Parafilm (an inert material) and unprotected skin substrates served as controls. The discharged nematocysts on the skin and Parafilm surfaces were counted using scanning electron microscopy. The percentage of discharged nematocysts on test substance protected skin surfaces ranged from only 7.7 to 38.2%, compared to 100% on the unprotected control skin. In addition to this marked reduction in nematocyst discharge, the relatively few discharged nematocysts on protected skin showed malfunctions, and the injection of venom would have failed because the tubules of the nematocysts did not penetrate the skin. The results indicate a general possibility that human skin may be protected against nematocyst discharge of jellyfish with the application of sunscreen or lotion.     

Ingestion of a medusa (Aegina citrea) by the nematocyst-containing ctenophore Haeckelia rubra (formerly Euchlora rubra): phylogenetic implications

The rare ctenophore Haeckelia rubra (formerly Euchlora rubra) has long been known to have nematocysts rather than colloblasts in its tentacles. Five specimens were collected in the San Juan Archipelago, Washington State, USA in 1980 and 1981, and their feeding behavior was observed in the laboratory. We found that H. rubra readily eats the tentacles of a medusa, Aegina citrea, whose nematocysts (apotrichous isorhizas) are nearly identical in morphology to the nematocysts of the ctenophore. When H. rubra was offered 16 other species of hydromedusae and 1 siphonophore in the laboratory, the ctenophores showed little or no tendency to ingest these potential prey items. In addition to its routinely positive response to A. citrea, the ctenophore could be induced by manipulation and starvation to accept and ingest bits of the bodies of 4 additional species of hydromedusae and 1 siphonophore. These results, combined with the histological and rearing experiments of other investigators, leave little doubt that the nematocysts in H. rubra are not endogenous, but are kleptocnidae similar to those nematocysts retained and subsequently used by some species of nudibranchs that feed on Cnidaria. A close phylogenetic link between the Cnidaria and the Ctenophora is most unlikely.     

Discharge characteristics of nematocysts isolated from acontia ofCalliactis parasitica

Experiments were performed on specimens ofCalliactis parasitica collected near Plymouth in April 1987 and in the Mediterranean between September 1987 and April 1988. Undischarged nematocysts (basitrichous isorhizas) were separated from the acontia of by using 1M glycerol, 1M citrate, 0.5% Nonidet or 0.5% Triton X-100 as isolating agents, or by freezing the acontial tissue. The mechanisms of extrusion were not studied. The effectiveness of 50 mM thioglycolate in discharging nematocysts isolated by the above methods and suspended in Ca- and Mg-free artificial sea water (ASW) was investigated. Nematocysts extruded in glycerol were more responsive to thioglycolate than those extruded in citrate. Capsules isolated in non-ionic detergents, however, and those obtained by freezing were not discharged by thioglycolate. If capsules extruded in glycerol were suspended in ASW and this was replaced with distilled water, they did not respond to thioglycolate. The effect of distilled water was only partly reversible. It was observed that these nematocysts, unlike those ofPelagia noctiluca andAiptasia mutabilis previously investigated, did not release measurable amounts of free Ca²⁺ during discharge. It is suggested that if calcium is not involved in the mechanism of discharge of isolatedC. parasitica nematocysts then some other ionic species may stabilize the resting condition of these capsules.     

Laboratory observations on the life history of Rhopilema verrilli (Scyphozoa: Rhizostomeae)

The life history of the scyphozoan Rhopilema verrilli is described from the planula to the young medusa stages. Planulae are retained within the gonadal tissue of the medusa until fully developed. On liberation, most planulae set and developed into small scyphistomae within 7 to 10 days. The scyphistoma differs from those of other described species in having an unusually large, clavate manubrium. The only means of asexual reproduction observed in the scyphistoma cultures involved the formation of podocysts. Strobilation was usually of the monodisc variety, although polydics strobilation was not infrequent. The process of strobilation was completed within 7 days at 20°C. Newly liberated ephyrae typically had 8 pairs of lappets, and 8 rhopalia. Ephyral development resembled that of the closely related rhizostome Rhizostoma pulmo. The cnidome of the planula and scyphistoma consisted of atrichous isorhizas (a atrichs) and microbasic heterotrichous euryteles, while that of the strobila and ephyra consisted of a atrichs, euryteles, and holotrichous haplonemesContribution No. 546 from the Virginia Institute of Marine Science, Gloucester Point, Virginia 23062, USA.     

Distribution of a population of annelid polychaetes in the “trottoir” of the midlittoral zone on the coast of North-East Spain,Western Mediterranean

The discharge and mode of action of the microbasic mastigophores (b-Rhabdoids) of Anemonia sulcata Pennant (Anthozoa) tentacles were examined using light and electron microscopical techniques. The statistical analysis of different morphological parameters of 100 nematocysts before and after their complete discharge revealed four different phases in the process of discharge. The first phase consists of the opening of the three opercular flaps. The second phase is characterized by the extrusion of the shaft. Osmotic pressure generated by the nematocyst propagates the extremely fast shaft discharge. The third phase, the evagination of the long tubule, is propagated by intrinsic mechanical forces stored in the tubule's twists and coils. During the process of evagination the tubule length increases by 95%, its circumference by 17%. Its forward motion is produced by the dilatation of its circumference and undoing of its twists. This motion is supported by the pressure which is generated by the hydration of the capsular matrix. The fourth phase consists in the extrusion of soluble capsular material at the tip of the evaginated tubule. During discharge the net volume of the nematocyst increases by 182%. This expansion is only possible if a high osmotic pressure is generated within the cyst. The extrusion of the shaft is the crucial phase in the discharge of the microbasic mastigophores, because without the puncture produced by the shaft the tubule is not able to overcome even surfaces of low viscosity media. The function of these tentacular nematocysts is the mechanical capture of prey organisms and, most probably, the introduction of unknown toxins into the latter.     

Population dynamics of Eudendrium racemosum (Cnidaria, Hydrozoa) from the North Adriatic Sea

Benthic suspension feeders in shallow waters develop in relation to the food availability and the variation of physical parameters giving rise to complex communities that act as a control factor on the plankton biomass. The aim of the work is to establish the role of the hydrozoan Eudendrium racemosum in the energy transfer from the plankton to the benthos in marine food chains of the North Adriatic Sea. This study highlighted that the hydroid biomass changed over time in relation to temperature and irradiance, and the highest abundance was observed during summer with about 400,000 polyps m⁻² (about 19 g C m⁻²). The population suffered an evident summer decrease in relation to a peak of abundance of its predator, the nudibranch Cratena peregrina, whose adult specimens were able to eat up to 500 polyps day⁻¹ and reached an abundance of 10 individuals m⁻². The gut content analysis revealed that the hydroid diet was based on larvae of other benthic animals, especially bivalves and that the amount of ingested preys changed during the year with a peak in summer when it was estimated an average predation rate of 13.7 mg C m⁻² day⁻¹. In July, bivalves represented over 60 % of the captured items and about 18 mg C m⁻² day⁻¹. Values of biomass of E. racemosum are the highest ever recorded in the Mediterranean Sea, probably supported by the eutrophic conditions of the North Adriatic Sea. Moreover, our data suggest that settling bivalves provide the greater part of the energetic demand of E. racemosum.     

Microalgal communities epibiontic on the marine hydroid <Emphasis Type="Italic">Eudendrium racemosum</Emphasis> in the Ligurian Sea during an annual cycle

The microalgal community associated with Eudendrium racemosum, a marine hydroid widely distributed in the Mediterranean Sea, was studied during an annual cycle, at monthly frequency, in a coastal station of the Ligurian Sea. Microalgae were represented mainly by diatoms, which exhibited higher abundance and biomass values between autumn and spring (max 46,752 cells mm⁻² and 1.94 μg C mm⁻², respectively), while during summer a significant decrease was observed (min 917 cells mm⁻² and 0.013 μg C mm⁻²). High levels of abundance of filamentous cyanobacteria were observed in summer. Spatial distribution of epibiontic microalgae showed a markedly increasing gradient from the basal to the apical part of hydroid colonies. Considering the growth forms of diatom communities, motile diatoms (mainly small naviculoid taxa) were the most abundant in all the periods. Adnate (Amphora and Cocconeis spp.) were distributed mainly in the basal and central part of hydroid colonies and showed two peaks (autumn and summer). Erect forms (mainly Tabularia tabulata, Licmophora spp., Cyclophora tenuis) were mainly distributed in the apical part of the colonies and showed their maximum densities in spring–summer. Tube-dwelling (Berkeleya rutilans, Parlibellus sp.) were observed at low densities throughout the study period, without any significant temporal or spatial variability. Comparing the microalgal communities on marine hydroid to those grown on mimic substrata placed in the sampling station during summer, significantly higher values of abundance were observed in the hydroid, suggesting that microalgae may benefit from the polyp catabolites. This fact was particularly evident for the adnate diatoms, whose temporal trend paralleled the cycle of hydroid host.     

Ultrastructural observations on prey capture and digestion in the scyphomedusa Aurelia aurita

Aurelia aurita medusae are able to catch their prey with their entire body surface. Catch efficiency in medusae caught in Kiel harbour in May 1985 was found to be highest at the tentacles and lowest at the subumbrella. Surface structures of the medusa as well as the cnidom are described by SEM observations. Microbasic heterotrichous euryteles and atrichous isorhizas were found. Discharged nematocysts on the prey's skin indicate different functions of the two types. The villi in the gastral cavity show a characteristic morphological differentiation that consists of a ciliated distal and a basal area covered by vesicles. Four types of glandular cells were identified by TEM observations. Mucous cell types preferably occur in densely ciliated areas. The presence of serous cells is restricted to the basal region of the gastral villi and gastral cavity where the extracellular predigestion takes place. The time of food passage in young medusae of A. aurita decreases from 19 h at 4°C to 4 h at 22°C.     

Marginal tentacles of the corallimorpharian Rhodactis rhodostoma. 1. Role in competition for space

The corallimorpharian Rhodactis rhodostoma (Ehrenberg, 1934) forms aggregations that dominate patches on some coral reef flats in the Red Sea. The outcomes and mechanisms of competition for space between this corallimorpharian and other sessile organisms are poorly understood. Polyps of R. rhodostoma were observed to overgrow zoanthids, hydrozoan corals, sponges and encrusting macroalgae on a fringing reef at Eilat, northern Red Sea. R. rhodostoma polyps also damaged, and in some cases overgrew, reef-building corals in the families Poritidae, Acroporidae and Pocilloporidae, most of which form branching colonies with small polyps that are subordinate in coral competitive hierarchies. In contrast, most stony corals in the families Faviidae and Mussidae had standoff interactions with R. rhodostoma, in which they prevented the corallimorpharians from damaging them or approaching closer than 1 to 3 cm. The latter corals are ranked at the top of competitive hierarchies for Indo-Pacific corals, and they form massive colonies of large polyps which may develop aggressive organs termed sweeper tentacles. Some soft corals that exude allelopathic chemicals also avoided overgrowth by the corallimorpharians. Tentacles along the oral disk margin of R. rhodostoma polyps were swollen and bulbous during contacts with cnidarians. These bulbous marginal tentacles had significantly thicker ectoderm and a higher proportion of holotrichous nematocysts than did the normally filiform marginal tentacles of R. rhodostoma polyps. It is concluded that, on the reef flat at Eilat, this corallimorpharian damages and overgrows a variety of sessile competitors, including branching stony corals, via the application of specialised marginal tentacles filled with penetrating nematocysts. R. rhodostoma is an intermediate competitor in the aggressive hierarchy among Indo-Pacific Anthozoa, including the reef-building corals. Received: 1 July 1998 / Accepted: 24 March 1999     

Nematocyst complements of nudibranchs in the genus <Emphasis Type="Italic">Flabellina</Emphasis> in the Gulf of Maine and the effect of diet manipulations on the cnidom of <Emphasis Type="Italic">Flabellina verrucosa</Emphasis>

Aeolid nudibranchs maintain functional nematocysts, which are sequestered from the nudibranchs’ cnidarian prey and provide protection against predators. Some species exhibit extensive variation in incorporated nematocysts, while others maintain a limited number of types. This study examines the apparent diversity in uptake and patterns of nematocyst incorporation among related species. Nematocyst complements were described for four Gulf of Maine nudibranch species in the genus Flabellina exhibiting a variety of feeding strategies and prey specificities. Diet manipulations were performed to examine the response to changing nematocyst availability using a generalist consumer, Flabellina verrucosa, to assess nematocyst uptake based on diet. The flabellinid species examined exhibited significant differences in nematocyst incorporation, reflecting differences in their specificity as predators and nematocyst types available in their natural prey. The nematocyst complement of F. verrucosa was the most variable and differed among collection regions. When diet was manipulated, nematocyst uptake depended on the prey the nudibranchs consumed, but when offered a variety of prey, F. verrucosa selectively preferred nematocysts from scyphistomae. The observed variation in nematocyst uptake among species and regions probably relates to environmental disparities among populations.     

A histological and chemical study of the cerata of the opisthobranch molluscTethys fimbria

Tethys fimbria is a poorly studied nudibranch that, recently, has been shown to possess large amounts of unusual prostaglandin (PG) derivatives. The cerata ofT. fimbria collected in the Bay of Naples (Italy) between November 1990 and January 1991 were submitted to histological analysis. This revealed the presence of both longitudinal and transverse smooth muscle fibres together with multicellular defensive glands, responsible for the typical spontaneous contractions of the cerata and for the extrusion of a hyaline secretion, respectively. A role forT. fimbria cerata in mollusc defense behaviour is suggested. Chemical analysis of the cerata led to the isolation of high levels of prostaglandin free acids (PGs) and of PG-1,15-lactones of the E, A and F series, whereas the secretion contained only PG-1,15-lactones of the E and A series. The latter were found to be toxic to mosquito fish at concentrations ranging between 1 and 10 µg ml^–1, whereas PG free acids and PG-1,15-lactones of the F series were not toxic. The possibility that PGE- and PGA-1.15-lactones areT. fimbria defense allomones is discussed.     

Mechanosensitivity in the model sea anemone <Emphasis Type="Italic">Nematostella vectensis</Emphasis>

Tentacles of the sea anemone, Nematostella vectensis, are covered with hair bundles. Hair bundles were deflected by water jets to test whether they are mechanoreceptors. Electrophysiological recordings confirm that deflections of hair bundles induce transients in membrane current. In a different species of anemone, hair bundle mechanoreceptors are known to change shape and responsiveness according to the activity of chemoreceptors that bind prey-derived compounds including N-acetylated sugars. In Nematostella, hair bundles significantly elongate upon exposure to NANA, an N-acetylated sugar. Based on a bioassay in which discharged nematocysts are counted in gelatin-coated test probes touched to tentacles, we find that NANA shifts vibration dependent discharge of basitrich nematocysts to lower frequencies overlapping those produced during swimming by known prey including planktonic crustaceans. Furthermore, we find for the first time that vibration detection extends at least 2.5 cm beyond the tentacle tips. Thus, Nematostella likely employs its hair bundles to detect swimming movements of nearby prey.     

Histological and biochemical bases of defense mechanisms in four species of Polybranchioidea ascoglossan molluscs

The multiple defensive strategies of four Mediterranean ascoglossan molluscan species, belonging to two different Polybranchioidea families, Cyerce cristallina (Trinchese, 1881) and Caliphylla mediterranea (Costa A., 1869) (Polybranchiidae), and Ercolania funerea (Costa A., 1867) and Placida dendritica (Alder and Hancock, 1843) (Stiligeridae), were studied. E. funerea, P. dendritica and C. mediterranea were collected from the lake Fusaro (Arco Felice, Naples) in 1992. C. cristallina was collected from Capo Miseno (Bay of Naples) in 1991. C. cristallina and E. funerea easily undergo autotomy of dorso-lateral appendages (cerata) followed by an extraordinarily quick (8 to 10 d) regeneration of the latter. Histological analyses showed the presence, at the basis of both normal and regenerated cerata of these species, of a muscular sphincter which facilitates the autotomic process. C. mediterranea and P. dendritica, which do not undergo autotomy, lack this anatomical feature but feed upon algae belonging to the Chaetomorpha and Bryopsis genera, and, as shown by electron microscopy studies, retain large quantities of chloroplasts which they use as camouflage amidst algae and to escape predation. E. funerea also exploits this behavioral defense mechanism. Histological investigations also revealed in the cerata of all four species several multi-cellular mucous glands responsible for the secretion of the slime typical for these molluscs. C. cristallina, E. funerea and P. dendritica secrete large amounts of slime, whose extracts displayed ichthyotoxic activity when assayed by the Gambusia affinis ichthyotoxicity assay, while extracts of C. mediterranea slime were not toxic. A chemical analysis of the slime, mantle and cerata led to the isolation of polypropionate - and -pyrones from all species except C. mediterranea. These secondary metabolites possess structures that differ only by the degree of methylation and the geometry of double bonds of the side chain and are specifically distributed in cerata and slime of C. cristallina and E. funerea. The pyrones also display different activities in the Hydra vulgaris regeneration assay and in the G. affinis ichthyotoxicity test and, depending on their structural features and tissue distribution, are likely to play a role either as defense allomones or as supportive inducers of cerata regeneration. In conclusion, combined biological observations and histological and chemical techniques generated valuable information on the defensive behavior of these four ascoglossan species which exploit various combinations of the same behavioral and/or chemical defensive strategies and thus successfully avoid predation.     

Host effect on size structure and timing of sex change in the coral-inhabiting snail<Emphasis Type="Italic"> Coralliophila violacea</Emphasis>

The distribution, size and reproductive characteristics of the snail Coralliophila violacea (Lamarck), which inhabits the surface of both the branching coral Porites nigrescens and the massive corals P. lobata and P. lutea, were surveyed to examine the host effect on: (1) population structure and (2) reproductive characteristics, including the size at sex change of symbionts. On branching hosts, most snails were solitary, whereas on massive hosts, most had formed multiple-snail patches. Significantly smaller snails as well as proportionally more females were found on branching than on massive hosts. Furthermore, the fecundity of the females on the branching hosts was significantly lower than that on the massive hosts. The size at sex change (male to female) of the snails was smaller on the branching hosts than on massive hosts. Patch composition differences can partly explain the smaller size at sex change for snails on branching hosts; however, there was also evidence that host morphology had a significant effect on the timing of sex change.Communicated by T. Ikeda, Hakodate     

Protective skin structures in shell-less marine gastropods

Skin and stomach epithelia of the four suborders of nudibranch gastropods (Mollusca, Opisthobranchia) are characterized by large intracellular ovoid disks, here called spindles. These spindles are an evolutionary novelty in the nudibranchs; in shell-less sacoglossan species they are missing. We here examined whether the distribution and occurrence of the spindles is consistent with the assumption of a protective role against discharging nematocysts of co-habiting and prey Cnidaria. Spindles were abundant in epidermal cells of regions exposed to nematocysts, such as the cerata, the lips, the edges of lamellate rhinophores, the surfaces of non-retractile gill-like organs, as well as in the stomach epithelium of eolid and dendronotacean species which feed on Cnidaria. While cells packed with spindles almost exclusively formed the epidermis of eolid and arminacean species, they were less numerous in the skin of dendronotacean and dorid species, where glandular cells predominated. The preponderance of either glandular or spindle cells suggests a dual complementary defense strategy, on the one side the production of mucus coats and aversive glandular secretions, on the other structural defensive devices that are cushion-like entities filled with inert grains.     

Transport of brine shrimps via the digestive system of migratory waders: dispersal probabilities depend on diet and season

Waterbirds are known to disperse invertebrate propagules that survive gut passage, but there is very little information about how the probability of dispersal changes at different times of the annual cycle when birds move in different directions, or how it is affected by changes in diet. We studied internal transport of brine shrimp Artemia cysts by migratory waders in the Odiel saltworks in south-west Spain. Viable cysts of parthenogenetic Artemia were abundant in the faeces and regurgitated pellets of redshank Tringa totanus, pellets of spotted redshank T. erythropus, and faeces of black-tailed godwit Limosa limosa during spring and/or autumn migrations in 2001–2002, but were not recorded during winter. Godwits did not produce pellets, and spotted redshank faeces were not sampled. Significant correlations between the number of cysts in a pellet or faecal sample and the proportion of that sample constituted by Artemia adults suggested that most cysts were ingested while in the ovisacs of gravid females. The proportion of cysts destroyed during digestion increased when accompanied by harder food items or grit, and when fewer cysts were ingested. The median number of intact cysts was higher in redshank faeces than in their pellets, but cysts extracted from pellets were more likely to hatch. A higher proportion of redshank pellets contained Artemia cysts in spring than in autumn, but more redshank migrated through the area in autumn. Significantly fewer cysts were recorded in redshank pellets in winter than in spring or autumn. Our results confirm that there is potential for long-distance dispersal of Artemia cysts via waders during both northwards (spring) and southwards (autumn) migrations.     

The role of isopods and amphipods in the initial fragmentation of eelgrass detritus in Nova Scotia,Canada

Daytime observations on the isopods Idotea phosphorea and I. baltica and the amphipod Gammarus oceanicus held in laboratory microcosms showed that I. phosphorea and G. oceanicus spent 45% and 30% respectively, of their active time feeding on dead, intact eelgrass leaves which had been recently released from plants. I. baltica spent 41% of its active time consuming intact green leaves. The shredding of intact dead leaves by I. phosphorea and G. oceanicus resulted in production of small detrital particles which were liberated from the faeces of the invertebrates and this type of feeding led to the breakdown of whole leaves. Field experiments which separated the effects of shredding by invertebrates and grinding by waves and ice on the loss of weight from leaf packs showed that relative to controls isopods significantly increased weight loss from dead leaves. Loss of weight from leaf packs exposed to both biotic and physical shredding forces was not significantly different from that found on those exposed only to shredding by isopods. However, trends in the data indicated that fragmentation of whole, dead leaves in the field probably is a result of the synergistic effects of shredding by invertebrates and physical factors, particularly ice grinding. The role played by invertebrates in fragmenting intact, dead leaves is discussed in the light of energy flow and nutrient cycling within seagrass systems.     

Lack of sequestration of host plant glucosinolates in <Emphasis Type="Italic">Pieris rapae</Emphasis> and <Emphasis Type="Italic">P. grarricae</Emphasis>

Summary. Sequestration of plant toxins in herbivores is often correlated with aposematic coloration and gregarious behaviour. Larvae of Pieris brassicae show these conspicuous morphological and behavioural characteristics and were thus suggested to sequester glucosinolates that are characteristic secondary metabolites of their host plants. P. rapaeare camouflaged and solitary, and are thus not expected to sequester. To test this hypothesis and to check the repeatabi-lity of a study that did report the presence of the glucosinolate sinigrin in P. brassicae, larvae were reared on three species of Brassicaceae (Sinapis alba, Brassica nigra and Barbarea stricta), and different leaf and insect samples were taken for glucosinolate analysis. The major host plant glucosinolates could only be found in traces or not at all in larval haemolymph, bled or starved larvae, faeces or pupae of both species or P. brassicae regurgitant. Haemolymph of both Pieris spp. was not rejected by the ant Myrmica rubra in dual-choice assays; the regurgitant of P. brassicae was rejected. This suggests the presence of compounds other than glucosinolates that might be sequestered in or produced by P. brassicae only. In faeces of both Pieris spp. a compound which yielded 4-hydroxybenzylcyanide (HBC) upon incubation with sulfatase was detected in high concentrations when larvae had been reared on S. alba. This compound may be derived from hydrolysis of sinalbin, the main glucosinolate of that plant. The unidentified HBC progenitor was apparently not sequestered in the two Pieris spp., and was not detected in faeces of larvae reared on B. nigra or B. stricta. Received 18 July 2002; accepted 11 September 2002.