Variability in the chemical defense of the sponge Chondrilla nucula against predatory reef fishes

Chondrilla nucula is a common Caribbean demosponge that grows in a range of habitats, from coral reefs to mangrove swamps. On reefs, C. nucula grows as a thinly encrusting sheet, while in mangrove habitats it surrounds submerged mangrove roots as fleshy, lobate clumps. Previous feeding experiments using predatory reef fish revealed a high degree of variability in the chemical defenses of C. nucula. The present study was undertaken to determine whether a relationship exists between habitat, growth form, and chemical defense of C. nucula. Both laboratory and field feeding-assays of crude extracts confirmed that C. nucula possesses a chemical defense with high intercolony variability, but there was no significant variation in feeding deterrency between reef and mangrove habitats at either geographic location (Bahamas and Florida). Extracts of C. nucula collected during September and October 1994 from the Bahamas were significantly more deterrent than those collected during August 1993, May 1994, and May 1995 from Florida, and extracts of these spring and summer Florida collections were more deterrent than extracts of C. nucula collected in December 1994 and February 1995 in the same locations. There was no evidence that deterrent compounds were concentrated in the surface tissues of the sponge, or that chemical defense could be induced by simulated predation. Laboratory and field assays of the fractionated crude extract revealed that feeding deterrency was confined to the most polar metabolites in the extract. Field transplants were used to determine whether predation influenced the growth form of C. nucula. Uncaged sponges transplanted from the mangrove to the reef were readily consumed by spongivorous reef fishes. Lobate mangrove sponges became thinner after being caged on the reef for 3 mo, but encrusting reef sponges did not become thicker after being caged in the mangroves for the same period of time. Reef sponges that were caged for 3 to 15 mo thickened by only a small amount (<1 mm) compared to uncaged and open-caged (i.e. in cages lacking tops) sponges. Simulated bite marks on both reef and mangrove sponges were repaired at a rapid rate (0.8 to 1.6 mm d⁻¹). Fish predation has an important impact on the distribution and abundance of C. nucula, but the thin growth form common to reef environments may be more the result of hydrodynamics than of grazing by spongivorous fishes. Received: 6 October 1997 / Accepted: 19 March 1998     

Species-specific defense strategies of vegetative versus reproductive blades of the Pacific kelps <Emphasis Type="Italic">Lessonia nigrescens</Emphasis> and <Emphasis Type="Italic">Macrocystis integrifolia</Emphasis>

Chemical defense is assumed to be costly and therefore algae should allocate defense investments in a way to reduce costs and optimize their overall fitness. Thus, lifetime expectation of particular tissues and their contribution to the fitness of the alga may affect defense allocation. Two brown algae common to the SE Pacific coasts, Lessonia nigrescens Bory and Macrocystis integrifolia Bory, feature important ontogenetic differences in the development of reproductive structures; in L. nigrescens blade tissues pass from a vegetative stage to a reproductive stage, while in M. integrifolia reproductive and vegetative functions are spatially separated on different blades. We hypothesized that vegetative blades of L. nigrescens with important future functions are more (or equally) defended than reproductive blades, whereas in M. integrifolia defense should be mainly allocated to reproductive blades (sporophylls), which are considered to make a higher contribution to fitness. Herein, within-plant variation in susceptibility of reproductive and vegetative tissues to herbivory and in allocation of phlorotannins (phenolics) and N-compounds was compared. The results show that phlorotannin and N-concentrations were higher in reproductive blade tissues for both investigated algae. However, preferences by amphipod grazers (Parhyalella penai) for either tissue type differed between the two algal species. Fresh reproductive tissue of L. nigrescens was more consumed than vegetative tissue, while the reverse was found in M. integrifolia, thus confirming the original hypothesis. This suggests that future fitness function might indeed be a useful predictor of anti-herbivore defense in large, perennial kelps. Results from feeding assays with artificial pellets that were made with air-dried material and extract-treated Ulva powder indicated that defenses in live algae are probably not based on chemicals that can be extracted or remain intact after air-drying and grinding up algal tissues. Instead, anti-herbivore defense against amphipod mesograzers seems to depend on structural traits of living algae.     

Carbon conversion and metabolic rate in two marine sponges

The carbon metabolism of two marine sponges, Haliclona oculata and Dysidea avara, has been studied using a ¹³C isotope pulse-chase approach. The sponges were fed ¹³C-labeled diatoms (Skeletonema costatum) for 8 h and they took up between 75 and 85%. At different times, sponges were sampled for total ¹³C enrichment, and fatty acid (FA) composition and ¹³C enrichment. Algal biomarkers present in the sponges were highly labeled after feeding but their labeling levels decreased until none was left 10 days after enrichment. The sponge-specific FAs incorporated ¹³C label already during the first day and the amount of ¹³C label inside these FAs kept increasing until 3 weeks after labeling. The algal-derived carbon captured by the sponges during the 8-h feeding period was thus partly respired and partly metabolized during the weeks following. Apparently, sponges are able to capture enough food during short periods to sustain longer-term metabolism. The change of carbon metabolic rate of fatty acid synthesis due to mechanical damage of sponge tissue was studied by feeding sponges with ¹³C isotope–labeled diatom (Pheaodactylum tricornutum) either after or before damaging and tracing back the ¹³C content in the damaged and healthy tissue. The filtration and respiration in both sponges responded quickly to damage. The rate of respiration in H. oculata reduced immediately after damage, but returned to its initial level after 6 h. The ¹³C data revealed that H. oculata has a higher metabolic rate in the tips where growth occurs compared to the rest of the tissue and that the metabolic rate is increased after damage of the tissue. For D. avara, no differences were found between damaged and non-damaged tissue. However, the filtration rate decreased directly after damage.     

Oxygen dynamics and transport in the Mediterranean sponge Aplysina aerophoba

The Mediterranean sponge Aplysina aerophoba kept in aquaria or cultivation tanks can stop pumping for several hours or even days. To investigate changes in the chemical microenvironments, we measured oxygen profiles over the surface and into the tissue of pumping and non-pumping A. aerophoba specimens with Clark-type oxygen microelectrodes (tip diameters 18–30 μm). Total oxygen consumption rates of whole sponges were measured in closed chambers. These rates were used to back-calculate the oxygen distribution in a finite-element model. Combining direct measurements with calculations of diffusive flux and modeling revealed that the tissue of non-pumping sponges turns anoxic within 15 min, with the exception of a 1 mm surface layer where oxygen intrudes due to molecular diffusion over the sponge surface. Molecular diffusion is the only transport mechanism for oxygen into non-pumping sponges, which allows total oxygen consumption rates of 6–12 μmol cm⁻³ sponge day⁻¹. Sponges of different sizes had similar diffusional uptake rates, which is explained by their similar surface/volume ratios. In pumping sponges, oxygen consumption rates were between 22 and 37 μmol cm⁻³ sponge day⁻¹, and the entire tissue was oxygenated. Combining different approaches of direct oxygen measurement in living sponges with a dynamic model, we can show that tissue anoxia is a direct function of the pumping behavior. The sponge-microbe system of A. aerophoba thus has the possibility to switch actively between aerobic and anaerobic metabolism by stopping the water flow for more than 15 min. These periods of anoxia will greatly influence physiological variety and activity of the sponge microbes. Detailed knowledge about the varying chemical microenvironments in sponges will help to develop protocols to cultivate sponge-associated microbial lineages and improve our understanding of the sponge-microbe-system.     

Jasmonic acid treatment and mammalian herbivory differentially affect chemical defenses and growth of wild mustard (Brassica kaber)

Summary. Jasmonic acid (JA) is a wound-related hormone found in most plants that, when applied exogenously, can induce increases in levels of chemical defenses in patterns similar to those induced by mechanical damage or insect feeding. Relative to responses to insect and pathogen attack, chemical responses of herbaceous plants to mammalian herbivore attack have been little studied. In a field experiment, we compared the effects of JA treatment and naturally occurring mammalian herbivory on the expression of trypsin inhibitors, glucosinolates, peroxidase activity and growth of wild mustard (Brassica kaber). Exogenous JA significantly increased trypsin inhibitor activity and glucosinolate concentration, and moderately increased peroxidase activity in the eighth true leaves of five-week-old plants, relative to untreated controls. In contrast, levels of these chemical defenses in the eighth true leaves or in regrowth foliage of plants that had ∼80% of their leaf area removed by groundhogs (Marmota monax) did not differ from that in undamaged and untreated controls. Although exogenous JA significantly elevated levels of chemical defenses, it did not affect height of plants through the season and only slightly reduced time to first flower. Groundhog herbivory significantly reduced height and delayed or abolished flowering, but these effects were not substantial unless coupled with apical meristem removal. We hypothesize that the lack of effect of groundhog herbivory on chemical defenses may be due in part to the speed and pattern of leaf area removal by groundhogs, or physiological constraints caused by leaf area loss. Despite having no effect on chemical defense production, leaf area loss by groundhogs was more costly to growth and fitness than the effects of JA application in this study, but only substantially so if coupled with apical meristem removal. We suggest that in general, costs of defense production in plants are likely to be minimal when compared to the risk of losing large amounts of leaf area or primary meristematic tissue. Thus, if they are effective at deterring herbivory, the benefits of inducible defense production likely outweigh the costs in most cases. Received 20 December 2000; accepted 3 May 2001     

Biodiversity and rockfish recruitment in sponge gardens and bioherms of southern British Columbia,Canada

In the Strait of Georgia and Howe Sound, British Columbia, colonies of individual cloud sponges, growing on rock (known as sponge gardens) receive resource subsidies from the high biodiversity of epifauna on adjacent rock habitats. Bioherms are reefs of glass sponges living on layers of dead sponges. In the same area as the sponge gardens, newly discovered bioherms in Howe Sound, BC (49.34.67 N, 123.16.26 W) at depths of 28- to 35-m are constructed exclusively by Aphrocallistes vastus, the cloud sponge. The sponge gardens had much higher taxon richness than the bioherms. The sponge garden had 106 species from 10 phyla, whereas the bioherm had only 15 species from 5 phyla. For recruiting juvenile rockfish (quillback, Sebastes maliger), the food subsidy of sponge gardens appears to be missing on bioherms of cloud sponge, where biodiversity is relatively low. While adult and subadult rockfishes (S. maliger, S. ruberrimus, S. proriger, and S. elongatus) were present on bioherms, no evidence for nursery recruitment of inshore rockfishes to bioherms was observed, whereas the sponge gardens supported high densities of newly recruited S. maliger, perhaps owing to the combination of both refuge and feeding opportunities. These results indicate that sponge gardens form a habitat for early stages of inshore S. maliger, whereas A. vastus bioherms are associated only with older juvenile and adult rockfishes.     

Parrotfish predation on cryptic sponges of Caribbean coral reefs

Some sponge species that live in crevices in the reef frame appear to be restricted to their cryptic habitat by predation. When cryptic sponges were excavated, on Guigalotupo reef, San Blas, Panama, exposing them to potential predators, they were eaten by fishes that are generally considered to be herbivores, primarily parrotfishes of the genus Sparisoma: S.aurofrenatum (Cuvier & Valenciennes), S.viride (Bonnaterre), and S.chrysopterum (Bloch & Schneider). Of the 9150 bites observed to be taken by these species during paired (i.e., with sponges versus without sponges) trials conducted in defined feeding areas during 1986, 1987, and 1988, 72% (i.e. 6581 bites) were on cryptic sponges, even though these were only offered during half of the total observation time and never constituted more than 7% of the cover of the feeding observation areas. Individual parrotfish returned over and over to take bites of the exposed cryptic sponges until they were entirely consumed. They vigorously chased each other away from the sponges, but exhibited no such defense of their usual algal foods. A total of 18 sponge species were tested. Of the cryptic and semi-cryptic sponge species tested, only one of six was rejected by the parrotfish. Two of these six sponge species were consistently consumed entirely, and two were consumed entirely whenever their surfaces were sliced off with a razor blade, demonstrating that these sponges concentrate defenses against predators in their surfaces. One semi-cryptic species and one semi-exposed species were fed upon, but not entirely consumed. By contrast, 11 of 12 of the exposed and semi-exposed species were rejected. Cryptic sponges grew out of their cavities in the reef only when protected by seasonally thick mats of macroalgae or by cages that excluded fish. Received: 10 January 1997 / Accepted: 4 February 1997     

Within-plant allocation of a chemical defense in Secale cereale. Is concentration the appropriate currency of allocation?

Summary. The among-leaves allocation of DIBOA, a hydroxamic acid associated with plant resistance, in the shoot of rye (Secale cereale) was evaluated over the vegetative development of the plant. The appropriateness of using the concentration of secondary metabolites, DIBOA in this case, as the parameter to evaluate defense allocation in plants is discussed. Both biological and statistical arguments are put forward to suggest that allocation of chemical defenses should refer to absolute content and not to concentration. Results showed that leaf age was significantly linked to leaf concentration of DIBOA, young leaves having higher concentrations. In contrast, leaf content of DIBOA, our proposed currency of allocation, was not significantly higher in younger leaves. Furthermore, a regression analysis showed that the DIBOA content of leaves was better explained by the leaf relative biomass (proportion of shoot biomass) than by leaf biomass itself. It is suggested that, rather than leaf age, leaf relative biomass is the major factor determining DIBOA allocation in rye shoots. It is proposed that studies addressing within-plant defense allocation should use chemical defense content as the currency, emphasizing the major factors driving this process and its underlying mechanisms. Likewise, it is proposed that studies aiming at characterizing optimal patterns of plant defense should use chemical defense concentration as the currency, and be accompanied by evaluations of the actual resistance against herbivores of the plant parts analyzed, together with the effect on plant fitness. Received 19 February 1999; accepted 28 April 1999.     

Assessing the antipredatory defensive strategies of Caribbean non-scleractinian zoantharians (Cnidaria): is the sting the only thing?

The relative importance of chemical, nematocyst, and nutritional defenses was examined for 18 species of Caribbean sea anemones (actinarians), zoanthids, and mushroom polyps (corallimorpharians) from the Florida Keys and the Bahamas Islands, 2008–2010. Feeding assays were performed using the fish Thalassoma bifasciatum with artificial foods containing crude organic extracts of cnidarian tissues. A novel behavioral assay using brine shrimp nauplii was used to assess nematocyst defenses. The nutritional quality of cnidarian tissues was examined using bomb calorimetry and soluble protein assays. In general, actinarians invested in nematocyst defenses, zoanthids in either nematocyst or chemical defenses, and corallimorpharians lacked both, except for 1 of 3 species that was chemically defended. Relative to other coral reef invertebrates, cnidarian tissues had similar caloric values but lower soluble protein concentrations. Trade-offs between chemical and nematocyst defenses were observed for 65% of species, while habitat and behavior provided a likely explanation for undefended species.     

Chemical defense in pelagic octopus paralarvae: Tetrodotoxin alone does not protect individual paralarvae of the greater blue-ringed octopus (Hapalochlaena lunulata) from common reef predators

Some pelagic marine larvae possess anti-predator chemical defenses. Occasionally, toxic adults imbue their young with their own defensive cocktails. We examined paralarvae of the greater blue-ringed octopus (Hapalochlaena lunulata) for the deadly neurotoxin tetrodotoxin (TTX), and if present, whether TTX conferred protection to individual paralarvae. Paralarvae of H. lunulata possessed 150 ± 17 ng TTX each. These paralarvae appeared distasteful to a variety of fish and stomatopod predators, yet food items spiked with 200 ng TTX were readily consumed by predators. We conclude that TTX alone does not confer individual protection to paralarvae of H. lunulata, and that they possess an alternative defense. In larger doses, tetrodotoxin is a deterrent to the predatory stomatopod Haptosquilla trispinosa (mean dose = 3.97 μg/g). This corresponds to 12–13 paralarvae per predator based on the TTX levels of the clutch we examined. Thus, the basic assumption that individual paralarvae of H. lunulata are defended by TTX alone was disproved. Instead, functionality of TTX levels in paralarvae may arise through alternative selective pathways, such as deterrence to parasites, through kin selection, or against predator species not tested here.     

Susceptibility to oxidative stress of the Mediterranean demosponge Petrosia ficiformis : role of endosymbionts and solar irradiance

 The effects of elevated pO₂ and irradiance as inducers of prooxidant conditions have been investigated in the Mediterranean demosponge Petrosia ficiformis (Poiret, 1789). This species lives symbiotically with the autotrophic cyanobacterium Aphanocapsa feldmanni, the abundance of which is controlled by the intensity of light irradiance. In the presence of symbionts, tissues of P. ficiformis were characterized by a general enhancement of antioxidant defenses as compared to aposymbiotic specimens. The main differences included higher activities of several antioxidant enzymes and a greater capability to neutralize various forms of oxyradicals, as indicated by the total oxyradical scavenging capacity (TOSC) assay. Elevated pO₂, more than light, appeared to be the primary factor inducing prooxidant pressure in the Mediterranean sponge; in fact, irrespective of the solar irradiance experienced by the sponge, symbiotic specimens showed comparable activities of antioxidant enzymes and a similar scavenging capacity towards various reactive oxygen species. However, the potential toxicity of photodynamic production of reactive oxygen species was demonstrated in organisms from more irradiated sites, as the levels of antioxidant defenses were lowered in the outer layer of the sponge. The role of enhanced antioxidant defenses in protecting symbiotic specimens, also from oxyradical-mediated toxicity of light exposure, was supported by translocation experiments; aposymbiotic sponges did not survive when moved to conditions of elevated solar irradiance, while no effects were observed in symbiotic specimens if translocated and/or deprived of symbionts. Received: 23 November 1999 / Accepted: 13 June 2000     

High occurrences of brominated lipid fatty acids in boreal sponges of the order Halichondrida

Sponges of warm- and temperate- but also cold-waters are known to synthesize structurally diverse primary and secondary metabolites. These compounds fulfill a variety of functions including adaptations of the cell membranes to environmental conditions. We show here that boreal sponges of the order Halichondrida are rich sources of brominated lipid fatty acids. The comparison of lipid compositions of halichondrid Demospongiae from boreal and warmer waters indicates an accumulation of brominated fatty acids in sponges from colder settings. Moreover, the spatial distribution of brominated fatty acids in the sponge tissue of one widely distributed sponge of the North-East Atlantic (Phakellia ventilabrum) hints to a function related to membrane fluidity and permeability rather than to defense against predation. However, brominated fatty acids are diagnostic for the presence of bromoperoxidases in sponges and may therefore be potentially useful as markers in a chemical screening for secondary metabolites of pharmacological interest.     

Size-specific changes in amount of lipid and daily growth rate of early larval sardine, Sardinops melanostictus, in the main Kuroshio Current and its offshore waters off eastern Japan

The size-specific nutritional conditions of larval sardines, Sardinops melanostictus, from the main Kuroshio Current and its offshore waters off eastern Japan were assessed by lipid analysis. A rapid lipid analysis technique (diagnostic kits for human serum lipids) was used to measure the different lipid components of individual sardine larvae as indicators of their nutritional condition. Size-specific growth trajectories of individual larvae were estimated by the biological intercept method, and the recent daily growth rate of standard length (SL) was calculated from the 3 d outer increment width on the otolith. Relationships between the amount of larval phospholipid (PL; tissue weight indicator) and SL, and the recent daily growth rate of larva (Gr) and SL, could be expressed by the equations PL = 0.459 SL ^1.77 and Gr = 0.0809 SL − 0.341, respectively. There was no notable difference in these values between the two survey areas. The relationship between the amount of triglyceride (TG) and SL could be expressed allometrically (TG = 0.013 SL ^2.63). The relationship between the index of starvation tolerance (TG/PL) and SL could be expressed by the allometric equation TG/PL = 0.0288 SL ^0.865, suggesting that larger sardine have a higher starvation tolerance than smaller sardine. The TG of the 8 to 9 mm SL size-class larvae in the offshore area was higher than in the main Kuroshio Current. To test whether the TG for each larval sardine in the 8 to 9 mm SL size-class could be correlated with variables (temperature, chlorophyll a and distribution density of the sardine larvae) measured at the sampling stations, correlation analyses were performed. A highly significant negative correlation between TG and distribution density of the sardine larvae was found. A density-dependence effect seemed to influence the fluctuation of the larval storage energy component for short-term needs. Received: 12 March 1998 / Accepted: 26 March 1999     

Cytotoxic haplosclerid sponges preferred: a field study on the diet of the dotted sea slug<Emphasis Type="Italic"> Peltodoris atromaculata</Emphasis> (Doridoidea: Nudibranchia)

Trophic specializations are widespread among opisthobranch molluscs. One purported example from the Mediterranean Sea is the dotted sea slug Peltodoris atromaculata. It has been hypothesized that this species is strongly monophagous on the sponge Petrosia ficiformis. However, the small amount of evidence that has been found for this hypothesis is based just on laboratory tests. Here we study the feeding habits and the diet of Peltodoris atromaculata in its natural habitat. We observed and videotaped 161 individuals together with the organisms on which they were found (their living substrata). Feeding scars were identified and videotaped as well. Individuals and their living substrata were sampled for further analysis in the laboratory. The composition of faeces of Peltodoris, especially undigested sponge spicules, was analyzed by light and scanning electron microscopy and compared to the composition of the living substrata. Most of the faecal samples consisted of undigested sponge spicules. Although Peltodoris was found on 11 species of sponges, only 2 of them, Petrosia spp. and Haliclona fulva, form its diet (76% out of n=121 samples). In accordance with this, feeding scars in the habitats were exclusively observed on these two sponges. Estimation of electivity indices suggests that Haliclona is preferred over Petrosia. One remarkable feature of the exclusive feeding of Peltodoris on Petrosia and Haliclona is that both sponges share specific fulvinol-like polyacetylenes that show cytotoxic activity in bioassays. Potential benefits and evolutionary aspects of this trophic specialization are discussed. Besides sponge-containing faeces, we found spicule-free faeces (24%, n= 29). These were very small in volume compared to sponge-containing faeces, and only few distinct structures were present. However, the use of food other than sponges is not necessarily indicated by this, because the spicule-free faeces might also represent left-overs from the stomach and digestive gland after sponge spicules have been released.Communicated by O. Kinne, Oldendorf/Luhe     

Chemical defense and evolutionary trends in biosynthetic capacity among dorid nudibranchs (Mollusca: Gastropoda: Opisthobranchia)

Summary. An evolutionary scenario incorporating recent advances in phylogenetic research begins with an opisthobranch-pulmonate common ancestor that was herbivorous and had some diet-derived chemical defense. The Nudibranchia and their closest relatives, the Notaspidea, form a lineage the ancestors of which had switched to feeding upon sponges and deriving protection from metabolites contained in them. Subsequently there have been repeated shifts in food and defensive metabolites, and trends are evident in the ability to detoxify, sequester and utilize metabolites from food, as well as to synthesize defensive compounds de novo. The Notaspidea display a minor adaptive radiation that foreshadows a more extensive one in the various lineages of nudibranchs. This review emphasizes changes that have occurred within the Holohepatica, or dorid nudibranchs (order Doridacea). Their sister-group, the Cladohepatica, consists of three other orders, Dendronotacea, Arminacea, and Aeolidiacea, in which there has been a shift from sponges to Cnidaria as food. The Dendronotacea often feed upon Octocorallia, which combine spicules, chemical defense, and stinging capsules and thereby suggest a transition from feeding on sponges. A previous diet of Octocorallia is suggested by the defensive use of prostaglandins in the dendronotacean Tethys fimbria, which eats crustaceans. A shift to bryozoans in some Arminacea is accompanied by use of different metabolites. Dorid nudibranchs evidently began as sponge-feeders, but some lineages have shifted to a variety of other food organisms, and others have specialized in the kind of sponges they feed on and how they do it. There have been shifts to bryozoans (Ectoprocta) and ascidians (Chordata: Urochordata) that track metabolites rather than the taxonomy of the food. There is a crude correlation between the genealogy and the defensive metabolites of the sponge-feeding dorids. De novo synthesis is well documented in this order and the metabolites are appropriately positioned so as to have an adaptive effect. The hypothesis that the capacity for de novo synthesis was acquired by gene transfer across lineages is rejected, partly on the basis of different chirality of metabolites in the nudibranchs and their food organisms. Instead it is proposed that there has been a preadaptive phase followed by evolution in a retrosynthetic mode, with selection favoring enzymes that enhance the yield of end products that are already present in the food. Received 5 February 1999; accepted 26 July 1999     

Sponge-feeding fishes of the West Indies

In an analysis of the stomach contents of 212 species of West Indian reef and inshore fishes, sponge remains were found in 21 species. In eleven of these, sponges comprised 6% or more of the stomach contents; it is assumed that these fishes feed intentionally on sponges. Sponges comprise over 95% of the food of angelfishes of the genus Holacanthus, over 70% of the food of species of the related genus Pomacanthus, and more than 85% of the food of the filefish, Cantherhines macrocerus. Lesser quantities of sponges are ingested by the remaining fish species. Fishes that feed on sponges belong to highly specialized teleost families, suggesting that this habit has evolved in geologically late time. The small number of fish species that concentrate on sponges as food suggests that the defensive characters of sponges—mineralized sclerites, noxious chemical substances, and tough fibrous components—are highly effective in discouraging predation. The two sponges most frequently eaten by fishes have a low percentage of siliceous spicules relative to organic matter, but among the 20 next most frequently consumed species no striking correlation occurs with respect to spicule content. Color and form of the sponge show no special correlation with frequency of occurrence in fish stomachs. Three species of fishes appear to concentrate on one species of sponge, but in these cases over 60% of the food taken consists of a variety of other organisms. Those fishes, more than half of whose diet consists of sponges, tend to sample a wide variety of species. No strong evidence is provided by our data that fish predation is a significant factor in limiting sponge distribution in the West Indian region.     

Linking abundance and diversity of sponge-associated microbial communities to metabolic differences in host sponges

Many sponge species contain large and diverse communities of microorganisms. Some of these microbes are suggested to be in a mutualistic interaction with their host sponges, but there is little evidence to support these hypotheses. Stable nitrogen isotope ratios of sponges in the Key Largo, Florida (USA) area grouped sponges into species with relatively low δ¹⁵N ratios and species with relatively high δ¹⁵N ratios. Using samples collected in June 2002 from Three Sisters Reef and Conch Reef in the Key Largo, Florida area, transmission electron microscopy (TEM) and denaturing gradient gel electrophoresis were performed on tissues of the sponges Ircinia felix and Aplysina cauliformis, which are in the low δ¹⁵N group, and on tissue of the sponge Niphates erecta, which is in the high δ¹⁵N group. Results showed that I. felix and A. cauliformis have large and diverse microbial communities, while N. erecta has a low biomass of one bacterial strain. As the low δ¹⁵N ratios indicated a microbial input of nitrogen, these results suggested that I. felix and A. cauliformis were receiving nitrogen from their associated microbial community, while N. erecta was obtaining nitrogen solely from external sources. Sequence analysis of the microbial communities showed a diversity of metabolic capabilities among the microbes of the low δ¹⁵N group, which are lacking in the high δ¹⁵N group, further supporting metabolic differences between the two groups. This research provides support for hypotheses of mutualisms between sponges and their associated microbial communities.     

Exudation of biologically-active metabolites in the sponge Aplysina fistularis

Specimens of the demosponge Aplysina fistularis forma fulva (Pallas) from the intertidal zone at La Jolla, California, USA, contain two antimicrobial metabolites, aerothionin (1) and homoaerothionin (2). Both metabolites are exuded into seawater. The rates of exudation into artificial seawater were measured under near in situ conditions (7.7x10^-4 g aerothionin min^-1 g^-1 dry weight of sponge) and in an aquarium (8.9x10^-3 g min^-1 g^-1) for undamaged sponges. Sponges responded to a simulated injury by exuding 10 to 100 times as much of these metabolites. The exudation of aerothionin and homoaerothionin may inhibit surface overgrowth by fouling organisms and tissue damage by mobile animals.Please address requests for reprints to Dr. J. E. Thompson     

Redwood of the reef: growth and age of the giant barrel sponge Xestospongia muta in the Florida Keys

The growth of animals in most taxa has long been well described, but the phylum Porifera has remained a notable exception. The giant barrel sponge Xestospongia muta dominates Caribbean coral reef communities, where it is an important spatial competitor, increases habitat complexity, and filters seawater. It has been called the ‘redwood of the reef’ because of its size (often >1 m height and diameter) and presumed long life, but very little is known about its demography. Since 1997, we have established and monitored 12 permanent 16 m diameter circular transects on the reef slope off Key Largo, Florida, to study this important species. Over a 4.5-year interval, we measured the volume of 104 tagged sponges using digital images to determine growth rates of X. muta. Five models were fit to the cubed root of initial and final volume estimates to determine which best described growth. Additional measurements of 33 sponges were taken over 6-month intervals to examine the relationship between the spongocoel, or inner-osculum space, and sponge size, and to examine short-term growth dynamics. Sponge volumes ranged from 24.05 to 80,281.67 cm³. Growth was variable, and specific growth rates decreased with increasing sponge size. The mean specific growth rate was 0.52 ± 0.65 year⁻¹, but sponges grew as fast or slow as 404 or 2% year⁻¹. Negative growth rates occurred over short temporal scales and growth varied seasonally, significantly faster during the summer. No differences in specific growth rate were found between transects at three different depths (15, 20, 30 m) or at two different reef sites. Spongocoel volume was positively allometric with increasing sponge size and scaling between the vertical and horizontal dimensions of the sponge indicated that morphology changes from a frustum of a cone to cylindrical as volume increases. Growth of X. muta was best described by the general von Bertalanffy and Tanaka growth curves. The largest sponge within our transects (1.23 × 0.98 m height × diameter) was estimated to be 127 years old. Although age extrapolations for very large sponges are subject to more error, the largest sponges on Caribbean reefs may be in excess of 2,300 years, placing X. muta among the longest-lived animals on earth.     

Habitat use by sponge-dwelling brittlestars

Cryptic organisms often associate with sessile invertebrates for refuge in space-limited environments. To examine interspecific habitat associations on coral reefs, tube- and vase-shaped sponges were surveyed for associated brittlestars at six sites on the coral reefs off Key Largo, Florida. Of 179 sponges encountered, Callyspongia vaginalis was the most abundant (43.0%), followed by Niphates digitalis (39.7%), and Callyspongia plicifera (4.5%). Three of eight sponge species surveyed did not differ from C. vaginalis in two physical refuge characteristics: oscular diameter and inner tube surface area. Brittlestars (416 total), all of the genus Ophiothrix, were only found in C. vaginalis, N. digitalis, and C. plicifera. The most abundant brittlestar, O. lineata (326), occurred on C. vaginalis (99.0%) and N. digitalis (1.0%), while O. suensonii (67) occurred on C. vaginalis (79.1%), N. digitalis (19.4%), and C. plicifera (1.5%). There was no pattern of co-occurrence of O. lineata and O. suensonii on C. vaginalis. The abundance of O. lineata increased with surface area of C. vaginalis. Differential habitat use was observed in O. lineata, with small individuals (<5 mm disk diameter) located inside and on the surface of sponge tubes and large individuals (5 mm) solely inside tubes. The number of large O. lineata in C. vaginalis never exceeded the number of tubes per sponge, and tagged O. lineata remained in the same sponge for at least 3 weeks. In density manipulations, no pattern of intraspecific competition among large O. lineata was observed; however, there was evidence for interaction between size-classes. Brittlestars selected live sponge habitat over a non-living refuge, suggesting a mechanism for sponge habitat recognition. Sponge-dwelling brittle stars prefer some tube- and vase-shaped sponge species despite similar oscular diameters and surface areas. Surprisingly, these preferred sponge species are known from previous studies to be chemically undefended against generalist fish predators; therefore, brittlestars that inhabit these sponges do not gain an associational chemical defense. Sponge habitat use by O. lineata may be governed by intraspecific interactions to maintain habitat and access to food. While past studies have suggested that O. lineata is an obligate sponge commensal, the present study suggests that O. lineata has a species-specific association with the tube-sponge C. vaginalis.Communicated by O. Kinne, Oldendorf/Luhe