The outer layer is called the epidermis , whereas the inner layer is called the gastrodermis and lines the digestive cavity. Between these two layers is a non-living, jelly-like mesoglea. There are differentiated cell types in each tissue layer, such as nerve cells, enzyme-secreting cells, and nutrient-absorbing cells, as well as intercellular connections between the cells.
However, organs and organ systems are not present in this phylum. The nervous system is primitive, with nerve cells scattered across the body in a network. The function of the nerve cells is to carry signals from sensory cells and to contractile cells. Groups of cells in the nerve net form nerve cords that may be essential for more rapid transmission.
Cnidarians perform extracellular digestion , with digestion completed by intracellular digestive processes. Food is taken into the gastrovascular cavity , enzymes are secreted into the cavity, and the cells lining the cavity absorb the nutrient products of the extracellular digestive process.
The gastrovascular cavity has only one opening that serves as both a mouth and an anus an incomplete digestive system. Like the sponges, Cnidarian cells exchange oxygen, carbon dioxide, and nitrogenous wastes by diffusion between cells in the epidermis and gastrodermis with water. The phylum Cnidaria contains about 10, described species divided into four classes: Anthozoa, Scyphozoa, Cubozoa, and Hydrozoa.
The class Anthozoa includes all cnidarians that exhibit a sessile polyp body plan only; in other words, there is no medusa stage within their life cycle. Examples include sea anemones, sea pens, and corals, with an estimated number of 6, described species. Sea anemones are usually brightly colored and can attain a size of 1. These animals are usually cylindrical in shape and are attached to a substrate. A mouth opening is surrounded by tentacles bearing cnidocytes [Figure 5].
Scyphozoans include all the jellies and are motile and exclusively marine with about described species. The medusa is the dominant stage in the life cycle, although there is also a polyp stage. Species range from 2 cm in length to the largest scyphozoan species, Cyanea capillata , at 2 m across. Jellies display a characteristic bell-like body shape [Figure 6]. Identify the life cycle stages of jellies using this video animation game from the New England Aquarium.
Cubozoans are anatomically similar to the jellyfish. A prominent difference between the two classes is the arrangement of tentacles. Cubozoans have muscular pads called pedalia at the corners of the square bell canopy, with one or more tentacles attached to each pedalium. In some cases, the digestive system may extend into the pedalia.
Cubozoans typically exist in a polyp form that develops from a larva. The polyps may bud to form more polyps and then transform into the medusoid forms. Watch this video to learn more about the deadly toxins of the box jellyfish. Hydrozoa includes nearly 3, species, 1 most of which are marine.
Most species in this class have both polyp and medusa forms in their life cycle. Many hydrozoans form colonies composed of branches of specialized polyps that share a gastrovascular cavity. Other species are solitary polyps or solitary medusae.
The characteristic shared by all of these species is that their gonads are derived from epidermal tissue, whereas in all other cnidarians, they are derived from gastrodermal tissue [Figure 7] ab. Section Summary Animals included in phylum Porifera are parazoans and do not possess true tissues. These organisms show a simple organization. Sponges have multiple cell types that are geared toward executing various metabolic functions.
Cnidarians have outer and inner tissue layers sandwiching a noncellular mesoglea. Cnidarians possess a well-formed digestive system and carry out extracellular digestion. The cnidocyte is a specialized cell for delivering toxins to prey and predators. Cnidarians have separate sexes. They have a life cycle that involves morphologically distinct forms—medusoid and polypoid—at various stages in their life cycle. Describe the feeding mechanism of sponges and identify how it is different from other animals.
The sponges draw water carrying food particles into the spongocoel using the beating of flagella in the choanocytes. The food particles are caught by the collar of the choanocyte and brought into the cell by phagocytosis. Digestion of the food particle takes place inside the cell. The difference between this and the mechanisms of other animals is that digestion takes place within cells rather than outside of cells.
It means that the organism can feed only on particles smaller than the cells themselves. Poriferans do not possess true tissues, whereas cnidarians do have tissues. Because of this difference, poriferans do not have a nerve net or muscle cells for locomotion, which cnidarians have. Skip to content Chapter Diversity of Animals. Learning Objectives By the end of this section, you will be able to: Describe the organizational features of the simplest animals Describe the organizational features of cnidarians.
Compare the structural differences between Porifera and Cnidaria. In tropical ecosystems, biodiversity and productivity are maintained through efficient recycling pathways, such as the sponge loop. In this pathway, encrusting sponges recycle dissolved organic matter DOM into particulate detritus. Subsequently, the sponge-produced detritus serves as a food source for other organisms on the reef. Alternatively, the DOM stored in massive sponges was recently hypothesized to be transferred to higher trophic levels through predation of these sponges, instead of detritus production.
However, for deep-sea sponges, the existence of all prerequisite, consecutive steps of the sponge loop have not yet been established. Here, we tested whether cold-water deep-sea sponges, similar to their tropical shallow-water counterparts, take up DOM and transfer assimilated DOM to associated fauna via either detritus production or predation.
During the subsequent 9-day chase in label-free seawater, we investigated the transfer of the consumed food by sponges into brittle stars via two possible scenarios: 1 the production and subsequent consumption of detrital waste or 2 direct feeding on sponge tissue.
We found that particulate detritus released by both sponge species contained C from the previously consumed tracer DOM and POM, and, after 9-day exposure to the labeled sponges and detritus, enrichment of 13 C and 15 N was also detected in the tissue of the brittle stars. These results therefore provide the first evidence of all consecutive steps of a sponge loop pathway via deep-sea sponges. We cannot distinguish at present whether the deep-sea sponge loop is acting through a detrital or predatory pathway, but conclude that both scenarios are feasible.
We conclude that sponges could play an important role in the recycling of DOM in the many deep-sea ecosystems where they are abundant, although in situ measurements are needed to confirm this hypothesis. In the deep-sea, sponges and cold-water corals CWC form complex reef structures, which support rich communities of suspension-feeding fauna and play crucial roles as habitat and feeding grounds for motile taxa, including commercial fish species Miller et al. These ecosystems are amongst the most productive deep-sea habitats and they are responsible for significant carbon C and nitrogen N cycling van Oevelen et al.
In fact, CWC reefs and sponge grounds have been identified as benthic biodiversity hotspots, even comparable to tropical coral reefs in terms of grams organic C m —2 and kg dry weight km —2 Polovina, ; van Oevelen et al. Paramount to the productivity of benthic ecosystems in oligotrophic waters is their capacity to efficiently retain and recycle resources. The largest organic resource in the oceans is dissolved organic matter DOM Benner et al. However, DOM is known to be processed by bacterioplankton and then returned to the classic food chain through planktonic grazing, a pathway termed the microbial loop Azam et al.
Essentially, by consuming DOM, bacteria remineralize nutrients that would otherwise be lost to the environment Fenchel, Within shallow-water tropical coral reefs, an additional DOM recycling pathway has been established: the sponge loop de Goeij et al.
In this pathway, encrusting sponges that dominate the surface of cryptic habitats e. This detritus subsequently feeds the detrital food chain de Goeij et al. However, no detritus production was found for several sponges with a non-encrusting, but massive, emergent growth form, that generally occur on the exposed reef McMurray et al.
Therefore, a complementary sponge-loop pathway was hypothesized, in which sponge-assimilated DOM is transferred to higher trophic levels via direct predation on sponge tissue McMurray et al. To date, this predatory sponge loop has not yet been confirmed.
Whether via detritus production or predation, the sponge loop, together with the microbial loop, helps to explain how tropical shallow-water coral reefs maintain a high productivity and biodiversity in otherwise oligotrophic marine environments de Goeij et al. The re cycling and transfer of DOM could be of particular importance for benthic deep-sea ecosystems as, for large parts of the year, particulate phytodetritus transported from the ocean surface cannot fulfill the carbon demands of these systems Gooday, ; Duineveld et al.
Recently, first evidence was found that the sponge-loop pathway may not just operate on tropical shallow-water coral reefs, but also in the deep-sea Rix et al. The capacity to take up DOM represents the first step of the sponge loop de Goeij et al. The second step of the sponge loop is the assimilation of DOM into particulate organic matter POM , leading to either the release of detritus including pseudo faeces or an increase in sponge biomass. Using stable isotope tracers, multiple studies have shown that deep-sea sponges are capable of assimilating DOM into biomass Rix et al.
However, the third—ecologically critical, but most difficult to experimentally identify—step of the sponge loop has not been established in the deep-sea to date: the transfer of assimilated DOM by sponges to higher trophic levels. Note also that all the aforementioned studies on DOM cycling by deep-sea sponges are based on ex situ measurements in controlled laboratory settings. The existence and ecological relevance of a deep-sea sponge loop has therefore not been established to date.
In fact, they are established as very efficient filter-feeders of organic particles, such as bacterio-, and phytoplankton e. Food bacteria, as part of their POM diet, were found to be assimilated more efficiently into sponge tissue compared to DOM Kazanidis et al. Currently, it is unknown how the processing of DOM and POM by encrusting and massive sponges affects the subsequent steps of the sponge loop.
The present study aims to test the hypothesis that, similar to their tropical counterparts, deep-sea sponges transfer assimilated DOM and POM to associated fauna, following the prerequisite, consecutive steps of the sponge loop. They are known to feed on pseudo- fecal droppings of bivalves Maier et al. Brittle stars may also directly feed on sponge tissue Morison, ; McClintock, The transfer of assimilated DOM and POM by the two sponges via either detritus or direct predation into brittle stars was investigated during the subsequent 9-day chase, after transfer of the pulse-labeled sponges to label-free running seawater aquaria containing brittle stars.
This study investigated the nutritional relationship between two North-Atlantic deep-sea sponge species, the massive species Geodia barretti and the encrusting species Hymedesmia sp. Figure 1A and Supplementary Table 1. Figure 1. Experimental set up. A Organisms used in this study. The remaining three sponge individuals or 2 for encrusting POM were then transferred to individual L running seawater flow-through aquaria for 24 h washing phase.
Lastly, the individuals were pooled per species and food source and transferred to L running seawater flow-through aquaria with 3 brittle stars Ophiura sp. The Barents Sea is a shelf sea with an average depth of m Sundfjord et al.
Hymedesmia sp. This cold-water coral reef is characterized by the presence of the reef framework-forming scleractinian coral Lophelia pertusa and reef-associated fauna such as sponges, crustaceans, and other corals Rovelli et al. North-Atlantic seawater was pumped in from 6 m water depth at 30 L h —1.
All individuals were transported without air-exposure to the laboratory facilities at the University of Bergen, Norway, where the experiments took place. Each holding tank contained a maximum of five sponge individuals and five brittle stars. All sponges were acclimatized for a minimum of 1 week prior to the incubation experiments and all sponges and their attached rocky substrates were cleared from epibionts prior to incubations.
Non-labeled axenic P. After 10 day, diatoms were concentrated on a 0. Lastly, the DOM solution was filtered over a 0. The filtrate was collected, lyophilized, and analyzed for C and N content and isotopic composition. Isotopically-enriched POM was prepared by labeling ambient seawater bacterioplankton de Goeij et al. Briefly, seawater bacteria were concentrated by prefiltering natural seawater over a 0.
The inoculum was added to M63 medium Miller, , amended with thiamine 0. A schematic of the pulse-chase experimental set-up is shown in Figure 1B and comprised three phases: a pulse, a washing, and a chase phase. From here on, for simplicity and to distinguish between the two sponge loop pathways scenarios—i. Prior to the incubations, chambers were acid washed 0.
Incubation water was replenished every 8 h 24 h total incubation time to ensure that sponge individuals were regularly receiving fresh seawater after which new, labeled substrate was added. At the end of the incubation, sponges were removed from the chamber and rinsed in 0. After drying, tissue was homogenized with mortar and pestle and stored in a desiccator until further analysis by EA-IRMS.
Phase 2: washing— Post-labeling, sponge individuals were transferred to individual L label-free running seawater flow through aquaria for 24 h to ensure no residual 13 C- and 15 N-labeled substrate remained on the inner and outer surface of the sponges.
Phase 3: chase— Post-washing, DOM-fed and POM-fed massive and encrusting sponges were placed in four L label-free running seawater flow-through aquaria together with 3 unlabeled brittle stars per tank: one tank with three DOM-fed massive sponges plus brittle stars, one with three DOM-fed encrusting sponges plus brittle stars, one with three POM-fed massive sponge plus brittle stars, and one with two POM-fed encrusting sponges plus brittle stars.
After placing the brittle stars at random positions in each aquarium, they settled themselves on the surface of the sponges Figure 2. After 9-day, all sponges and brittle stars were rinsed in 0. Then, tissue was homogenized with mortar and pestle and stored in a desiccator until further analysis for C and N content and stable isotope enrichment 13 C and 15 N by EA-IRMS see below sections for details on flux calculations. Background detritus samples and tissue samples from each species were collected prior to the pulse-chase experiment and served as non-labeled controls.
Figure 2. Geodia barretti and Ophiura sp. After placing them at random position in the aquaria, brittle stars settled themselves on the surface of the sponges. DOM and POM substrates, labeled and non-labeled sponge tissue, brittle star, and detritus samples were analyzed for organic C and total N content on an elemental analyzer [Elementar Isotope cube Elementar GmbH , Langenselbold, Germany ] coupled to a BioVision isotope ratio mass spectrometer Elementar ltd , Manchester, United Kingdom for simultaneous measurement of organic carbon and nitrogen content and 13 C: 12 C and 15 N: 14 N ratios.
Before analysis, samples were lyophilized for 24 h in a FD Ilchin Biobase freeze drier. After freeze-drying, approximately 10 mg per sample was weighed out into separate tin capsules and acidified. The peak area from the elemental analyzer to content ratio was calculated with respect to several replicates of a standard acetanilide of known C and N content. To calculate the 13 C: 12 C and 15 N: 14 N ratios, C and N stable isotope ratios are expressed in standard delta notation as:.
The above background enrichment of 13 C or 15 N in the samples was calculated as the excess fractional abundance of 13 C or 15 N in the samples compared with the background i. Unfortunately, the 15 N-detritus background measurements could not be analyzed and therefore no 15 N-enrichment of detritus samples was calculated. Rates were then normalized to time and tissue C or N content of the sponges or detritivores. Both detritus detrital pathway and the sponge tissue itself predatory pathway are considered stable-isotope-enriched food sources to brittle stars during the label-free chase.
We calculated brittle star tracer uptake rates for two hypothetical scenarios: in scenario 1 with detritus as food source using the labeling efficiency of detritus , and in scenario 2 with sponge tissue as food source using the labeling efficiency of the sponge tissue. These incorporation rates were calculated assuming transfer from sponge to brittle star via detritus or via direct predation.
Adjustments for multiple tests were made using the Bonferroni procedure. Full statistical output is available in Supplementary Table 2. Stable isotope enrichment of sponge tissue did not significantly change between the end of the pulse and end of the chase phase Figures 3A,B , left panels; Supplementary Tables 2 , 3 for both sponge species, except for POM-fed massive sponges, where tissue isotopic enrichment decreased significantly during the chase for 13 C.
Figure 3. Sponge-driven transfer of dissolved organic matter DOM A and particulate organic matter POM B , to sponge associated fauna in an ex situ aquarium set-up. The dashed and solid lines represent the fate of 13 C red line and 15 N blue line incorporated by massive sponges and encrusting sponges, respectively.
After an initial h pulse gray shading; data obtained from Bart et al. Unfortunately, 15 N-detritus data were not analyzed. Note the difference in scale on the Y -axis between panels. Data is also shown in Supplementary Table 3. Released detritus showed a continuous increase in above-background 13 C-enrichment no 15 N data available during the chase Figures 3A,B , middle panels , demonstrating turnover of DOM and POM by sponges.
After 9-day exposure to the labeled sponges and detritus, above-background enrichment of 13 C and 15 N was also detected in the tissue of the brittle stars Figures 3A,B , right panels. Both detritus and the sponge tissue itself are possible sources of enrichment found in brittle stars after the label-free chase phase.
In scenario 1 Figure 4A and Supplementary Table 4 , brittle star uptake rates of detrital-C from POM-fed massive and encrusting sponges were higher compared to DOM-fed sponges, however, these differences were not significant Supplementary Table 2. Figure 4. In scenario 1 A brittle star tracer uptake rates were quantified with enriched detritus as food source, in scenario 2 B , uptake rates were calculated with the enriched sponge tissue as food source.
Data are also shown in Supplementary Table 4. For the transfer of tissue-N via predation, no significant differences were found in brittle star uptake rates between DOM- and POM-fed sponges, for both massive and encrusting sponges Supplementary Tables 2 , 4. The C:N ratios of uptake by brittle stars under scenario 2 direct tissue predation were lower for DOM-fed sponges 1.
This study provides the first evidence of all three consecutive steps of the sponge loop in deep-sea sponges. The two investigated deep-sea sponge species take up and assimilate DOM and POM, subsequently turn sponge-assimilated DOM and POM into detritus, and transfer carbon and nitrogen derived from both food sources to associated fauna.
Transfer of assimilated food to associated fauna by sponges is possible via two scenarios: 1 via the production of detrital waste or 2 via direct predation on sponge tissue. The plausibility of both scenarios, and their potential ecological relevance for deep-sea ecosystems, are discussed below. At present, DOM cycling by various types of sponges and its relevance for marine ecosystems, is a heavily debated topic e.
The original sponge-loop hypothesis proposed that coral reef sponges recycle DOM by converting it into particulate detritus, which is then used by various detritivorous organisms and thereby re-enters the classical food chain de Goeij et al. This pathway was tested on sponge species with mm-thin sheet to cm-thick e.
Based on encrusting sponges alone, sponge-loop carbon cycling is estimated to amount to the gross primary production rates of an entire coral reef ecosystem de Goeij et al. Interestingly, massive upright growing sponges, living on the exposed parts of the reef, were not found to produce significant quantities of detritus McMurray et al. Massive sponges may allocate the majority of assimilated C in three-dimensional upward tissue growth, while mm to cm-thin encrusting species are restricted to space-limited, two-dimensional growth.
Consequently, encrusting sponges may invest relatively more carbon in cell turnover, shedding and detritus production compared to massive species. This hypothesis was strengthened by recent work of Maier et al. McMurray et al. Based on the limited number of replicates used in our study due to the difficulties obtaining and experimenting with live deep-sea organisms, we cannot rule out one of these scenarios, and will discuss both.
Deep-sea detrital sponge loop —Multiple studies have shown that sponge species from various deep-sea ecosystems produce particulate waste material in the form of detritus or fecal pellets Witte and Graf, ; Rix et al. Only few studies have quantified the release of detritus. In comparison, the massive sponge G. Yet, from an ecological perspective, detritus production by massive sponges can still have a significant effect on C turnover in marine ecosystems.
The average total organic C content C org of the massive G. Consequently, even when the relatively small G. Although little is known about the relative contribution of encrusting versus massive sponges in deep-sea ecosystems, the carbon standing stock of G. Geodia barretti can thus potentially produce 66 mg detritus per m —2 d —1 , which amounts to an abundant supply of particulate organic carbon to the local deep-sea benthos.
For example, Piepenburg and Schmid estimated a mineralization rate of Deep-sea predatory sponge loop— Spongivory is a strategy performed by various animals, including echinoderms Randall and Hartman, ; Pawlik, , and sponge spicules have been found in the stomachs of various brittle star species Pearson and Gage, A study on deep-sea ophiuroid individuals of six different species by Pearson and Gage showed that deep-sea brittle stars are trophic generalists lacking dietary specialization.
Feeding strategies ranged from detritivory to scavenging and even suspension feeding. Yet, rates of direct tracer incorporation were up to three orders of magnitude lower compared to the incorporation of sponge-derived C and N. Some massive sponge species are known to produce metabolites that deter predation Chanas et al.
In contrast, chemically defended individuals that grow relatively slowly may be long lived, and therefore important for the sequestration and storage of C as biomass McMurray et al. The massive HMA sponge used in our study, G. Furthermore, it is difficult to make a genuine comparison with spongivory in shallow-water tropical systems, as deep-sea sponge metabolism is much slower due to lower ambient temperatures Bart et al. A comparison of potential deep-sea sponge-loop scenarios and comparing dissolved and particulate food sources reveals two interesting trends: when sponges feed on POM, relatively more C is transferred to associated fauna, compared to DOM.
Secondly, relatively more DOM-derived C appears to be transferred via detritus compared to predation. This implies that POM i. This corresponds with the observations made by Kazanidis et al. This strengthens our hypothesis that DOM and POM are both essential parts of deep-sea sponge diet, as source of maintenance and as building blocks, respectively.
This could also have ecological consequences for the wider associated ecosystem and food web, since both C and N may transfer at different rates according to the diet of sponges and the chosen feeding strategy of associated fauna. It is important to note that both the detrital and predatory scenario are plausible for both types of sponges i. DOM and POM , and we cannot distinguish between the two scenarios under the current experimental approach.
Moreover, our limited replication, in both sponge species and species of associated fauna, and the effect of concentration and lability of ambient DOM in time and space available to sponges living in different deep-sea ecosystems, make it very difficult to extrapolate our data to deep-sea benthic ecosystem-wide processes.
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