Individuals of Xestospongia bergquistia are straight standing, barrel shaped, leuconoid sponges. The body size is highly variable, but they belong to one of the biggest sponge genus. Full grown individuals are usually around 1 m long and 30 cm wide (Fromont 1991, Froman 1995). A large, cup-shaped, apical introversion forms a central hollow in the sponge’s body, which can take up as much as one third of the total sponge height. See figure 1 for a general example.

The outer surface of the sponge is crested and full of ridges and extensions, forming various shapes and forms, from rounded knobs to thin finger-like branches, which extend from the sponge’s sides.

Live specimens present an external red-brown to maroon colour, with the internal surface of the cup which can be paler in colour. The tips of the ridges and extensions can be white.

A thin external membrane gives to the sponge a smooth to the touch surface. The sponge itself has a firm and glassy texture. The sponge tissue can be easily pulled apart, and, when compressed, will not rebound. This peculiarity is a consequence of the spicule and choanosome composition (see Skeleton section), and it’s a useful tool to distinguish between X. bergquistia and its sister and neighbour species, X. testudinaria, when in the field (Fromont 1991).

They are gonochoristic animals, having separate sexes but no sexual dimorphism. When reproductive, female individuals are easy to distinguish, for their high number of eggs visible in the body (Figure 2). It’s quite harder to distinguish between males and non reproductive individuals, because sperm cysts are not very big and are only present for short periods of time (see Sexual Reproduction and Life Cycle section).

X. bergquistia presents siliceous monoaxonal spicules of high variable size and thickness (around 300 µm x 12 µm). They are usually gently curved, while some are centrally bent or undulating (Figure 3e). Oxeas, strongyles and stepped-ended spicules can all be found (Figure 3d), with some spicules presenting knob-like protuberances close to their endpoints.

Xestospongia bergquistia are benthic, sessile, filter feeding animals. Their bodyplan is heavily centred around the leuconoid aquiferous system (see Aquiferous System section), which allows them to intake and filter water very efficiently, even with their large body size (Ruppert et al. 2004). They are very slow growing and long lived animals, with some individuals belonging to the same genus estimated to be 2000 or more years old (McMurray et al. 2008).

Inhabiting inshore fringing reefs and living on both dead coral and rock, X. bergquistia can be found in sympatry with X. testudinaria, in areas with high sediment load, as well as caves and open light spaces. Giant barrel sponges have a high impact on their ecosystem, in a way which is strongly linked to their lifestyle and bodyplan. Bell (2008) reviewed many of the ways sponges shape and affect their surroundings: 1) They bioerode, build, consolidate and stabilise the benthic substrate by boring in it, binding separate fragments together and creating new substrate by scratch (biomineralisation); 2) They have a strong role in linking the benthic and pelagic environments, by filtering the water for food uptake, which helps clearing the water column and cycle nutrients and carbon; 3) They have many interspecific relationships with a wide range of taxa, through which sponges become important for primary production by hosting photosynthetic bacteria, as well as providing a food source and a microhabitat for settlement and predator avoidance. Given their large size, X. bergquistia are likely to filter large amounts of water and provide a protective environment for many smaller animals. Observing parts of the sponge under a dissecting microscope, animals from many different taxa could be observed living in its tissue, most evidently small crustaceans and polychaete worms. In Figure 4, we can see some polychaetes which inhabit the animal.

No studies have been conducted on the asexual reproduction of X. bergquistia. High regeneration capacity is common in all sponges, and is given by the totipotency of their archeocytes. They can regenerate an adult sponge even from single cells (Ruppert et al. 2004), which is fundamental when reproducing asexually by fragmentation. This capacity is probably fundamental for a slow growing genus like the barrel sponge, and determining for the early stages of habitat colonisation. As they are not common traits of all sponges, I cannot say if X. bergquistia can reproduce by budding or forming gemmules.

X. berguistia is an oviparous, gonochoristic species. These traits are somewhat peculiar within the Haplosclerid order (see Evolution and Systematics section), and, because of them, their sexual reproduction is quite interesting to analyse, in an evolutionary and ecological prospective. A detailed description of their life cycle, reproductive timing and early development will be given based on the first description made by Fromont and Bergquist (1994). With the aim of looking at similarities and differences between oviparous and viviparous strategies, the observations from two studies, looking at one oviparous (Maldonado and Riesgo 2009) and two viviparous (Abdo et al. 2008) species, will briefly be reported. Finally, a small conclusion about these strategies will be discussed. 

Female individuals start producing their eggs in May, probably cued by the temperature decrease typical of fall months. Egg development lasts for around 5 months, with a rapid increase of oocyte size around one month before spawning. Eggs can be easily seen equally distributed in high densities within the female sponge’s tissues (Figure 2), and have differentiated yolk and a visible nucleolus (Figure 5). Sperm cysts in male individuals are also evenly distributed at high densities across the animal. The sperm develop synchronously within the single cyst, but the different cysts develop asynchronously within the same animal. A very rapid development was estimated, with really small cysts starting to be recorded just around a week before spawning.

Spawning occurs in the months of October and November, after the increase in temperature, but before the reaching of the summer highest temperatures. The temperature recorded for spawning was of 27°C-28°C. They do not seem to spawn consistently with lunar or tide cycles, but always in the advanced morning, between 7 am and 11 am, probably guided by the shift from darkness to light. Even though they don’t seem to follow usual spawning cues like lunar cycle or tides, spawning is highly synchronous between males and females. Females spawn negatively buoyant eggs in a pale mucous film (Figure 6) and males spawn a positively buoyant cloud of sperm (Figure 7), probably the night before the females’ spawning event. This video shows a female egg spawning event: 

A fertilisation rate of 71.4% was estimated for the ~1.4 billion eggs spawned from each female. Fertilised eggs have an oval shape and undergo total and equal cleavage in the early stages of development (Figures 8 and 9). Around 5 hours after fertilisation, the embryos have already gone through many cell divisions, forming morulae. It takes them around 4 days to become rounded, ciliated, unpigmentated larvae (Figure 10). Not much is known about the biology of the larval stage, its duration is estimated to be short, based on dispersal distance (Bell et al. 2014). No information is known about its settlement mechanisms, but a fair guess would be that it’s guided by chemical signals and cues, because of the larva’s lack of pigmentation and the high levels of self recruitment. Images of juveniles of different ages (Figure 11 and Figure 12) presented here, have been taken by Dr. Claire Larroux. As we can see, skeleton and oscule (Figure 13) are among the first things to be developed by the juvenile, possibly in order to support the feeding and structure of the growing animal.

Maldonado and Riesgo (2009) described the reproduction of Petrosia ficiformis, another oviparous gonochoristic haplosclerid sponge. The total time of gametogenesis is indeed longer than in X. bergquistia. Oogenesis lasts 7-8 months, but spermatogenesis lasts just a few weeks, still a relatively short period. Eggs were spawned in isolation and not in the mucous threads typical of the barrel sponges. Embryonic development is also slower, with the merula stage reached after many more hours. Larval stage was reached after around 36 hours.

Abdo et al. (2008) described the reproduction of two Haliclona spp., viviparous hermaphroditic haplosclerid sponges. Oogenesis time is longer than in the barrel sponge but similar to P. ficiformis, lasting 6-7 months and producing larger eggs than both the oviparous species. On the other hand, spermatogenesis is much longer than both species, lasting up to 4 months. This long duration, along with increased asynchrony between sperm development within and across individuals, gives the sponge multiple chances of spawning, reducing sperm dilution in the water. An important difference noted in the study is the lower reproductive output of female gametes in this sponge. Embryos were observed in the tissue of the sponge for 4 to 5 months.

The main conclusion we can get from these observations about the reproductive strategies, is that some reproduction traits really seem to covariate. A longer spermatogenesis, functional for multiple spawning, and bigger egg size, with higher energy investment into each egg, are appropriate for a viviparous strategy. Short spermatogenesis, with one spawning each year, and smaller egg size, with high fecundity, are appropriate for oviparous species. Given the small number of species compared here, it is hard to take big conclusions, but a trend in the covariance of reproductive traits can surely be observed.

The body of giant barrel sponges, as other sponges, is made up of cells organised into two tissue layers, ephitelioid and connective (Ruppert et al. 2004). The ephitelioid layer is characterised by three major cells types, the pinacocyte, the porocyte and the choanocyte. Pinacocyte are flattened cells that are positioned one next to the other to form a pavement structure that covers most of the body externally and internally. Porocytes are small cells that surround pores through which water can flow, resembling a sphincter by regulating the opening and closure of the pore. Choanocytes are flagellated cells with a collar with the important function of creating water current through the sponge and absorbing the food and nutritious particles. The connective layer is also known as mesohyl and it’s composed by a proteinaceous extra-cellular matrix containing the skeletal elements and both differentiated and undifferentiated cells. The undifferentiated totipotent archeocytes play many key roles in the sponge’s physiology and survival, including digestion and nutrient transport, protection from parasites, and gametogenesis. The skeletal elements are secreted by the differentiated sclerocytes (see Skeleton section). For higher detail on how these cells are organised and work together the Aquiferous System section, on how their organisation is functional to the body plan.

Fromont (1991) and Froman (1995) described the organisation of the skeleton of giant barrel sponges. Reticulated spicule tracts, uniformly organised in all directions, with spicules interlocked in a triangular pattern, form the endoskeleton of X. bergquistia. Oval meshes of spicules are formed both in the inner and in the outer parts of the skeleton. In the inner part (Figure 3c), skeleton meshes of up to 720 µm diameter are formed with tracts 100-200 µm wide. Smaller meshes are formed in the outer skeleton (Figure 3b), ranging from 100 µm to 200 µm. Large internal canals with no spicules are found, up to 2000 µm wide. One of the most peculiar and defining character of X. bergquistia is the complete lack of spongin fibre development along the spicule tracts. This absence results in the sponge lacking elasticity and resistance to shredding, a useful identification tool.

The aquiferous system is the key feature of a sponge’s filter feeding body plan. It accounts not only for the nutrition of the sponge, but also for gas exchange, waste disposal and sexual reproduction (Ruppert et al. 2004).  X. bergquistia, along with other demosponges, have a leuconoid design of the aquiferous system. In this type of system water flow through incurrent and excurrent canals, coated by pinacocytes, in through the ostia and out through numerous oscula. Flagellated chambers, alveolar structures made from the aggregation of choanocytes (Figure 14), are in high densities between incurrent and excurrent canals, and are the engine for the water flow through the sponge. The leuconoid design allows high surface area per body volume, playing the key role in allowing the giant barrel sponges to reach their big sizes.

Being sessile, soft animals with no exoskeleton, sponges have to adopt different strategies to deter predators and parasites. The major source of defence for all sponges are the bioactive and toxic compounds found within their bodies, as spicules, counterintuitively, do not seem to contribute to the sponge’s impalatability (Chanas and Pawlik 1996; Waddell and Pawlik 2000). These secondary metabolites act both as antifoulant (Nguyen et al. 2013) and as toxic deterrent for predators (Waddell and Pawlik 2000). The chemical compounds extracted from Xestospongia spp. have been shown to be bioactive in an array of different ways, having cardiovascular, cytotoxic, enzyme inhibitory, antimicrobial and insecticidal activities (Zhou et al. 2010). The antifoulant action of the secondary metabolites tested by Nguyen et al. (2013), is believed to be a valid alternative to the current commonly used toxic antifoulants, with way fewer drawbacks. Many, if not all, of these compounds are believed to be synthesised by the symbiotic bacteria and fungi living in the sponges’ tissues (Lee et al. 2001). Given the important applications and value that the metabolites can have, further studies on these interactions could be beneficial in many fields of science.

I would like to thank Dr. Claire Larroux for the images, videos and general suggestions she gave me, which helped improve and refine this webpage.

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