Leucetta sp. is an encrusting sponge with a firm tubular body, homogenous organization, and leuconoid form (Fig. 1). Complex folding of the choanoderm creates ridges on the sponge’s surface. Leucetta sp. has a three-dimensional choanoskeleton made of uniform, triradiate calcareous megascleres (Fig. 2). These spicules provide structural support for the choanosome, or inner part of the sponge (Borojevic et al., 2002). Additionally, a layer of freely distributed spicules forms the ectosome on the outer surface of the sponge (Voigt et al., 2012). In Leucetta sp., there is no clear distinction between the ectosome and choanoskeleton (Borojevic et al., 2002; Hooper, 2003). 
​

Leucetta sp. is white in color, which is characteristic of calcareous sponges. While many demosponges are colorful due to their photosymbionts, calcareous sponges are typically not photosynthetic so they are pale yellow or white (Rossi et al., 2011; Fromont et al., 2016). My sponge sample was roughly 3 cm in length and 5 mm in height with a crusty texture. The oscula were not as large or obviously visible as those on Leucetta chagosensis and Leucetta micrographis.

Spicules are mineralized structures that provide skeletal support in sponges (Brusca et al., 2016). They come in a variety of shapes, sizes, and arrangements, so spicule morphology is often used in sponge identification (Hooper, 2003). Calcareous sponges have spicules composed of calcium carbonate, while demosponges contain spicules made of silica. Microscleres are small reinforcing spicules, and megascleres are larger spicules that function as the primary support element (Brusca et al., 2016). Calcareous sponges only have megascleres (Bergquist, 1972).
​

To isolate and analyze my sponge’s spicules, I used the bleach digestion method described by Hooper (2003). Small sections of sponge were extracted and immersed in bleach solution (125 g/L hypochlorite) for two hours until all organic tissue had been dissolved, leaving only the mineral skeleton (Hooper, 2003). The following suspended spicules were diluted with distilled water and centrifuged. Spicules were extracted and prepared for observation under a compound light microscope and Scanning Electron Microscope (SEM).
​

Leucetta sp. has regular triradiate spicules, meaning they have three identical rays at equal angles to each other (Fig. 3-5). This spicule form is common in calcareous sponges and is a defining characteristic of the family Leucettidae (Borojevic et al., 2002; Hooper, 2003). Each Leucetta sp. ray was between about 0.1–0.35 mm in length.

The nature of my sponge’s spicules was a key feature used for identification. Here, I describe how I identified my sponge to the Leucetta genus. 
Class: Calcareous sponges only contain megascleres and are considered much more limited in their spicule morphology compared to demosponges, which typically have multiple spicules of diverse shapes and sizes (Bergquist, 1972; Manuel, 2006; Brusca et al., 2016). My sponge only has one type of spicules; they are triradiate, which is the most form among the Calcarea (Manuel, 2006). 
Subclass: Like all members of the Calcinea subclass, my sponge has spicule rays that are equiangular and roughly equal in length. The other calcareous subclass, Calcaronea, generally has spicules with two short rays and one elongated ray, and the rays are not arranged at equal angles to each other (Bernquist, 1978). 
Order: My sponge was further classified in Clathrinida because its skeleton contains freely distributed spicules (Hooper, 1998). 
Family: Sponges in the Leucettidae family have a leuconoid body form, thin ectosome, and triradiate or quadriradiate spicules, which all describe features of my sponge (Hooper, 1998). 
Genus: Finally, my sponge sample was classified in the genus Leucetta, which is comprised of sponges that have triradiate spicules and no clear distinction between their choanoskeleton and ectosome (Hooper, 1998). 
​
Dr. John Hooper and Dr. Merrick Ekins, who are both sponge taxonomists at the Queensland Museum, helped identify and confirm my sponge to the genus level, for which I am very grateful. Unfortunately, I was not able to identify the species of my sponge. The Calcarea comprise less than 5% of all known sponges, and they have not been studied extensively due to their small, inconspicuous nature and cryptic lifestyle (Manuel, 2006). Still, I analyzed records of documented Leucetta sponges from the World Record of Marine Species, Atlas of Living Australia, Encyclopedia of Life, and the literature in an attempt to identify my specimen to the species level.
​
Leucetta chagosensis and Leucetta micrographis are the most common Leucetta sponges on the Great Barrier Reef and throughout the Indo-Pacific region, but their morphologies are very different from that of my sponge. L. chagosensis, known as the lemon sponge, is bright yellow in color and has visible oscula. L. micrographis is is white, but it has a very large oscula and it is not as tubular or reticulate as my specimen. The other Leucetta species recorded on the Great Barrier Reef are Leucetta homoraphis and Leucetta villosa, but neither looks like my sponge. Other sponges in the Leucetta genus have been recorded in Antarctica, Florida, the Caribbean, Brasil, South Africa, California, and the Red Sea, but there is very limited information about these sightings. Since none of them were reported anywhere near the Great Barrier Reef or Indo-Pacific region, I could not infer that my sponge was any of these species. 
​
Leucetta sulcata appears most similar to my sponge. It is white with complex folding and the same firm, tubular structure as my sponge. However, L. sulcata has only been recorded in Madagascar. I did not want to identify my sponge too hastily, given that it does not share the same geographic range as L. sulcata. I hope that one day, DNA sequencing can be used to identify my sponge to the species level.

The ecology of calcareous sponges is poorly understood, especially when compared to demosponges (Fromont et al., 2016). As Wörheide & Hooper (1999) explain, calcareous sponges are difficult to study given their relatively poor fossil record and cryptic nature. 
​

My sponge was collected from an ARMS plate that had been deployed at Manly Boat Harbor in Moreton Bay, Queensland, Australia. Leucetta spp. are marine sponges best adapted to sheltered waters from the intertidal zone up to 100m (Bergquist, 1978; Borojevic et al., 2002). They are restricted to shallow depths due to factors controlling calcium secretion (Bergquist, 1978). Leucetta spp. are mostly found in temperate and tropical regions, and they are the most common Calcinea sponges in the tropics (Borojevic et al., 2002). They live in cryptic habitats, attached to firm substrata such as coral rubble, rock crevices, and overhangs (Hooper & Wiedenmayer, 1994; Borojevic et al., 2002; Brusca et al., 2016). 

Although the specific ecology of Leucetta sp. is not known, we can still infer its potential ecological and commercial value. Sponges are critical to marine environments (Wörheide et al., 2005). They filter waste and toxins from the water, and form symbiotic relationships with bacteria, algae, and other reef organisms (Brusca et al., 2016). Sponges that contain cyanobacteria can become net primary producers and add oxygen to seawater (Brusca et al., 2016). However, there is limited information about calcarean symbionts, and most calcareous sponges do not appear photosynthetic, especially since they are typically white like my sponge (Fromont et al., 2016).
​
Sponges produce allelochemicals (via their bacterial symbionts) to fight predators, compete for space, and prevent infection (Brusca et al., 2016). Their spicules and toxic compounds deter most predators, except Hawksbill sea turtles and nudibranchs, which are known spongivores (S. Degnan, pers. comm.). Allelochemicals can also act as an anti-foulant and shape the benthic community. Calcareous sponges typically do not bore into reef substrate like demosponges do.
​
Research has demonstrated that sponges contain antibacterial agents, which may offer opportunities for new pharmaceutical drugs in the future (Brusca et al., 2016). L. chagosensis has been shown to contain imidazole alkaloidal compounds that are active against the pathogen Cryptococcus neoformans, which causes an opportunistic infection in AIDS patients (Dunbar et al., 2000). Other Leucetta sponges also have antimicrobial activity, so perhaps my sponge does as well (Hassan, 2009; Loaëc et al., 2017). 

Leucetta sp. is a filter feeder, meaning that organic food particles in the water are captured and phagocytized. Flagella on choanocyte cells create currents that drive water flow through the sponge (Brusca et al., 2016). As water enters the sponge, the arrangement of the aquiferous system (dermal pore --> incurrent canals --> choanocytes) forms a series of sieves that filter incoming food particles less than 50μm in diameter (Bergquist, 1972; Leys & Eerkes-Medrano, 2006; Brusca et al., 2016). Sticky, mucous-covered choanocyte tentacles trap particles and bring them through the choanocyte collar (Fig. 6). Food particles are initially processed in choanocyte cells but then transferred to archaeocytes, which continue digestion in the mesohyl and transport nutrients throughout the sponge (Brusca et al., 2016). Mobile archaeocytes can also directly phagocytize particles in the 2-5μm range (Bergquist, 1978). Pinacocytes that line the outer surface of sponges phagocytize larger food particles (Brusca et al., 2016). Sponges may also absorb dissolved organic matter from the surrounding water to supplement filter feeding (Bergquist, 1978; Brusca et al., 2016). 

Sexual reproduction in Leucetta sp. is exclusively viviparous, whereby eggs are fertilized inside the female sponge (Hooper, 1998). Leucetta sp. is hermaphroditic, meaning it produces both male and female gametes, but it lacks dedicated germ cells and gonads (Burton, 1963). Instead, sperm arise from choanocyte cells, while eggs derive from archaeocytes (Brusca et al., 2016). Oocytes form in the mesohyl and grow by phagocytizing neighboring amoeboid cells (Brusca et al., 2016). Sperm is released through the oscula into the water column and captured by a choanocyte cell on a female sponge. The sperm must cross the choanoderm, enter the mesohyl, reach the oocytes, and penetrate the follicle barrier in order to fertilize the egg (Fig. 7) (Brusca et al., 2016). Zygotes are brooded within the female sponge and undergo direct development, before emerging as mature larvae. Leucetta sp. larvae are parenchymal (ciliated) and lecithotrophic, meaning they do not feed on plankton but instead rely on yolk from maternal provisions as their sole energy source (Bergquist, 1972). As a result, the larval stage is very brief; larvae must reach developmental competency quickly and settle soon after being released from their parent. Although sponges have no nerves or muscles, they still have sophisticated sensory capabilities that allow them to find suitable habitat (Hooper, 2003). They are negatively phototactic and geotactic, enabling them to settle on the benthos, where they remain for the rest of their life (Brusca et al., 2016). 

The primary advantage of my sponge’s form of reproduction is that offspring have a greater chance of survival. Fertilized eggs grow safely inside the mother instead of floating freely and developing in the open ocean where they would be vulnerable to predation. However, as a consequence, Leucetta sp. produce few offspring because of the high maternal investment required for brooding their young. Additionally, Leucetta sponges have less genetic diversity than those that undergo broadcast spawning and oviparous reproduction, and limited opportunities for dispersal due to their short-lived larval stage.

Sponges are a loose, asymmetrical aggregation of cells around a water canal system (S. Degnan, pers. comm.). They have three main layers:

1. Pinacoderm: outer epithelial-like surface of a sponge comprised of flattened, unciliated cells called pinacocytes
2. Mesohyl: gelatinous middle layer important for nutrient transport, structural support, and reproduction. Includes a mesoglea containing spicules, collagen, and archaeocytes (Brusca et al., 2016)
3. Choanderm: inner surface of a sponge, composed of flagellated choanocyte cells

Leucetta sp. has a solid body with an inorganic mineral skeleton composed exclusively of large spicules known as megascleres (Fig. 8) (Borojevic et al., 2002; Hooper, 2003). The calcium carbonate spicules in Leucetta sp. have a triradiate shape and are produced by specialized mesohyl cells called sclerocytes (Hooper, 2003). Many sponges also have an organic skeleton consisting of a fibrous collagen framework called spongin (Brusca et al., 2016). However, calcareous sponges do not have enough collagen to form true spongin, so it is unclear to what degree collagen provides skeletal support in Leucetta sp.  

Sponges have an extensive network of pores and canals through which water is continuously pumped for feeding, nutrient exchange, and excretion (Brusca et al., 2016). Leucetta sp. has a leuconoid aquiferous system, which is the most complex and common sponge body form (Fig. 9). In this construction, the choanoderm is highly folded into discrete, oval choanocyte chambers that open into branching channels (Hooper, 1998). The advantage of the leuconoid form is that it increases the sponge's surface area to volume ratio, allowing for more efficient gas, waste, and nutrient exchange (Bergquist, 1978).
​

The flow of water through Leucetta sp. is as follows: dermal pores --> incurrent canal --> prosopyle     --> choanocyte chamber --> apopyle --> excurrent canal --> atrium --> osculum (Brusca et al., 2016)
​

Water enters the sponge through ostia, or dermal pores, and passes through incurrent canals to the mesohyl and choanocyte chambers via prosopyle pores. Water movement slows over the choanoderm to allow for gas and nutrient exchange (Brusca et al., 2016). In Leucetta sp., the atrium is often reduced to a series of canals that lead directly into the osculum, where water and waste materials are ultimately expelled (Borojevic et al., 2002). 

Although sponges do not have true tissues, organs, or cellular coordination, they have a variety of specialized cell types (Fig. 10):

• Archaeocytes (also known as amoebocytes) are totipotent stem cells that can differentiate into other cell types (Brusca et al., 2016). These remarkable cells allow the sponge to reconfigure its body plan, regenerate, and perform different functions. Archaeocytes are highly mobile and found in the mesohyl. They are important in phagocytosis, digestion, nutrient transport, reproduction, and waste excretion.
• Choanocytes are specialized, flagellated cells with a collar of microvilli actin filaments (Fig. 11) (Bergquist, 1972). Choanocytes drive water currents throughout the sponge by beating their flagella in an uncoordinated manner. They lie next to the mesohyl and also perform phagocytosis.
• Pinacocytes are flat, compact, epithelial cells that line the outer surface of sponges as well as all inner canals (Fig. 11) (Bergquist, 1972; Hooper, 2003).
• Sclerocytes produce spicules that provide skeletal support for the sponge (Hooper, 2003).
• Lophocyte and collenocyte cells secrete collagen fiber that supports the mesohyl (Brusca et al., 2016).
  
• Spongocytes produce a fibrous collagen matrix known as spongin that provides skeletal support in the mesohyl (Brusca et al., 2016). Typically, calcareous sponges do not have have spongin.
  
• Porocytes are cells on the pinacoderm that control water flow through the ostia and dermal pores (Brusca et al., 2016).

Sponges do not have a central nervous system, muscles, heart, gut, digestive cavity, or any of the complex organs seen in higher Metazoa (Brusca et al., 2016). However, they still have incredible functionality from their aquiferous system and plasticity due to their totipotent archaeocyte cells (S. Degnan, pers. comm.).
​

Sponges can respond to environmental stimuli and prevent the uptake of potentially dangerous particles (Leys, 2015). Cilia on the oscula can detect changes in water current, so the oscula will contract in response to reduced water flow (Brusca et al., 2016). Sponges also constrict their bodies in response to chemical irritation, sediments, lack of oxygen, and temperature change (Leys & Hill, 2012). 
​

Sponges are aerobic, and gas exchange with incoming water typically occurs through diffusion across the choanoderm (Hadas et al., 2008; Brusca et al., 2016). Waste is continuously excreted through oscula (Brusca et al., 2016). Sponges exhibit phototaxis, which is critical for larval settlement, as well as chemotaxis. Cells can communicate with each other by releasing hormones, known as paracrine signaling (Brusca et al., 2016). 

I would like to thank Professors Sandie and Bernie Degnan for their guidance in Marine Invertebrates class and lab. Special thanks to Dr. John Hooper and Dr. Merrick Ekins from the Queensland Museum for helping with sponge identification. I would also like to thank our tutors, Eunice and Davide, as well as the other PhD students and post-docs who assisted in lab.

Atlas of Living Australia. ‘GENUS: Leucetta.’ Available at https://spatial.ala.org.au/?q=lsid:urn:lsid:biodiversity.org.au:afd.taxon:99aed4f8-3496-43b8-b416-15e7a50b69c9 [accessed 22 May 2019].

Bergquist, P.R. (1972). Phylum Porifera. In ‘Textbook of Zoology Volume I: Invertebrates.’ 7^thEdn. (Eds A.J. Marshall & W.D. Williams). pp. 76-103. (Macmillan Press: London).

Bergquist, P.R. (1978). ‘Sponges.’ (University of California Press: Berkeley, CA).

Borojevic, R., Boury-Esnault, N., Manuel, M., and Vacelet, J. (2002). Order Clathrinida Hartman, 1958. In ‘Systema Porifera: A Guide to the Classification of Sponges.’ (Eds J.N.A. Hooper & R.M. Van Soest). pp. 1141-1152. (Kluwer Academic Plenum Publishers: New York, NY).

Brusca, R.C., Moore, W., and Shuster, S.M. (2016). Two basal metazoan phyla. In 'Invertebrates.' 3^rd Edn. (Eds R.C. Brusca, W. Moore, and S.M. Shuster). pp. 212-263. (Sunderland, Massachusetts U.S.A. Sinauer Associates, Inc: Sunderland, MA). 

Burton, M. (1963). ‘A revision of the classification of the calcareous sponges with a catalogue of the specimens in the British Museum (Natural History).’ (British Museum: London).

Degnan, S. ‘L5-6: Phylum Porifera.’ 12 Mar 2019 [personal communication].

Dohrmann, M., Voigt, O., Erpenbeck, D., and Wörheide, G. (2006). Non-monophyly of most supraspecific taxa of calcareous sponges (Porifera, Calcarea) revealed by increased taxon sampling and partitioned Bayesian analysis of ribosomal DNA. Molecular Phylogenetics and Evolution 40, 830-843.

Dunbar, D.C., Rimoldi, J.M., Clark, A.M., Kelly, M., and Hamann, M.T. (2000). Anti-cryptococcal and nitric oxide synthase inhibitory imidazole alkaloids from the calcareous sponge Leucetta cf chagosensis. Tetrahedron 56, 8795-8798.

Encyclopedia of Life. ‘Leucetta Haeckel 1872.’ Available at https://eol.org/pages/33757/maps [accessed 25 May 2019].

Fromont, J., Huggett, M.J., Lengger, S.K., Grice, K., and Schonberg, C.H.L. (2016). Characterization of Leucetta prolifera, a calcarean cyanosponge from south-western Australia, and its symbionts. Journal of the Marine Biological Association of the United Kingdom 96, 541-552.

Hadas, E., Ilan, M., and Shpigel, M. (2008). Oxygen consumption by a coral reef sponge. Journal of Experimental Biology 211, 2185-2190.

Hassan, W. H., Al-Taweel, A. M., and Proksch, P. (2009). Two new imidazole alkaloids from Leucetta chagosensissponge. Saudi Pharmaceutical Journal 17, 295–298. 

Hooper, J.N.A. (1998). 'Guide to the families and genera of Calcarea.' (Queensland Museum: Brisbane).

Hooper, J.N.A (2003). ‘Sponguide: Guide to Sponge Collection and Identification.' (Queensland Museum: Brisbane).

Hooper, J.N.A. and Wiedenmayer, F. (1994). Porifera. In ‘Zoological Catalogue of Australia.’ Vol 12. (CSIRO: Melbourne).

Leys, S.P. (2015). Elements of a ‘nervous system’ in sponges. Journal of Experimental Biology 218, 581-591.

Leys, S.P. and Eerkes-Medrano, D.I. (2006). Feeding in a Calcareous Sponge: Particle Uptake by Pseudopodia. The Biological Bulletin 211, 157-171.

Leys, S.P. and Hill, A. (2012). Chapter One: The Physiology and Molecular Biology of Sponge Tissues. In ‘Advances in Marine Biology’. Vol 62. (Eds. M.A. Becerro, M.J. Uriz, M. Maldonado, and X. Turon). pp. 1-56. (Academic Press: London).

Loaëc, N., Attanasio, E., Villiers, B., Durieu, E., Tahtouh, T., Cam, M., Davis, R.A., Alencar, A., Roue, M., Bourguet-Carreaux, M., Proksch, P., Limanton, E., Guiheneuf, S., Carreaux, F., Bazureau, J., Klautau, M., and Meijer, L. (2017). Marine Derived 2-Aminoimidazolone Alkaloids Leucettamine B-Related Polyandrocarpamines Inhibit Mammalian and Protozoan DYRK & CLK Kinases. Marine Drugs 15, 1-15.

Manuel, M. (2006). Phylogeny and evolution of calcareous sponges. Canadian Journal of Zoology 84, 225-241.

Pineda, M., Strehlow, B., Sternel, M., Duckworth, A., den Haan, J., Jones, R., and Webster, N.S. (2017). Effects of sediment smothering on the sponge holobiont with implications for dredging management. Scientific Reports 7, 1-15.

Reece, J.B., Urry, L.A., Cain, M.L., Wasserman, S.A., Minorsky, P.V., and Jackson, R.B.. (2011). Campbell Biology. Pearson Education Inc. Boston

Riesgo, A., Farrar, N., Windsor, P.J., Giribet, G., and Leys, S.P. (2014). The Analysis of Eight Transcriptomes from All Poriferan Classes Reveals Surprising Genetic Complexity in Sponges. Molecular Biology and Evolution 31,1102–1120.

Rigby, J.K. (1987). Phylum Porifera. In ‘Fossil Invertebrates.’ (Eds. R.S. Boardman, A.H. Cheetham, and A.J. Rowell). pp. 116-139. (Blackwell Science: Cambridge, MA).

Rossi, A.L., de Moraes Russo, C.A., Solé-Cava, A.M., Rapp, H.T., and Klautau, M. (2011). Phylogeneticsignal in the evolution of body colour and spicule skeleton in calcareous sponges. Zoological Journal of the Linnean Society 163, 1026–1034.

Voigt, O., Wulfing, E., and Worheide, G. (2012). Molecular phylogenetic evaluation of classification and scenarios of character evolution in calcareous sponges. PLOS ONE 7, 1-16.

Webster, N.S. (2007). Sponge disease: a global threat? Environmental Microbiology 9, 1363-1375.

Wörheide, G., and Hooper, J.N.A. (1999). Calcarea from the Great Barrier Reef. 1: Cryptic Calcinea from Heron Island and Wistari Reef. Memoirs of the Queensland Museum 43, 859-891.

Wörheide, G., Sole-Cava, A.M., and Hooper, J.N.A. (2005). Biodiversity, molecular ecology and phylogeography of marine sponges: patterns, implications and outlooks. Integrative and Comparative Biology 45, 377–385.

WoRMS (World Register of Marine Species). ‘Leucetta Haeckel, 1872.’ Available at http://www.marinespecies.org/aphia.php?p=taxdetails&id=131731 [accessed 24 May 2019].