By Rebecca Gibbel, MS, DVM
ANCIENT SPONGES
Marine sponges have been on Earth longer than any other animal, first appearing in the fossil record between 700 and 890 million years ago [1,2]. Although it can be difficult to interpret fossils of animals that lack a bony skeleton, occasionally, a fossil imprint can be captured of softer-bodied animals like sponges [2]. Dating can also be done using “molecular clock” calculations based on the rate of accumulated genetic mutations [2]. Another clever way to indirectly document that sponges were present in the distant past is to sample rocks or oils of a known age and screen them for steroid biomarker substances uniquely associated with sponges [3].
Living sponges have not changed their anatomy much since the Paleozoic Era, so their body plan must work well for them! Perhaps their long tenure is due to their simple structure – essentially a thorny outer scaffolding of interlocking calcium or silica spicules enclosing a cell-lined cavity that conveniently contains both male and female gonads. Maybe their success is attributable to their tolerance to changes in ocean chemistry and temperature. Or it may be that sponges simply took a fortunate evolutionary path. They may wind up as the surviving winners of the reef, persisting long after the more sensitive corals are gone.
“Hexactinellae” from Ernst Haeckel’s 1904 Kunstformen der Natur, shows some of the remarkable diversity of sea sponges. The name Hexactinellae refers to the six rays that make up the structure of the sponge skeleton.
In addition to their ancient ancestral lineage, individual sponges win the animal longevity award. Although they look very different from familiar terrestrial animals, they are multicellular filter feeders, and some of the deep-sea sponges are carnivorous (which is scary if you’re a copepod). Recently, a glass sponge of the Monorhaphis chuni species was collected in the East China Sea after having lived for 11,000 cold, dark years at a depth of 1100 meters [4]. In addition, a living specimen of a different sponge species, Scolymastra joubini, was harvested and determined to be 15,000 years old. However, there is some controversy about the methods used to make this claim [5]. Other famous superannuated animals seem young in comparison- such as the oldest black corals at 4000 years of age and the Galapagos tortoises and Greenland sharks that can live for several hundred years, which is quite impressive for vertebrates with so many moving parts. Then there is the “immortal’ jellyfish that cycles between life stages in a continual renewal, but that somehow feels more like a magic trick than a single life.
SECRETS of the SPONGE SKELETON
Sponges may appear to be unassuming and often gaily colored blobs anchored to the reef, but don’t be fooled. Recent research has revealed a number of astonishing qualities that are beneath their spiky surface. Some of the extremely long-lived glass sponges have been quietly witnessing millennia of changes in the ocean’s climate and composition. Glass sponges consist of an outer tube of interlocking silica spicules, with associated animal cells that do the work of capturing planktonic food, digesting and reproducing. The glassy spicules have shapes that are characteristic of each species, and their composition reflects the temperature and chemistry of the surrounding ocean through the years of their growth [4]. The silica-based skeletons of sponges are considered “living climate archives”, and they are like live dinosaurs carrying meteorologic and geologic instruments around!
Sponge spicules and the skeletal scaffolding created from them. Photos courtesy of Wikipedia
Glass sponges are particularly useful as recording devices since they are deep-sea animals inhabiting depths of 1100 meters, which is a very difficult area to study. The deepest ocean has fairly constant temperatures across the globe, so samples in one location allow generalized conclusions. Using isotopic and elemental analysis of an 11,000-year-old specimen of Monorhaphis chuni sponge, scientists determined the seawater temperatures and chemistry during the sponge’s long life. In addition to providing historical climate data, the sponges revealed several large fluctuations in temperature over the past millennia that were attributed to nearby hydrothermal vent eruptions [4].
An additional secret of the glass sponge Euplectella aspergillum, known as the Venus flower basket, is that the woven skeleton of the sponge often houses a male and female pair of tiny shrimp-like Stenopodidea crustaceans, who are permanently imprisoned within it [6]. Surrounded by glass chain-link walls, the shrimp have both a secure home and a mated partner, which are scarce in the deep ocean. Floating planktonic food can flow through the sponge lattice, and the tiny progeny of the shrimp are small enough to swim away to strike out on their own. As the pair of Stenopodidea grow, they become permanently closed in within the silica skeleton and perform janitorial services for life.
An Euplectella aspergillum glass sponge, commonly known as a “Venus flower basket”
Photo courtesy of NOAA
PHARMACIES OF THE SEA
Other sponge treasures hiding in plain sight include the myriad chemicals these simple animals create. It may seem odd that an unpalatable sessile organism with almost no predators would evolve so many antimicrobial chemical defencesdefenses, but one speculation is that immobile animals have a greater need to wage chemical warfare against threats since they do not have the ability to move away. Although most of a sponge’s body consists of mineral spicules, over half of its body mass is composed of the microbial communities that live on the sponge’s inner and outer surfaces [7].
Researchers are screening sponges for potential antimicrobial pharmaceuticals that might help counter the growing resistance to current antibiotics, in a process called “bioprospecting”. An astonishing 30% of the promising drugs from all marine sources are derived from sponges and their microbiome organisms [8]. Over 200 new compounds from sponges are discovered each year, and many of them have antiviral, antibacterial, and anti-inflammatory properties, which makes them suitable chemical candidates for drug development. Some examples of drugs isolated from sponge compounds include the antiviral nucleoside known as AZT, the breakthrough treatment for human AIDS, and Cytarabine and Halaven chemotherapy drugs. Remdesivir is another anti-viral drug from sponge chemicals that effectively treats both Ebola virus and Covid-19. Humans owe the lowly sponges some appreciation for this!
The rather unattractive Techtitethya crypta sea sponge has yielded nucleoside compounds from which vital antiviral treatments were developed. Photo credit: https://guide.poriferatreeoflife.org/sp_35.html
LEARNING ENGINEERING FROM THE SPONGES
Sometimes when people think they’ve invented something novel, it turns out that nature had the idea first! There is a lot to learn from natural structures and functions, and the study of biological materials with the goal of repurposing them for human use is called biomimetics. Well-known examples of this include the development of Velcro®, which was copied from the surface hooks of plant burrs that attach themselves to passing animals. Similarly, Leonardo Da Vinci and the Wright Brothers created airplane designs after studying the structure and mechanics of birds’ wings.
When glass sponge skeletons were closely examined, they were found to possess sophisticated optical properties very similar to fiber optic cables. It is not known whether the deep-sea sponges benefit from these light-gathering characteristics, but there may be an advantage to having a body with a faint glow from the tiny amounts of light that pass through to the depths. Or the sponge’s complex scaffold of intertwined silica spicules may have evolved simply for strength, and its optical properties are just a coincidence.
The lattice of the sponge’s glass-like spicules acts as a “wave guide”, confining light waves to the core of the structure, reducing scatter and allowing light to be transmitted down the length of the structure even more efficiently than a man-made fiber optic cable! While the sponges build their scaffolding of fused silica in the cool ocean, commercial fiberoptic cables must be fabricated at high temperatures. Optical physicists examined the structure of the sponges, which contain additive ions that enhance light transmission. However, these ions cannot be incorporated into manufactured waveguides at the high temperatures necessary to fabricate them [9].
Although they are made from glass-like materials, the natural sponge skeleton has a crack-arresting structure that is stronger than synthetic fiberoptic cables and has superior buckling resistance. Biomimetic engineers are studying them to improve the design of bridges and aerospace materials [10].
Skeletal detail of the deep-sea Euplectella aspergillum sponge, showing the basket-like cage structure.
Photo credit: Matheus Fernandes Harvard SEAS
CONCLUSION
Most of us don’t give much thought to sponges other than wondering how long to put a kitchen sponge in the microwave and maybe some late-night musings about whether Sponge Bob Square Pants is too violent for children. Snorkelers often see a colorful underwater creature on the reef and wonder if it’s a coral, only to be disappointed when told: “It’s only a sponge.” But now that we’ve caught a glimpse of some of the hidden wonders of sponges, maybe we can revisit their reputation as the most boring marine inhabitant and nominate sea cucumbers for that role instead!
REFERENCES
1. Turner, E.C. (2019). Possible poriferan body fossils in early Neoproterozoic microbial reefs. Nature, 596, 87–91. https://doi.org/10.1038/s41586-021-03773-z-
2. Wang, X., Liu, A.G., Chen, Z., Wu, C., Liu, Y., Wan, B., Pang, K., Zhou, C., Yuan, X., Xiao, S. (2024). A late-Ediacaran crown-group sponge animal. Nature, 630, 905–911. https://doi.org/10.1038/s41586-024-07520-y
3. Zumberge, J. A., Love, G. D., Cárdenas, P., Sperling, E. A., Gunasekera, S., Rohrssen, M., Grosjean, E., Grotzinger, J., Summons, R. E. (2018). Demosponge steroid biomarker 26-methylstigmastane provides evidence for Neoproterozoic animals. Nature ecology & evolution, 2(11), 170.
4. Jochum, K. P., Wang, X., Vennemann, T. W., Sinha, B., & Müller, W. E. G. (2012). Siliceous deep-sea sponge Monorhaphis chuni: A potential paleoclimate archive in ancient animals. Chemical Geology, 300–301, 143–151. https://doi.org/10.1016/j.chemgeo.2012.01.009
5. Gatti, S. (2002). The role of sponges in high-Antarctic carbon and silicon cycling-a modelling approach. Reports on Polar and Marine Research, 434.
6. NOAA. (2024, June). What is a Glass Sponge? National Ocean Service, https://oceanservice.noaa.gov/facts/glass-sponge.html
7. Liang J, She J, Fu J, Wang J, Ye Y, Yang B, Liu Y, Zhou X, Tao H. (2023). Advances in Natural Products from the Marine-Sponge-Associated Microorganisms with Antimicrobial Activity in the Last Decade. Marine Drugs, 21(4):236. https://doi.org/10.3390/md21040236
8. Hall, Danielle. (2019). Sea Sponges: Pharmacies of the Sea. Smithsonian Ocean. https://ocean.si.edu
9. Sundar, V., Yablon, A., Grazul, J., & Aizenberg, J. (2003). Fibre-optical features of a glass sponge. Nature, www.nature.com/nature
10. Frazier, S. (Oct. 2020). Deep-Sea Sponge Skeletons Could Inspire Better Bridges. Scientific American.
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