With more than 8,000 known species spanning across the globe, sea sponges are among the most populous and mysterious creatures on the planet. Living in both seawater and fresh water, in the shallows and the deep sea, they have been found thriving in nearly every type of habitat. Sea sponges are multicellular heterotrophs that belong to the Porifera phylum. Porifera means “pore-bearing,” and refers to the thousands of tiny holes that speckle their surface.
These holes are their means of existence – their entire circulatory, respiratory, digestive, and excretory systems are supported by the flow of water through their pores. Sponges are primitive in both structure and behavior. They are mostly detritivores that eat microscopic life forms and organic debris that is carried to and through them by the water’s current. Within the pores of the sponge are specialized cells called choanocytes, also known as collar cells, that make up a special layer called the choanoderm.
Choanocytes are cells with a ring of microvilli surrounding an undulated cilium that is used for particle capture. While most sponges are filter feeders, there are a few exceptions. The deep-sea harp sponge Chondrocladia lyra was discovered by researchers 20 years ago and belongs to the carnivorous Cladorhizidae family of demosponges. The harp sponge earned its name due to its several horizontal branches supporting parallel vertical branches, akin to the shape of a harp. Its shape is not only mesmerizing but advantageous. The sponge’s widespread branches increase its surface area, thus increasing its reach for food and for reproductive purposes. Instead of sucking in water and filtering out microorganisms, the harp sponge captures tiny animals in its branches.
On its limbs, there are thousands of small spikes that can be used to snare prey. Once they’re trapped, the victim is covered by a thin membrane to hold them in place while they are slowly digested. The harp sponge’s unique shape and location give it a biological advantage for reproduction over other sponges. At the tip of its many appendages is a small ball where sperm is stored and produced. When the sperm is released from the ball into the strong deep-sea current, it is captured by the branches of other nearby sponges. Not only do sea sponges have multiple forms of feeding, but they also employ several different types of reproduction.
Most sponges reproduce sexually like the harp sponge, but they can also reproduce asexually through budding, fragmentation, and various forms of regeneration. All sponges are capable of regeneration, which vastly improves their overall survivability. Once a piece of the body removed, the original sponge is able to regenerate the lost appendage, and the portion that was removed can form an entirely new sponge.
Their ability to regenerate is so powerful that a new sponge can generate from even a small mass of cells. When environmental conditions are particularly adverse, such as in winter, the sponge will disintegrate into tiny masses of amoebocytes covered externally by a pinacoderm and spicules referred to as reduction bodies (Chandra, n.d.
). When favorable conditions return, the reduction bodies can regenerate into fully formed sponges within a matter of days. This process has been replicated in a laboratory when reduction bodies from Ephydatia fluviatilis were formed by moving a sponge from a pH of 7 to 6. When the reduction bodies were returned to their ideal pH of 7, the cells accumulated to form a sponge with choanocyte chambers, spicules, and ostia within five days (Bisbee, Francis, & Harrison, 1989).
Their powerful regenerative abilities have resulted in sponges being a focal point for interest in both science and medicine. Since the 1950s, researchers have been finding ways to use the unique compounds found in sea sponges to revolutionize treatment. One of the most significant discoveries has been in the treatment of advanced or recurrent metastatic breast cancer. Advanced or recurrent metastatic breast cancer is defined as disease that has continued beyond two courses of chemotherapy. For these patients, few viable options existed prior to the discovery of eribulin.
Once they stopped responding to traditional methods of anthracycline-based treatment or docetaxel monotherapy, the goal of cancer treatment would be changed to palliative care and efforts to comfortably prolong life. However, in 2010 the Federal Drug Administration approved a new drug for breast cancer treatment under the brand name Havalen, also known as eribulin mesylate. Eribulin mesylate is a synthetic analog of the molecule halichrondrin B, found in a Pacific sea sponge known as Halichondria okadai. Though halichondrin B’s tumor-fighting benefits were discovered in 1986, little came of its discovery due to its inherent complexity. In addition to having a compound structure that was difficult to replicate, halichondrin B is only found at very low concentrations in a rare sea sponge that was collected in Japan. It wasn’t until Dr.
Yoshito Kishi, a Harvard University chemistry professor, with funding from the U.S. National Cancer Institute, discovered its synthesis that we were able to create eribulin mesylate (Fogarty, 2011). Erbulin was approved for cancer treatment following the 2011 EMBRACE study published in Lancet.
In this study, 752 women were randomly allotted to treatment groups, with 508 receiving eribulin and 254 receiving the physician’s drug of choice. Overall survival was significantly increased in eribulin, with a median age of survival of 13.1 months, over the 10.
6 months on the physician’s choice (Cortes, et al., 2011). The incidence of severe side effects between the groups was roughly the same, albeit women treated with eribulin experienced more severe cases of neuropathy, neutropenia, and leukopenia. Eribulin fights cancer by way of its macrocyclic ketone structure. Like other cancer-fighting agents, such as taxanes and vinca alkaloids, eribulin is a microtubule inhibitor that induces apoptosis of cancer cells by stopping mitosis in the G2/M phase of the cell cycle (Park, Kitahara, Kadoya, & Kato, 2013).
What separates eribulin from conventional microtubule inhibitors is that it promotes antitumor activity at lower concentrations. This is due to selective binding with a high affinity to microtubule plus ends and inhibiting only microtubule polymerization without affecting depolymerization (Park et al., 2013). The drug’s induction of antitumor activity lower doses means that it is more cost effective with the potential of fewer or less severe side effects. In addition to their cancer-fighting properties, sea sponges have been utilized in a number of medical treatments. From the species Tectitethya crypta, found in the Caribbean, doctors isolated the chemicals spongothymidine and spongouridine, which were later used in the development of anti-viral and anti-cancer drugs. Among those created from these chemicals was the HIV drug azidothymidine.
According to the National Oceanic and Atmospheric Administration, we have only explored roughly five percent of the ocean (NOAA, 2018). In that five percent, we have already discovered and cataloged thousands of unique sea sponges. From these, we’ve been able to revolutionize certain forms of medicine and improve the quality of thousands of lives. As technology advances, so too will our ability to explore the ocean and from it develop new techniques and treatments.
Sea sponges present a unique opportunity for humanity to understand the creation and extension of life by way of their regenerative abilities. It took twenty-five years from the discovery of halichondrin B to the synthesis and creation of eribulin mesylate. In another twenty-five years, who knows what incredible discoveries and creations science will have made as an extension of the billions of years of evolution that went into so seemingly simple, yet masterfully designed as sea sponges.