Cranosina Colombiana

From Colombia’s Caribbean coast emerges one of the ocean’s most overlooked architects—Cranosina colombiana, a tiny bryozoan that builds intricate mineral structures in the depths. Though invisible to the naked eye, this colonial marine animal plays a vital role in reef ecosystems, encrusting hard surfaces with delicate calcified chambers that house hundreds of interdependent individuals.

Identification and Appearance

As a member of the order Cheilostomatida, the most diverse group of bryozoans, Cranosina colombiana possesses mineralized exoskeletons and forms single-layered sheets that encrust over surfaces. Like other members of the family Calloporidae, this species lives in marine environments and tends to encrust on hard surfaces.

Each individual zooid—the tiny animal unit within the colony—measures only about half a millimeter in length. Each zooid consists of a “cystid,” which provides the body wall and produces the exoskeleton, and a “polypide,” which holds the organs. Members of Calloporidae possess extensive calcification under epithelial cell layers within the frontal wall of individual zooid organisms, with the addition of spines in many species.

The colony itself can grow to impressive proportions despite the microscopic size of individual members. While the size of an individual bryozoan zooid is quite small, averaging half a millimeter in length, total colony sizes can range from one centimeter to over a meter across.

Habits and Lifestyle

Bryozoans are typically sessile, colonial animals. Cranosina colombiana spends its entire adult life anchored to a substrate, unable to move from its chosen location. All colonies have “autozooids”, which are responsible for feeding, excretion, and supplying nutrients to the colony through diverse channels.

The colony functions as a unified organism despite being composed of genetically identical clones. Single animals, called zooids, live throughout the colony and are not fully independent, and these individuals can have unique and diverse functions. Communication between zooids occurs through specialized pores, allowing the colony to coordinate feeding, reproduction, and defense.

Once established, colonies persist indefinitely if conditions remain favorable. Colonies, once established, will continue to bud and thrive indefinitely, assuming conditions are favorable.

Distribution

The genus Cranosina has been recorded in Brazil and represents new records for that region. Cranosina colombiana is endemic to Colombia’s Caribbean waters, where it was first described by Osburn in 1950. The species occurs in the tropical western Atlantic, preferring hard substrates at various depths along the continental shelf.

Members of Calloporidae tend to encrust on hard abiotic and biotic surfaces. Colonies usually grow on rocky substrates, but many other solid surfaces are used as well, from the shells and exoskeletons of other invertebrates to floating chunks of Antarctic ice.

Diet and Nutrition

Bryozoans use a lophophore—a ring of cilia-lined tentacles—to generate currents that assist in feeding on diatoms and other planktonic organisms. Each zooid extends its delicate lophophore into the water column, creating microscopic currents that trap food particles.

Each zooid’s tiny, mucous-coated tentacles trap diatoms and other microscopic organisms, which are swept to the mouth via cilia that line the tentacles, with the mouth at the base of the tentacles and a digestive tract shaped like a U, with the anus just below the ring of tentacles. This filter-feeding strategy allows the colony to extract nutrition continuously from the surrounding seawater.

Mating Habits

Bryozoans are hermaphroditic. Cranosina colombiana, like other members of Calloporidae, employs internal brooding to nurture its young. Members of Calloporidae engage in internal brooding within an internal brooding sac that originates from autozooid tissues, specifically derived from autozooid tissue that the maternal zooid engulfs.

Calloporidae engage in hermaphroditic reproduction in which fertilisation happens within the brooding cavity rather than broadcast spawning in the vast majority of species. Calloporidae reproduction is unique in the development of the ovicell which typically houses zooid embryo and consists of the hyperstomial, or reproductive zooid—ovicells are a globular chamber that serves to incubate embryos in a durable skeletal like calcified structure.

Once larvae develop, they are released into the water column where they disperse before settling on suitable hard substrates to establish new colonies.

Population and Conservation

Very little is known about the population status of Cranosina colombiana. The species has been recorded only from Colombian Caribbean waters, with limited documentation in scientific literature since its original description. No formal conservation assessment has been conducted, and the species is not currently listed on the IUCN Red List.

Bryozoans were recently recognized as major reef builders in the Southwestern Atlantic, but their diversity remained unknown until recent comprehensive sampling, highlighting the biodiversity significance of reef systems and the need for more comprehensive sampling to clarify the role of bryozoans in modern turbid-zone reefs and rhodolith beds. As marine ecosystems face mounting pressures from climate change, ocean acidification, and coastal development, the role of bryozoans like Cranosina colombiana in maintaining reef integrity becomes increasingly important.

Fun Facts

• Individual bryozoan zooids are typically about 0.5 millimetres long.

• Some bryozoan colonies can creep very slowly by using spiny defensive zooids as legs.

• Bryozoans are capable of withdrawing their lophophores into their zoeciums in order to avoid predation.

• Zooids can “clone” themselves by budding, but they can also create eggs and sperm and reproduce sexually.

• Mineralized skeletons of bryozoans first appear in rocks from the Early Ordovician period, making it the last major phylum to appear in the fossil record.

• Cheilostomata is the most diverse order of bryozoan, possibly because its members have the widest range of specialist zooids.

• Bryostatins—compounds originally isolated from marine bryozoa—comprise a structurally and functionally novel group of macrocyclic lactones that have shown significant activity against cancer cell lines and bryostatin 1 has entered phase II clinical trials for treatment of melanoma, non-Hodgkin’s lymphoma, renal cancer and colorectal cancer.

References

• Animal Diversity Web. “Bryozoa (moss animals).” University of Michigan Museum of Zoology. Accessed 2024.

• Chimonides, P.J. & Cook, P.L. (1994). “Notes on the genus Cranosina (Bryozoa, Cheilostomida).” Zoologica Scripta, 23, 43–49.

• Montoya-Cadavid, E., Flórez-Romero, P., & Winston, J.E. (2007). “Bryozoans of the Colombian Caribbean.” Biota Colombiana, 8(2), 159-184.

• Wikipedia. “Bryozoa.” Wikimedia Foundation. Accessed 2024.

• Wikipedia. “Calloporidae.” Wikimedia Foundation. Accessed 2024.