Specimen Showcase | The Bioluminescent Sentinel of the Seafloor

Imagine a feather anchored to the seafloor, gently swaying with the ocean’s currents. This is the world of 𝘗𝘵𝘦𝘳𝘰𝘦𝘪𝘥𝘦𝘴 𝘴𝘱𝘢𝘳𝘮𝘢𝘯𝘯𝘪 Kölliker, 1869, a remarkable species within the Pennatulacea order, commonly known as sea pens. 𝘗𝘵𝘦𝘳𝘰𝘦𝘪𝘥𝘦𝘴 𝘴𝘱𝘢𝘳𝘮𝘢𝘯𝘯𝘪 belongs to the octocorals, a group that includes corals and sea anemones, but it stands out due to its unique appearance and specialized adaptations to the marine environment. This lineage has existed for over 450 million years, with origins dating back to the Cambrian period—more than 200 million years before the emergence of dinosaurs.🦕

Like other sea pens, 𝘗. 𝘴𝘱𝘢𝘳𝘮𝘢𝘯𝘯𝘪 resembles an old-fashioned quill or feather🖋️. However, what appears to be a single organism is actually a colony of polyps👨‍👩‍👧‍👦, which are small, soft-bodied organisms related to corals and jellyfish. Each polyp in the colony performs a specific role for 𝘗. 𝘴𝘱𝘢𝘳𝘮𝘢𝘯𝘯𝘪 has a peduncle, a specialized structure that acts as an anchor, embedding itself into soft, sandy, or muddy sediments for stability. Above the peduncle, the colony extends into feathery branches covered with feeding polyps that open and close like tiny hands, capturing plankton and organic particles drifting in the water.

One of the most remarkable features of 𝘗. 𝘴𝘱𝘢𝘳𝘮𝘢𝘯𝘯𝘪 is its bioluminescence—the ability to produce light through chemical reactions in its tissues💙. This adaptation is particularly relevant as 𝘗. 𝘴𝘱𝘢𝘳𝘮𝘢𝘯𝘯𝘪 is typically found at depths of 22 to 72 meters, where light penetration is reduced. In these environments, bioluminescence becomes essential. When disturbed, the sea pen emits a faint blue or green glow, potentially deterring predators like sea stars and certain fish species. This light display may also serve as a form of communication within the species or to attract larger predators to distract smaller threats. Additionally, 𝘗. 𝘴𝘱𝘢𝘳𝘮𝘢𝘯𝘯𝘪 can retract its upper body into the sediment when threatened, adding a layer of physical protection.

Before reaching its bioluminescent, stationary adult form, 𝘗. 𝘴𝘱𝘢𝘳𝘮𝘢𝘯𝘯𝘪 begins life as a tiny, free-swimming larva called a planula. Unlike its adult form, the planula is covered with tiny cilia that allow it to move through ocean currents in search of a suitable habitat. When it finds an ideal spot—usually a nutrient-rich, soft-bottom area—the larva settles and develops into a juvenile sea pen, forming the roots of a new colony. This transformation marks the transition from a mobile larval phase🏃‍♂️ to a stationary adulthood🧎‍♂️. As an adult, 𝘗. 𝘴𝘱𝘢𝘳𝘮𝘢𝘯𝘯𝘪 relies on filter feeding, using specialized feeding polyps called autozoids to capture microscopic organisms. These polyps work together to nourish the entire colony, a stark contrast to the nutrient-dependent, drifting lifestyle of its larval phase.

𝘗. 𝘴𝘱𝘢𝘳𝘮𝘢𝘯𝘯𝘪 is highly adaptable and inhabits a range of marine environments, from shallow coastal waters to deeper zones. In Hong Kong, they are more commonly found in deeper, soft-bottom habitats where nutrient-rich currents support their feeding needs. The muddy or sandy substrates also provide a secure anchor, enabling them to thrive while capturing passing food particles.

This species also displays seasonal changes in behavior, adjusting its feeding activity and physical extension based on environmental conditions. During times of plentiful plankton, sea pens fully extend their polyps to maximize feeding🙆‍♀️. In nutrient-poor seasons, they may partially retract to conserve energy. This adaptability is crucial for survival in dynamic environments where food availability varies throughout the year.

As a filter feeder, 𝘗. 𝘴𝘱𝘢𝘳𝘮𝘢𝘯𝘯𝘪 plays a vital role in marine ecosystems. By filtering suspended particles and organic matter from the water, it helps maintain water clarity and quality. This filtering benefits other organisms, such as coral reefs and seagrasses, that rely on clear water for photosynthesis and respiration. Additionally, 𝘗. 𝘴𝘱𝘢𝘳𝘮𝘢𝘯𝘯𝘪 provides a microhabitat for other animals, such as juvenile fish and crabs seeking shelter within its branches, effectively creating a small ecosystem on the seafloor. Check out those two little cuties, 𝘗𝘰𝘳𝘤𝘦𝘭𝘭𝘢𝘯𝘦𝘭𝘭𝘢 𝘵𝘳𝘪𝘭𝘰𝘣𝘢 on our specimen!🦀

So, next time you marvel at the hidden wonders of the ocean, remember 𝘗. 𝘴𝘱𝘢𝘳𝘮𝘢𝘯𝘯𝘪. Although stationary and seemingly quiet, this remarkable sea pen plays a vital role in maintaining its marine ecosystem, embodying the resilience and quiet strength that define life in the depths. To learn more about the incredible diversity of life on our planet, consider visiting the Hong Kong Biodiversity Museum and following us for more weekly incredible biodiversity tales. Finally, don’t forget to share it with your friends! 🙌

Text: Amber Grace

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