What is a Gorgonian?

A gorgonian is a soft coral, a colonial, sessile animal. Gorgonians were initially believed to be ‘marine plants‘ and can still be misidentified today. Gorgonians are also called ‘sea fans’ due to many species that have an arborescent morphology, or fan-like shape. 

Below, are the top 10 questions frequently asked by those keen to learn more about these enigmatic creatures. Each point – and many more gorgonian topics – will be elaborated upon in more detail at Patreon. Sign up to learn more 🪸 

1. Why are they called Gorgonians?

The name ‘Gorgonian’ comes from ‘gorgonin’, a unique horn-like proteinaceous material that is present in the axis of most species. Gorgonians are typically characterised by having an internal skeletal axis (endoskeleton), and it is this semi-rigid proteinacous material that can provide flexibility to the colonies. Different species have different amounts of gorgonin as part of the central axis, often reinforced with varying quantities of calcareous material, including alternating configurations such as those seen in the Bamboo corals i.e., gorgonin nodes (yellow arrows) and calcareous internodes. Also note the bioluminescence on this deep-sea specialist.

2. Why are they soft corals?

It is true that gorgonians are not very soft! They have an internal axis and their soft tissues are filled with micro-skeletal elements called sclerites. But, all soft corals are filled with a variety of different sclerites that can characterise a particular genus or species. In fact, having a central axis doesn’t mean very much in terms of systematic interpretation – in other words, how these species have evolved and thus, related to each other. This is because having an axis has been gained and lost many times throughout their evolutionary history.

Therefore, whilst gorgonians are a highly diverse group of corals, the term ‘gorgonian’ is no longer scientifically valid. Furthermore, advances in research techniques have revealed that whilst gorgonian species are characterised by a central axis, they are just as related to soft corals without a central axis as to those that have. Therefore, gorgonians are soft corals and belong to the taxonomic class Octocorallia Haeckel, 1866 (Cnidaria: Anthozoa).

Why, then, do I just work on gorgonians? Because at mesophotic depths in particular they are the dominant coral group, highly diverse and therefore, have ecological and conservation significance as a functional group. They are also a full-time job!

3. What is unique about Octocorals?

Octocorals get their name from the latin word ‘Octo-‘ meaning eight; eight tentacles, eight mesentaries dividing the gut, all eight! This is in contrast to the Hexacorallia (e.g., the hard corals that you see on tropical reefs), which have hexaradial symmetry, with six or multiples of six tentacles (12, 18, 24 etc.. some corals even have 36 tentacles).

Each tentacle of an octocoral is hollow and on the lateral edges bear pinnules (red arrows), tentacles on tentacles! These increase the surface area of the tentacles for nutrient acquisition.

4. What Do Gorgonians Eat?

Gorgonian octocorals are suspension feeding heterotrophs, primarily feeding on particulate organic matter (POM) and dissolved organic matter (DOM) in the water column. The majority of species at the mesophotic depths of the Indo-Pacific have small polyps, and all gorgonians and soft corals have very weak nematocysts (stinging cells to capture things to eat). This means that they’re not very good at catching prey. Thus, their nutrient reliance is on smaller things and not exclusively on zooplankton as previously thought.

Some species can be mixotrophs, meaning that they can also harbour photosynthetic microalgae (i.e., they are zooxanthellate), the zooxanthellae that are also present in reef corals. Significantly less gorgonians are zooxanthellate, and particularly so in the Indo-Pacific. Indo-Pacific species do seem to be hardier, for example my doctoral thesis research on the zooxanthellate gorgonian Isis hippuris Linnaeus, 1758 contained one of the most durable Symbiodinium clades, and were far less susceptible to bleaching. They also adapted remarkably well to human impact resorting to coprophagy on degraded reefs. Thus they are the marine equivalent of the old adage; eat dung, millions of flies can’t all be wrong!

5. Do Gorgonians bleach?

Yes, but not easily. Only those species that possess photosynthetic algae – zooxanthellae – are vulnerable to bleaching. However, the tight relationship (symbiosis) between the algae and the host can lead to remarkable physiological responses and adaptations in the face of environmental change. Furthermore, the gorgonian host is less reliant on it’s photosynthetic dinoflagellates compared to the hard corals. For the Indo-Pacific shallow-water gorgonians possessing zooxanthellae, the future whilst challenging, is likely not as catastrophic as it is for their hard coral relatives.

6. How fast do Gorgonians grow?

Gorgonian growth is relatively slow averaging ~2 cm (± 1 cm) per year after an inital growth spurt of 6 cm in the first year. Interestingly, experimental research has shown that this growth spurt can also occur in a healthy colony that has had some branches removed. However, deep cold water species have been shown to grow an average of 5.08 μm (0.000508 cm) per year, with this study estimating the coral to be 1100 years old. The largest gorgonian on record was 5.7 m tall at 1366 m depth within the Papahānaumokuākea Marine National Monument.

With ~95% of the global marine environment still unexplored, one can only imagine the extraordinary discoveries these enigmatic creatures could disclose.

7. How do Gorgonians reproduce?

Gorgonian octocorals reproduce in a variety of ways. Sexually, they spread their seed through broadcast spawning, or brooding either internally or externally (inside the polyp or attached on the outside). The longer the larvae travel in the water column the greater the likelihood of settlement and colonisation in new and remote locations.  

Asexually, they can fragment their branches (typically the top few centimeters), which will then attach to the sea floor and grow upwards as a new gorgonian colony. This strategy ensures close proximity to other colonies to reproduce with. Several taxa go all out and reproduce both sexually and asexually, maximising their ability to travel to new locations and rapidly populate the habitats.

Most species are gonochoristic, female and male, identified only by inspecting the gametes – the eggs and sperm. Gametes are highly nutritious morsels for predators, the most voracious being nudibranchs.

8. Where do you find them?

Gorgonian octocorals are found in all oceans and virtually all marine habitats, from the shallow depths of coral reefs, to the deep sea, the freezing waters of the Arctic and Antarctic, the many fjords of the world, and I’ve even found them in the mangroves of Indonesia. The deeper it is the more you’ll find, and this is particularly true for the mesophotic depths of the Indo-Pacific.

There are over 5000 species of coral currently recognised, with many more yet to be described in the worlds oceans. Approximately 64% of all corals are Octocorals and 75% of all Octocorals are found below 50 m (164 ft) depth (see Cairns 2007). With the greatest diversity in the Indo-Pacific, and a tremendous abundance at depth, my hope is to use my diving and research collections to contribute to this knowledge.

9. Why are they important?

Gorgonian octocorals are often the dominant species group, most notably at mesophotic and deeper depths. These enigmatic creatures create animal forests providing secondary space and micro-habitats to a tremendous amount of biodiversity. They are host to the delightful pygmy seahorses, and act as nursery grounds for a variety of species of food fishes. Many are conservation ‘flagship’ species – a marine spokesperson if you will – to help bridge the gap between science and policy to protect the biodiversity of the oceans.

These ecosystem engineers are some of the oldest corals on the planet, having had millions of years to adapt to change. Many physiological and ecological characteristics of this group retain their ability to adapt and survive to environmental change; they are even blue carbon sinks in many regions. Yet, whilst the azooxanthellate gorgonians increase in diversity and abundance with increasing depth, they are still vulnerable to anthropogenic disturbance.

10. Conservation: Are Gorgonians in danger?

Yes! Not a single natural environment exists without the impact of humans. Historically, gorgonians have been used for money, and exploited for jewellery even to this day.

Gorgonians are vulnerable to environmental change. Whilst they have adapted to remarkable changes in ocean chemistry and conditions over the millennia, the multiple stressors they face today are unprecidented. Stressors include commercial dredging, deep-sea mining, heavy pollution, coral mining, disease, severe temperature fluctuations, and overfishing. All these stressors have direct and cascading effects leading to habitat destruction. Local communities want to protect their natural resources, however, lack of funding and overbearing administration can thwart effective conservation and research initiatives.

Whilst it has been difficult to narrow down the top 10 key points about gorgonian octocorals, the most consuming were the threats that they face. The deeper depths of the Indo-Pacific are no exception with the persistent signature of human destruction at mesophotic depths in remote locations, where I seek to find nature in it’s original form. The number of gorgonian species isn’t clear, and those that are yet to be discovered (dark taxa) may be lost before they are known; it has never before been so urgent to describe biodiversity before it is lost under our own influence.

Learn more about these enigmatic creatures, their antics, adaptations, and threats at Patreon. Image prints are available for purchase at Fine Art America.