Gorgonian Octocorals are one of the most abundant, diverse, and eye-catching features on western Atlantic and Caribbean shallow-water Coral Reefs. The term ‘Gorgonian’’ commonly refers to Octocorals (Subclass Octocorallia, Order Alcyonacea) with an internal supporting axis and branching or tree-like appearance. The upright three-dimensional structure of Gorgonians provides essential habitat, food, and protection for a variety of organisms, including commercially important species. Gorgonians have also been heavily studied as a source for marine natural products (i.e., anticancer, antitumor, anti-inflammatory, antifungal, and antimicrobial properties) and their potential as bioindicators and bioarchives. Despite their importance and abundance, Gorgonians have received relatively little attention in research and monitoring efforts, compared to the focus on Hard Corals (Scleractinians), in part because of difficulties with field identification.
Like Scleractinian Corals, shallow-dwelling Gorgonians exist in a variety of reef habitats (i.e., hardbottoms, patch reefs, transitional reefs, and bank reefs) throughout the Caribbean; however, Gorgonians seem to thrive in a wider range of environmental conditions. Distribution patterns of species have been well documented and are shaped primarily by (1) temperature, (2) light, (3) depth, (4) substratum type, (5) hydrodynamics, and (6) sedimentation. In addition, numerous morphological and physiological mechanisms, including growth rates, skeletal structure, reproduction, recruitment, feeding modes, and immune response, further filter species into habitats based on sensitivities to the aforementioned abiotic factors.
The earliest records of data collection on shallow-water Octocorals date back as far as Louis Agassiz in the nineteenth century. More extensive research on octocorals began in the late 1950s and early 1960s, with an increasing interest in octocorals as potential sources of marine natural products after scientists noticed the strong aromatics of Eunicea mammosa. Research on the biology and ecology ofGorgonians gained momentum in the 1970s and continued through the 1980s in conjunction with the establishment and designation of several marine parks and sanctuaries. Ecological studies conducted through the 1990s and 2000s were mostly sporadic in nature and were focused primarily on growth and reproduction. In recent years, there has been an increased interest in gorgonians with reports of increased abundances, and their abilities to withstand climate-change impacts.
Although past research has provided a strong foundation to build upon, many topics require further development and investigation given the recent reports of increased abundances and evidence of Gorgonian resistance to global stressors such as elevated temperatures, ocean acidification, and nutrient enrichment.
In a paper published in the journal Coral Reefs on 31 January 2020, Selena Kupfner Johnson and Pamela Hallock of the College of Marine Science at the University of South Florida, present a synthesis of knowledge gained from more than a hundred years of widely scattered (1) taxonomical, (2) biological, and (3) ecological research on shallow-water symbiotic Gorgonians in the western Atlantic and Caribbean region, with the primary goals of providing a comprehensive resource document with a brief summary of past research, accessible bibliography, and suggestions for future work that can be used by researchers and resource managers interested in Gorgonians.
Representative photographs of shallow-water Gorgonian habitats taken along the Florida Keys Reef Tract in 2019: (a) nearshore hardbottom, (b) patch reef, (c) shallow fore reef, (d) shallow spur and groove, (e) deep fore reef, (f) deep spur and groove. Florida Fish and Wildlife Conservation Commission Fish and Wildlife Research Institute in Kupfner Johnson & Hallock (2020).
The Subclass Octocorallia is a monophyletic group that contains approximately 3000 species currently divided into three orders: Order Pennatulacea (Sea Pens), Order Heliopaoracea (Blue Corals), and Order Alcyonacea (Soft Corals, Gorgonians, and Stoloniferans). The Order Alcyonacea is the largest of the three orders and has been subdivided into six subordinal groups based mainly on skeletal structure.
Members of the suborders Alcyoniina, Protoacluonaria, and Stolonifera, which lack an internal skeletal axis, are most abundant in the Indo-Pacific region and commonly referred to as ‘Soft Corals’. Most Octocorals found in shallow Caribbean waters have a supporting internal axis composed of proteinaceous material called gorgonin and varying amounts of calcite and are commonly referred to as ‘Gorgonians’. This term encompasses members of three suborders, Calcaxonia, Holaxonia, and Scleraxonia, that were previously assigned to the Order Gorgonacea, which is now synonymized with the Order Alcyonacea. Calcaxonians have a solid axis with calcite in the loculi, Holaxonians have a hollow axis with varying amounts of calcite, and Scleraxonians have an axis made of fused sclerites. Shallow-water Gorgonian communities are dominated by members of the Suborder Holaxonia, with a few members from Suborder Scleraxonia, while members of Calcaxonia are found mostly in deeper habitats. Eleven genera are commonly found in shallow-water Caribbean reef habitats less than 25 m in depth and include 54 species that host algal symbionts belonging to the Symbiodiniacae. Holaxonian genera are divided among two families: the Family Gorgoniidae (Gorgoniids) includes Antillogorgia, Gorgonia, and Pterogorgia, and the Family Plexauridae (Plexaurids) includes Eunicea, Muricea, Muriceopsis, Plexaura, Plexaurella, and Pseudoplexaura. The suborder Scleraxonia includes two genera: Briareum and Erythropodium.
Representative photographs of the 11 common Gorgonian genera reported in shallow waters of the western Atlantic and Caribbean: (a) Antillogorgia, (b) Gorgonia, (c) Pterogorgia, (d) Eunicea, (e) Muricea, (f) Muriceopsis, (g) Plexaura, (h) Plexaurella, (i) Pseudoplexaura, (j) Briareum (digitate form), (k) Briareum (encrusting form), and (l) Erythropodium. Selena Kupfner Johnson in Kupfner Johnson & Hallock (2020).
Current systematics are based on morphological characters that include size and shape of colony, branching pattern, distribution of polyps, axis structure, and sclerite morphology. Many Gorgonians can be field identified to genus level using external features; however, species-level field identification can be quite subjective and microscopic examination of sclerites is often needed. This has led some researchers to recomend grouping species that are commonly confused in the field when comparing data collected by multiple observers. Additionally, Kupfner Johnson and Hallock recommend grouping subspecies, such as Eunicea calyculata typica and Eunicea calyculata coronata, when conducting cross-study comparisons to minimize inconsistencies with identification between studies.
Representative photograph of (a) Eunicea tayrona with magnified views of the (b) branch tip and (c) sclerite morphology. Kupfner Johnson & Hallock (2020).
Several taxonomic species-level revisions have occurred since 1981. Plexaurella dichotoma and Plexaurella fusifera were synonymised in 1985; however, many studies still refer to them as separate species. Plexaura kuna was identified as a new species in 1996. Prior to that, Plexaura kuna was commonly confused with Plexaura homomalla. A revision of the Candelabrum Octocorals of the genus Eunicea and added Eunicea tayrona as a new species in 2009, which closely resembles Eunicea fusca but has distinctly reduced sclerites. Based upon molecular studies, Plexaurella flexuosa was reassigned to Eunicea flexuosa in 2007. The genus name Pseudopterogorgia was changed to Antillogorgia in 2012.
Molecular phylogenetic studies continue to make progress toward further taxonomic resolution; however, efforts are still in their infancy and a lack of morphological characters and species-level molecular markers is inhibiting a full revision of the subclass. An exceptional and detailed review of the ongoing molecular phylogenetic studies including an evaluation of molecular markers previously used to evaluate subfamilial relationships and future progress toward the development of new markers was provided in 2010.
Gorgonians are morphologically diverse, differing in axis structure, colony form, flexibility, biochemical composition, and growth rate, allowing them to inhabit numerous habitats from inshore shallow waters to offshore deepwaters. With skeletons composed of a highly durable protein surrounded by calcareous sclerites, colonies can grow to heights up to 2 m, produce a variety of growth forms, and recent evidence suggests they are resistant to ocean acidification. Scleraxonians (e.g. Briareum asbestinum) have an axis composed of tightly fused calcareous sclerites, called the axial medulla, with varying levels of protein surrounded by a thin layer of coenchyme. Holaxonians (e.g. Gorgoniids and Plexaurids) have an internal axis composed of a hollow central cord surrounded by concentric layers (like tree rings) made of highly durable insoluble protein called gorgonin. The axis is surrounded by an axial sheath and coenchyme. In some Holaxonian species (e.g. Plexaurella spp.), calcite is deposited between concentric layers in the loculi.
Schematic of basic Gorgonian anatomy: (a) cross-section schematic through a Scleraxonian; (b) cross section of a Holaxonian; (c) axial structure of Holaxonian. Kupfner Johnson & Hallock (2020).
The tree-like nature has some distinct advantages: (1) upright growth minimizes the need to compete for bottom space, (2) enhanced ability to capture light, and (3) branching allows for easier particle capture in the water column. Although general morphology can be attributed to genetics, environmental conditions can also influence colony form, making identification difficult. For example, Eunicea flexuosa has been reported as tall and slender in calmer, deeper waters, and broad and bushy in shallow waters. Growth form is a trait that showcases Gorgonians’ high adaptability. Gorgonians exhibit the following colony forms: encrusting (e.g. Briareum and Erythropodium), unbranched (e.g. Briareum), plumose or pinnate (e.g. Antillogorgia and Muriceopsis), reticulate (e.g. Gorgonia), whip-like (e.g. Pterogorgia), and branched (e.g. Eunicea and Plexaura). Branched colonies can be further categorised as candelabra, bushy, or branched. Holaxonians are arborescent (i.e. tree-like) and have a variety of shapes and sizes. Scleraxonians are mostly encrusting or digitate forms.
Schematic of different Gorgonian colony forms found throughout the western Atlantic and Caribbean. Selena Kupfner Johnson in Kupfner Johnson & Hallock (2020).
Flexibility is another functional trait that allows Octocorals to withstand a diversity of habitats from deep to shallow with varying hydrodynamic regimes. Flexibility is highly dependent on the skeletal composition, as well as the shape and arrangement of sclerites in the coenchyme. Axes that are heavily mineralized with calcite between axial layers are stiffer, and those that have little mineralization are generally more flexible. For example, Gorgonia ventalina, which are often found in high-energy waters, are flexible with a stiff base and have little or no mineralisation in the axes, while taxa characterised by more rigid colonies, such as Plexaurella nutans, which are heavily mineralised, are generally found in calmer waters. Taxa exhibiting moderate stiffness, such as most Eunicea spp., are found in areas with moderate wave energy.
Growth rates are typically measured in situ as changes in colony height over time. Alternatively, annual growth bands can be measured, but they require removal of whole colonies at the base. However, it is important to note that while annual banding is assumed for symbiotic species, it has only been validated in temperate and deepwater species. Among those taxa for which data are available, Plexaurids have an average growth rate of 5 cm/year and Gorgoniids often double the rate of Plexaurids. To date, the oldest shallow-water Gorgonians recorded have been approximately 30–40 years, although the maximum age is still unknown.
Growth rates are variable between species, within species, and even within individual colonies, making studies on age and growth rates of shallow-water Gorgonians challenging. Gorgonians exhibit determinate growth. As such, growth rates decrease as the colony matures and are generally highest during the first 5 years. Rapid growth in the first few years makes the recruits less susceptible to mortality from burial and when the colony reaches an optimal size for gamete production and energy is diverted from growth to reproduction. Growth rates may also increase post-disturbance or after partial mortality at the site of injury. Eunicea flexuosa has been shown to twice as long to heal as Pseudoplexaura porosa after injury. Additionally, habitat-related differences in growth rates and morphology (e.g., thickness of branches, polyp density, stiffness, and branching patterns) have been documented in widely distributed species, such as Eunicea flexuosa, Briareum asbestinum, Antillogorgia spp., and Gorgonia spp. For example, thicker branches have been observed in shallow fore-reef areas where wave energy is generally higher.
Gorgonians are known to be gonochoristic (have separate sexes) and are able to reproduce both sexually and asexually. Sexual reproduction is considered the dominant mode for most Caribbean symbiotic species. However, new colonies of some speces, for example Plexaura kuna and Eunicea fusca, may also form asexually via vegetative propagation.
Sexual reproductive modes can be divided into three categories: broadcast spawning, internal brooding, and external brooding. Reproductive mode heavily influences connectivity and diversity. Most Caribbean Gorgonians studied thus far are either broadcast spawners or brooders with varying synchronized gametogenic cycles. Timing is dependent on environmental conditions and is species-specific. For instance, Gorgonia ventalina spawns year-round, whereas other Gorgonians seem to have a narrower window to reproduce. Broadcasters tend to be more widely distributed than brooders, as larvae are transported mainly by currents. Broadcasters release gamete bundles into the water column where they break apart and are fertilized. After fertilization, the zygotes develop into planktonic larvae that remain in the water column for several days to weeks, then settle and metamorphose into polyps (aka: spat). Once settled, the polyp begins to form a colony through the process of budding, a form of asexual reproduction. Eunicea and Plexaura species that have been studied thus far are broadcasters. Brooding is thought to promote recruitment and survival in frequently disturbed habitats, because larvae settle quickly and are frequently fully equipped with Algal symbionts. Internal brooders have larvae that develop within the females; then, days to weeks later, the larvae are released when they are ready to metamorphose, leaving little time in the water column to be eaten by predators. In external brooders, fertilization and partial development occur in mucus pouches on the surface of the female colonies. The larvae are released from the colony when they are ready to settle, resulting in settlement within the same area as the mother colony. Brooding species studied thus far include Briareum asbestinum, Pterogorgia anceps, Antillogorgia bipinnata, and Antillogorgia elisabethae.
Reproductive cycle of brooding and broadcast spawning Gorgonians. Selena Kupfner Johnson in Kupfner Johnson & Hallock (2020).
Studies of asexual propagation in Caribbean Gorgonian species are sparse. The only well-studied species thus far are Plexaura kuna and Eunicea fusca. Asexual propagation may allow for higher rates of population increase and therefore, the ability to survive and recover quickly after disturbances such as storms. However, asexual modes are a disadvantage for genetic diversity, leaving species more susceptible to disease.
No matter the mode of reproduction, substratum and light are limiting factors for settlement and recruitment. Larvae prefer a consolidated hard substrate for attachment, which is often in cracks and under ledges protected against sediment burial. Among the limited number of taxa for which data are available, roughly 60% of brooding species receive their Algal symbionts through vertical transmission, while the studied spawning species uptake their symbionts horizontally from the environment. The larvae that are equipped with symbionts require light and therefore settle on surfaces where light capture is optimal.
Additional reviews are needed for asexual reproduction and recruitment to gain a deeper understanding of mechanisms driving biodiversity on Coral Reefs, especially regarding environmental stressors.
Most tropical shallow-water Gorgonians host Algal endosymbionts and thereby have the ability to utilise both heterotrophic and photoautotrophic food sources. Heterotrophic capabilities have been linked to polyp size, branching pattern, and orientation to current. Gorgonians have been observed feeding on particulate organic matter, zooplankton, and microplankton from the water column; however, the relative dependence on heterotrophic feeding is still unknown in most taxa.
Traditional feeding experiments on Gorgonians date back as far as 1918, when Lewis Cary studied 11 species of Gorgonians and found species with the greatest surface-to-volume ratios had higher metabolisms than species with low ratios. A more recent study elaborated on Cary’s work using light–dark bottle experiments and carbon-isotope tracers. This study found significant differences in photosynthesis to respiration rates among 11 Gorgonian species. These differences negatively correlated with polyp size. Sea Fans and Sea Plumes (Family Gorgoniidae) reportedly acquire most of their energy from photosynthesis of their Algal symbionts, while Plexaurids (Branched Sea Rods) exhibited varying degrees of heterotrophy. Plumose and reticulate morphologies with small polyps likely maximise light exposure and optimise symbiont densities and nutrient exchange via increased surface area-to-volume ratios. Branched rod-shaped colonies with larger polyps might better support suspension-feeding by increasing water movement around the polyps. The relationship between polyp size and dependence upon photosynthesis suggests that some trade-offs may exist for different feeding modes. This is clearly a topic meriting additional research.
Another fundamental aspect of nutrition that is still unknown is variability in autotrophy and heterotrophy in relation to environmental change. Stable isotope analyses of Octocorals are becoming more routine in ecological studies as tracers of trophic level and source nitrogen. Bulk analyses are relatively simple and cost-effective, with potential to produce useful results, though confounding effects such as light can complicate interpretations. Recently, compound-specific stable isotope analysis has shown potential to distinguish nitrogen sources by use of amino acids, providing a tool to explore trophic interactions. Further studies on nutritional pathways and resource allocation could provide added insight into distribution patterns, physiological response to changing environmental conditions, as well as intraspecific and interspecific phenotypic variability.
Gorgonians, like other Cnidarians, have intriguingly complex immune systems that are equipped with an arsenal of bioactive compounds and various physiological mechanisms that help defend against foreign invaders and enhance resilience to environmental stress. These mechanisms include (1) mucus shedding, (2) melanisation, (3) secondary metabolites, (4) wound healing by amoebocytes, and (5) rapid lesion recovery. Main causes of death include, but are not limited to, detachment and burial from storms, overgrowth, predation, and disease.
General immune response pathways have been described in detail. The first line of defense for gorgonians includes physical barriers (e.g. mucus and sclerites) that help prevent threats from entering the organism, much like skin functions in Humans. Mucus has a range of defensive functions. It can act as sunscreen, can contain antipredatory compounds, and can be sloughed off to prevent sediment suffocation and entry of foreign material. Sclerites have also exhibited antipredatory qualities.
The internal immune response begins with the detection and recognition of ‘self versus non-self’ via pattern recognition receptors. After a threat has been identified, signaling pathways are activated to begin the associated effector response. These pathways activate cellular and chemical defenses with the end goal of destroying, isolating, eliminating the threat. Cellular responses include phagocytosis, encapsulation, cell lysis, and melanisation. Chemical responses include numerous secondary metabolites, antimicrobial peptides, enzymes, and reactive oxygen species. Once the threat has been eliminated, repair mechanisms are triggered that are essential to recovery and resilience and include apoptosis, antioxidants, and wound-healing amoebocytes.
Schematic of general components of immune response found in Gorgonians. Selena Kupfner Johnson in Kupfner Johnson & Hallock (2020).
An area of much-needed focus is how Octocoral immune function varies with changing environmental conditions. The detection of invaders requires resources to be allocated toward immunity. Prolonged stress could cause a reduction in available energy and lead to an immune-compromised health state (e.g., reproductive failure of infected colonies) To date, more than a dozen diseases have been reported to affect Gorgonians in the wider Caribbean. In addition, bleaching has been reported in eight of the 12 shallow-water genera: Muricea, Plexaurella, Pseudoplexaura, Pterogorgia, Briareum, Muriceopsis, Erythropodium, and Eunicea (only Eunicea flexuosa). However, bleaching can be difficult to assess as many Gorgonians have dark pigmentation in the tissues and sclerites that remain even after loss of symbionts.
Studies investigating the structure of Gorgonian assemblages began primarily in the early 1960s and have continued to the present. At least one distributional study was conducted in each of the seven eco-regions of the Caribbean basin between 1968 and 2018. Although direct comparisons of distributional studies are difficult because of differences in timing and sampling effort, some consistent trends can be extracted and used to guide further studies, especially those concerned with identifying potential bioindicator species.
Map marking locations of select distributional studies (stars). Numbered eco-regions are as follows: (1) Floridian, (2) Bahamian, (3) Greater Antilles (Northern Caribbean), (4) Eastern Caribbean, (5) Southern Caribbean, (6) Southwestern Caribbean, (7) Western (Meso) Caribbean. Kupfner Johnson & Hallock (2020).
Gorgonian population dynamics are principally driven by a range of interconnected environmental factors. Many of these are heavily influenced by Human activities, including temperature, substrate type and availability, structural complexity, water movement, sediment transport, depth, light intensity, and salinity. In addition, biotic factors such as competition, predation, symbioses, reproduction, settlement, and developmental properties provide local-scale refinement. Together, these abiotic and biotic factors have been shown to induce habitat filtering and morphologic variability.
Temperature controls many physiological and ecological processes (e.g., metabolic rates, reproduction, dissolved oxygen content, chemical reaction rates) and as such is one of the most widely recognized influences on the distribution and growth of marine organisms. All organisms have an optimal range, and their capacity to tolerate temperatures outside of this range directly affects survival. Few studies have investigated the temperature tolerances of gorgonians. Lewis Cary found the maximum upper limit of 12 species in Dry Tortugas to be between 34.5 and 38.2°C for a 24 hour exposure. Resistance was determined to be species-specific, with Plexaurids being least resistant and the Scleraxonian Briareum asbestinum being most resistant. A study in the 1970s determined the optimal ranges and extremes for six common species off Palm Beach, Florida. The optimal temperature range for Gorgonians was similar to that of Scleractinians, between 18 and 33°C, with a lower tolerance generally 15–17°C and an upper tolerance consistent with Cary’s findings. In general, Gorgonians appear to have a higher tolerance to warm temperatures and may be more restricted in distribution because of lower limits; however, more work is needed in this area.
The nature of the substratum (i.e., bottom relief), substratum availability, type of sediment, and sediment transport are proximal factors controlling settlement and survival of planulae. Excess sedimentation can impede recruitment and growth (e.g. by burial), reduce light attenuation, and increase abrasion. Gorgonians typically favor open areas of rough, solid bottom, with little to no inclination, for attachment. Recruits frequently find refuge in depressed areas between Scleractinians on reefs. However, it is not unusual for severe storms to fill these depressed areas with sediment, especially on patch reefs that are surrounded by a sand halo. This is likely why the tops of shallow-water patch reefs are often more populated with Gorgonians than the sloping sides.
Currents and wave energy control food and sediment transport, which have a marked influence on larval dispersal, recruitment, plankton dispersal, morphology, and orientation. The ‘tree-like’ nature and morphological plasticity of shallow-water Gorgonians allow them to inhabit a variety of flow regimes from shallow water with high wave action and surge, to deeper water with low energy, current-driven zones. Research has correlated axis stiffness with water-movement- based zonation in 13 species of Caribbean Gorgonians. The stiffest axes were found in deeper waters with low wave energy and surge. Shallow waters with moderate surge and wave energy had the most flexible axes. The orientation of fan-shaped and candelabrum-shaped Gorgonians perpendicular to net flow of water is so well documented that Gorgonians have been used as indicators of general flow patterns. This positioning is thought to enhance feeding efficiency and particle capture in deepwater species and optimize light capture in shallow-water species such as Gorgonia ventalina.
Nearly all shallow-water Caribbean Gorgonians host Dinoflagellate endosymbionts. The need for photosynthates therefore limits these species from growing in deeper waters with low light availability. Moreover, local bathymetry and nature of the substratum can cause significant variations in light intensity (i.e., turbidity, reflectance, and shading). For example, sandy bottoms increase reflectance and overhanging topography provides shade. Species with Algal symbionts are generally found in waters shallower than 16 m, while only a few species lacking Algal symbionts are found at depths of up to 25 m (e.g. Iciliogorgia schrammi).
Salinity ranges are much less understood, but the areas where reefs are best developed have an average salinity of 36.0 parts per thousand. In general, Gorgonians seem to be able to withstand hypersaline conditions more easily than reduced salinities, with an optimal range of 29.5–42.5 parts per thousand. There is also evidence that Gorgonians may be able to acclimate to lower salinities if the changes are gradual. However, this is one of many ecological parameters that needs further investigations.
Several ecological studies have recently emerged that provide evidence to support the hypothesis that Gorgonian Octocorals may be more resistant to stressors, such as elevated temperatures, bleaching, nutrient enrichment, and ocean acidification, and recover faster after disturbances than Scleractinian Corals. In fact, two locations in the Caribbean have recently reported shifts to Octocoral-dominated states based on long-term retrospective analyses of photographic data: (1) fore-reef environments of the Florida Keys and (2) reef sites in the US Virgin Islands. However, the extent of such population shifts and implications to ecosystem services are poorly understood. Possible explanations for these shifts that require further investigation include:
- The tree-like nature of gorgonians makes them better spatial competitors against Macroalgae because of rapid linear extension rates, which minimises the potential for smothering by Macroalgae.
- Higher post-disturbance recruitment rates allow Gorgonians to exploit space made available by declining populations of Scleractinians.
- Gorgonians and Scleractinians are biologically and morphologically diverse, and as such, their adaptive capabilities and threshold responses to changing environmental conditions may also vary considerably
The next step is to investigate these hypotheses regarding mechanisms favoring changes in gorgonian abundance. Quantitative studies that encompass more than Scleractinians are needed, in addition to studies that evaluate water quality data. Population models could be developed to predict future changes in Gorgonian communities due to environmental influences, based upon targeted surveys and additional analyses of historical records. With additional knowledge, documented trends in Gorgonian octocoral species abundance and richness could be dependable indicators for environmental conditions on reefs.
As global temperatures rise, storm frequency increases, and other climate-change-related stressors continue to impact Coral Reefs. Policy makers and resource managers urgently seek biological indicators that can be used as proxies to assess environmental conditions. Gorgonians are ideal candidates for bioindicators because they (1) are long-lived, (2) are sessile and cannot migrate, (3) build a protein axis which records conditions at the time of formation in annual bands, (4) are abundant and easily sampled, (5) have distributions strongly connected to abiotic factors, and (6) exhibit consistent sensitivities to changes in abiotic conditions.
The use of Gorgonians as effective bioindicators for identifying sources of anthropogenic pollution has been demonstrated in several studies around the Caribbean (e.g. sewage and agricultural fertilisers) Because the skeletal axes of Gorgonians are composed primarily of protein, large amounts of nitrogen are incorporated into the skeleton from the environment. This creates a record of the environmental conditions at the time of synthesis, giving the skeletal components immense value as bioarchives. Species used previously in the wider Caribbean for stable isotopic studies aimed at identifying sources of anthropogenic nitrogen include Gorgonia ventalina, Eunicea flexuosa, Plexaura homomalla, Antillogorgia spp., and Pseudoplexaura porosa. However, additional work is still needed to determine how isotopic values differ within and between species that are exposed to the same array of environmental conditions.
Studies that have utilised the whole Gorgonian assemblage as a tool for biomonitoring are limited to a few areas off of Cuba Higher abundances of ‘sensitive’ species are considered to be indicative of stable/favorable conditions, whereas higher abundances of ‘tolerant’ species are often indicative of suboptimal conditions. Some researchers have defined stress tolerators as ‘slow-growing organisms that are able to survive in nearly all habitats, but only dominate in habitats where physiological stress precludes or slows the growth of ruderals (rselected species) and competitors’. One researcher attempted to develop gorgonian indices based on the presence of tolerant species. He used the sum of relative abundances of 11 species, identified as tolerant to hydrodynamic stress, to infer the degree of hydrodynamic stress in a variety of reef habitats. The hydrodynamic index included Eunicea calyculata, Eunicea flexuosa, Eunicea mammosa, Eunicea tourneforti, Gorgonia flabellum, Gorgonia ventalina, Muricea muricata, Plexaurella dichotoma, Pterogorgia anceps, Pterogorgia citrina, and Pterogorgia guadalupensis. Another study applied similar principles to infer the degree of organic pollution based on presence of six species considered tolerant to polluted environments. The pollution index included Eunicea calyculata, Eunicea flexuosa, Eunicea mammosa, Eunicea tourneforti, Plexaura kukenthali, and Pseudoplexaua flagellosa. A later study proposed the addition of a seventh species, Pterogorgia citrina, to the pollution index.
To further validate which species or species groups are the best-suited indicators of specific environmental conditions, a deeper quantitative multivariate analysis of environmental parameters affecting distributions in all ecoregions of the Caribbean is needed. Additionally, because current taxonomy is based on several morphologic characters, exploring functional alternatives or coarser taxonomic linkages could reduce the need to identify individuals to species level or eliminate the need for taxonomic experts, thereby providing a more widely applicable biotic index.
Although the current state of knowledge on the biology, ecology, and taxonomy of symbiotic Gorgonians has grown considerably during the last century, this group continues to be underrepresented in long-term coral reef monitoring efforts. This neglect has primarily been attributed to difficulties in differentiating species in the field and the need for microscopic sclerite verifications. However, Gorgonians are abundant and integral components of Coral Reef communities and their functional importance is reason to overcome these challenges. With recent reports of increased abundances in parts of the Caribbean, as well as emerging evidence of resistance to elevated temperatures, ocean acidification, and nutrient enrichment, there is now a growing interest in the ecological and physiological mechanisms that contribute to their success under changing environmental conditions.
Because there are substantial amounts of published information on various aspects of Gorgonian biology and ecology, Kupfner Johnson and Hallock recommend that in-depth reviews on the specific topics included in this paper be undertaken. At present, sexual reproduction and molecular phylogenetics are the only topics for which reviews have been published within the past decade. Thus, the primary goals of this Kupfner Johnson and Hallock's paper was to emphasize the untapped potential of Gorgonians for detecting environmental change, and to gather widely scattered information into one reference document that can be used to promote and inform future studies. As reefs continue to lose Scleractinian coral cover, Kupfner Johnson and Hallock recommend that reef researchers more broadly consider Gorgonian ecology as a critical component of reef science.
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