June 2024:
The United Arab Emirates is blessed with access two significant bodies of water, the Persian Gulf and the Gulf of Oman, playing crucial roles in the region's ecology and economy.
The Persian Gulf is situated between Iran to the northeast and the Arabian Peninsula to the southwest. Its warm waters, with temperatures ranging from 24°C to 32°C, support a diverse marine ecosystem. Coral reefs, mangroves, and seagrasses thrive here, providing habitats for various species. Notable fauna include the dugong, hawksbill turtles, and a multitude of fish species such as the barracuda and groupers. The Gulf is also home to rich deposits of oil, making it economically vital.
To the southeast, the Gulf of Oman connects the Persian Gulf to the Arabian Sea. It borders Oman and Iran and is characterized by slightly cooler water temperatures, generally between 22°C to 30°C. The Gulf of Oman supports a similarly rich marine biodiversity, with its waters teeming with species like the Arabian humpback whale, dolphins, and various shark species. The coastline is lined with mangroves and seaweed beds, which serve as crucial breeding grounds for marine life.
Both gulfs are crucial for regional biodiversity; their unique ecosystems and warm waters make them captivating subjects for ecological study and conservation efforts. My fascination grew after two trips to Musandam, where these gulfs converge with the DNHG, particularly in observing the reproduction strategies of two species: the clownfish and the sea salp.
I- Clownfish and sequential hermaphroditism
Clownfish, famous for their symbiotic relationship with sea anemones, exhibit fascinating reproductive behaviors. These vibrant fish are protandrous hermaphrodites, meaning they are born male and can later change to female a type of sequential hermaphrodism. In a typical clownfish group, the dominant female is the largest fish, and the next largest is the breeding male, while the rest are smaller, non-breeding males. This group typically lives in a territory consisting of a single sea anemone, which provides protection and habitat.
Reproduction begins with the selection of a suitable nesting site, usually on a flat surface close to the sea anemone they inhabit. The breeding male meticulously cleans the site in preparation for the eggs. During the spawning process, the female lays hundreds to thousands of eggs, which the male then fertilizes externally.
After fertilization, the male takes on the primary role of guarding and caring for the eggs. He fans the eggs with his fins to ensure they receive enough oxygen and removes any debris or unfertilized eggs. The incubation period lasts about 6-10 days, depending on water temperature.
Once hatched, the larvae are planktonic, drifting in the open ocean for about 8-12 days before settling down and seeking out a host anemone. This reproductive strategy, along with their unique lifecycle and social structure, makes clownfish a subject of great interest in marine biology.
When the dominant female dies, a remarkable transformation occurs. The breeding male changes sex and becomes the new dominant female, the sex change seems to be controlled socially, i.e. male does not change sex when attaining a certain size, but only after the female disappearance (Moyer, 1978). The largest of the non-breeding males then matures into the new breeding male.
The sex transformation happens in two steps (Casas, 2016) :
- First, changes in gene expression occur in the male brain two weeks after the female disappears. These changes take about 30 days to complete. Interestingly, there's no initial change in the male's reproductive organs (gonads). The fish might be waiting to confirm the female's absence is permanent. This delay could be a way to avoid the costly (time and energy) process of sex change if the female returns.
- Secondly, these brain changes trigger a hormonal signal, likely through a pathway involving the hypothalamus, pituitary gland, and finally the gonads. This hormonal message instructs the gonads to break down the testicular tissue and develop an ovary, completing the sex change.
- First, changes in gene expression occur in the male brain two weeks after the female disappears. These changes take about 30 days to complete. Interestingly, there's no initial change in the male's reproductive organs (gonads). The fish might be waiting to confirm the female's absence is permanent. This delay could be a way to avoid the costly (time and energy) process of sex change if the female returns.
- Secondly, these brain changes trigger a hormonal signal, likely through a pathway involving the hypothalamus, pituitary gland, and finally the gonads. This hormonal message instructs the gonads to break down the testicular tissue and develop an ovary, completing the sex change.
Clownfish maximize reproductive potential and social stability by changing sex (Sadovy de Micheson, 2008). They start as males, avoiding the high energy cost of egg production while growing. Once large enough, they become females, handling reproduction. This strategy ensures efficient resource use within their anemone territory and reduces competition. When the dominant female dies, the breeding male quickly changes sex to replace her, ensuring continuous reproduction and maintaining group cohesion. This rapid adaptation enhances their survival and reproductive success.
Skunk clownfish - Amphiprion akallopisos, Tanzania
Clark's anemonefish - Amphiprion clarkii, Musandam
II- Sea Salps, from cloning to sequential hermaphroditism
On our last dhow trip in Musandam, while snorkeling in waters that were greener than usual—thick with algae and swirling with tiny marine life—I came across a creature I had never encountered before. At first, I thought it was just a drifting piece of jelly, a translucent barrel-like blob floating effortlessly in the current. But as I got closer, I realized it wasn’t alone—there were chains of them, pulsing gently in perfect synchrony.
This was my first encounter with a sea salp, one of the ocean’s strangest yet most fascinating creatures. Neither jellyfish nor plankton in the traditional sense, these gelatinous tunicates are nature’s tiny, living filters, silently drifting through the sea while playing a surprisingly big role in marine ecosystems. Intrigued, I had to find out more. What I discovered was even more fascinating than their ghostly appearance—these creatures have an extraordinary reproductive cycle that allows them to multiply rapidly and shape entire ecosystems.
Sea salps have a complex and highly efficient reproductive strategy, alternating between asexual and sexual phases, which correspond to two distinct life forms: solitary and aggregate (Andersen, 1998; Madin & Purcell, 1992). This dual strategy enables them to respond rapidly to environmental changes, ensuring both rapid population growth and genetic diversity.
In the solitary phase (oozoid), an individual reproduces asexually through budding, forming chains of genetically identical individuals known as blastozooids. These chains, sometimes composed of hundreds of individuals, remain connected and move as a synchronized unit, which not only enhances their ability to filter-feed on phytoplankton but also improves their chances of escaping predators (Madin, 1990).
In the aggregate form, sea salps switch to sexual reproduction. Each individual in the chain starts as a female and later transitions into a male, a reproductive strategy known as protogynous hermaphroditism. This prevents self-fertilization and enhances genetic diversity within the population. Unlike many marine organisms that release large numbers of gametes into the water, salps employ internal fertilization, where the developing embryo remains within the parent until it reaches a free-swimming stage. This increases the survival chances of offspring, particularly in nutrient-scarce environments (Godeaux, 1998).
Sea salps’ dual reproductive strategies enable them to rapidly exploit favorable environmental conditions, leading to population booms. Such blooms can occur within days in response to phytoplankton surges, with exponential population growth rates that surpass many other zooplankton species (Deibel & Lowen, 2012). Their ability to filter large volumes of water also makes them important players in carbon cycling, as they can transport carbon to deeper ocean layers when they die and sink (Henschke et al., 2016).
Sea Salps' cycle
Sea Salp - blastozooids phase, Musandam
(1) Moyer, J. T. & Nakazono, A. Protandeous Hermaphroditism in Six Species of the Anemonefish Genus Amphiprion in Japan. Japhanese J. Ichthyol. 25, 101–106 (1978)
(2) Casas, L. et al. Sex Change in Clownfish: Molecular Insights from Transcriptome Analysis. Sci. Rep. 6, 35461; doi: 10.1038/srep35461 (2016).
(3) Sadovy de Micheson, Y. & Liu, M. Functional hermaphroditism in teleosts. Fish Fish. 9, 1–43 (2008).
(4) Andersen, V. (1998). Salp and pyrosomid blooms and their importance in biogeochemical cycles. In Bone, Q. (Ed.), The Biology of Pelagic Tunicates (pp. 125-137). Oxford University Press.
(5) Madin, L. P., & Purcell, J. E. (1992). Feeding, metabolism, and growth of Cyclosalpa bakeri in the subarctic Pacific. Limnology and Oceanography, 37(6), 1236-1251.
(6) Madin, L. P. (1990). Aspects of jet propulsion in salps. Canadian Journal of Zoology, 68(4), 765-777.
(7) Godeaux, J. E. A. (1998). The relationships and systematic of the Thaliacea, with keys for identification. In Bone, Q. (Ed.), The Biology of Pelagic Tunicates (pp. 1-24). Oxford University Press.
(8) Deibel, D., & Lowen, B. (2012). A review of the life cycles and life-history adaptations of pelagic tunicates to environmental conditions. ICES Journal of Marine Science, 69(3), 358-369.
(9) Henschke, N., Everett, J. D., Doblin, M. A., Pitt, K. A., & Suthers, I. M. (2016). Population drivers of a salp (Thalia democratica) bloom in the Tasman Sea. Scientific Reports, 6, 32309.