Scuba diving can take you to the most extraordinary corners of the ocean, where you'll encounter diverse animal and plant species.
Fish employ various reproductive strategies, some of which are truly fascinating. In all cases, the goal remains the same: species survival, even when it costs them their lives.
Types of reproduction
Most fish are oviparous, meaning they release eggs into the environment. However, some species like certain sharks and freshwater fish are viviparous, with embryos developing inside the female's uterus. Other species such as tiger sharks are ovoviviparous - these fish hatch from eggs that develop internally within the female.
External fertilisation is most common among fish; females release eggs and males release sperm into the water where fertilisation occurs. In this scenario, many eggs risk being eaten by predators. Conversely, coelacanths and cartilaginous fish (sharks and rays) exhibit internal fertilisation. Male sharks and rays possess two copulatory organs called claspers (pterygopodia) which function as penises. This reproductive method is more secure and produces fewer offspring, as survival rates are significantly higher.
Hermaphroditism
Nearly all fish species have separate male and female sexes. However, many species are hermaphrodites, most being sequential hermaphrodites that undergo sex reversal as part of sexual development. Some mature first as females before transforming into males (protogynous hermaphrodites) like parrotfish and wrasses, including the Mediterranean rainbow wrasse (Coris julis) or the bird wrasse (Gomphosus varius). Species that begin their adult life as males before becoming females are called protandrous hermaphrodites, such as clownfish, gilthead seabream (Sparus aurata), etc.
Very few vertebrates exhibit simultaneous hermaphroditism, where individuals can function as both male and female during spawning. The painted comber (Serranus scriba) demonstrates this with a spectacular courtship display: two individuals rush toward each other while arching, with one fully extending its dorsal fin in an S-shaped posture. If the other responds, they ascend toward the surface to release eggs and sperm.
Attracting mates
Fish also perform courtship rituals or dances to attract partners, particularly species that mate. During shark courtship, for example, males swim close to females' cloacas while nibbling their pectoral fins - this prepares the female for mating until the male bites her pectoral fin to secure copulation. Sometimes a second male follows the pair to prevent the female escaping, as seen in nurse sharks (Ginglymostoma cirratum).
Broadnosed pipefish (Syngnathus typhle) exhibit unique reproductive behaviour: females dance around males before pressing their bellies together to transfer about twenty eggs into the male's brood pouch, where he fertilises and incubates them - similar to seahorses.
Some species produce sounds using their swim bladder to attract mates, particularly useful in deep waters with limited light. Certain deep-sea fish possess light-producing organs (bioluminescence) which sometimes serves to attract partners.
Protecting offspring
While most fish abandon their eggs to fate, some meticulously build sand nests or collect algae, like the East Atlantic peacock wrasse (Symphodus tinca). Here, males construct algae nests where females deposit eggs for subsequent fertilisation.
The garibaldi, inhabiting California's kelp forests, builds nests to attract females before tending the eggs until hatching, constantly fanning them for oxygenation.
Some fish incubate eggs in brood pouches (like seahorses) or other body areas. Male cardinalfish (Apogon imberbis and Apogonidae family) incubate eggs in their mouths, while other species carry eggs in gill chambers.
Metamorphosis
Some fish undergo dramatic changes during growth. Flatfish like flounder, sole, and turbot begin as symmetrical pelagic larvae with an eye on each side. As they mature, their bodies flatten and one eye migrates to join the other, transitioning to benthic life on the seafloor where both eyes remain on the upper side for environmental awareness while resting on sand.
Territorial behaviour
Nesting species must defend their territory. Some like many angelfish show extreme territoriality - mature males defend areas against other males. Consequently, juvenile angelfish often display different colouration, allowing them to coexist with territorial males until sexual maturity triggers adult colours and necessitates finding new territories for reproduction.
Colour changes
Some species change colours during development. The damselfish (Chromis chromis) juveniles exhibit electric blue colouring while adults darken. The longspine snipefish (Macroramphosus scolopax), living coastally at 50-150m depth, appears reddish for camouflage, whereas pelagic juveniles show blue-silver colouring for open-water concealment.
Species like the Mediterranean rainbow wrasse (Coris julis), ornate wrasse (Thalassoma pavo), and many wrasses display sexual dichromatism - males and females differ visibly in colour, with males typically showing brighter hues to attract mates.
Migratory patterns
Some species undertake extensive reproductive migrations. Fish migrating between saltwater and freshwater for breeding fall into two categories: anadromous and catadromous.
Salmon exemplify anadromous species - living in oceans but returning to rivers to spawn. These powerful swimmers navigate strong currents and leap 4-5m obstacles. Conversely, catadromous eels spend most lives in rivers before migrating downstream to the Sargasso Sea for spawning. They can travel overland using skin mucus to maintain moisture and cutaneous respiration. Their eyes enlarge for oceanic light conditions, and bodies shift from brown to silver.
Parasitic males?
Size differences between genders are common in nature. The deep-sea anglerfish (Melanocetus johnsoni) shows extreme sexual dimorphism - tiny males permanently attach to females' undersides, absorbing nutrients through her bloodstream. This adaptation proves effective in sparsely populated depths where finding mates is challenging.
Millions of eggs
Most pelagic species release thousands or even millions of eggs. Without parental protection, predators consume many, necessitating vast numbers to ensure some reach adulthood. The ocean sunfish (Mola mola), reaching 3 metres and 1 tonne, holds the record - one female contained an astonishing 300 million eggs.
