The amazing marine reproduction (Part II)

Since the dawn of life on Planet Earth (or the Blue Planet), species of both flora and fauna have continuously adapted to the ever-changing conditions of their surroundings to ensure their survival. Among the many factors that influence a species’ continuity, reproduction stands out as one of the most crucial. It is the biological process by which new organisms are created.

In the first part of our series on reproduction in the marine environment, we explored the various forms of asexual reproduction, a method in which a single individual can create new individuals without the need for sexual cells or gametes. This process includes modalities such as budding, fragmentation, and parthenogenesis, each with fascinating examples in marine fauna. 

However, asexual reproduction is only part of the story. In this second article, we will discuss sexual reproduction, which is used by many of the species currently known in the marine environment.

Sexual reproduction

As mentioned earlier, sexual reproduction requires two parents or adult individuals to produce new individuals. Like asexual reproduction, this method is very complex, with a wide variety of possibilities in the marine environment. Primarily, the sexual reproduction of each species is determined by how fertilisation takes place and how sex is expressed among that species’ individuals.

Types of fertilisation

Fertilisation is an inherent feature of sexual reproduction and involves the union of male and female reproductive cells. In nature, fertilisation can occur in two ways:

• Internal fertilisation: One of the parents introduces reproductive cells or gametes into the other.
• External fertilisation: The parents release the reproductive cells into an external environment. As these cells drift as part of the plankton, they eventually combine with those of another individual of the same species.

External fertilisation is most common for marine species that reproduce sexually. This is due to specific features of the marine environment, such as its vast size and dynamic nature, and the fact that many marine species have limited mobility or no limbs at all, among other factors. However, releasing gametes into the environment carries a risk, as the success of fertilisation depends on the reproductive cells meeting each other.

As a result, species have adapted to improve their fertilisation rates. In some species, adults gather in groups to increase the chances of fertilisation. However, the most common adaptation among species with external fertilisation is the mass production of reproductive cells, especially since many of these species do not move or have very limited mobility.

Sex expression in the marine environment

Sex expression is one of the most fascinating aspects of sexual reproduction in the marine environment. Depending on the species, the sexes may be separate, meaning that there are distinct male and female individuals, or both sexes may be present in a single individual. This is known as hermaphroditism.

• Separate sexes 

In species with differentiated genders, some do not show differences in external morphology between males and females. However, in other species, these variations do exist, a phenomenon known as sexual dimorphism. In some marine species with separate sexes, differences between males and females can be seen in characteristics such as colouration or body size. For example, in species such as the sperm whale (Physeter macrocephalus) and the long-finned pilot whale (Globicephala melas), males are significantly larger than females. Another example is the octopus (Octopus vulgaris), where the male has a specialised arm called the hectocotylus. This arm is used for reproductive purposes, as it is inserted into the female to fertilise her.

Octopus vulgaris, Dani San Román (SUBMON).

• Hermaphroditism

This phenomenon takes different forms in the marine environment. Some species exhibit simultaneous hermaphroditism, where an individual possesses both male and female sex organs throughout its fertile life, such as the dotted sea slug or sea cow (Peltodoris atromaculata), the comber (Serranus cabrilla) and the red sea-squirt (Halocynthia papillosa). 

Peltodoris atromaculata (left) y Halocynthia papillosa (right), Jordi Sánchez and Dani San Román (SUBMON).

Other species exhibit sequential hermaphroditism, where individuals change sex at some point in their life cycle. This phenomenon occurs in two main forms. Protandrous hermaphroditism occurs when a species begins life as a male and later becomes a female, as in the common two-banded sea bream (Diplodus vulgaris) and the salema porgy (Sarpa salpa). On the other hand, protogynous hermaphroditism occurs when individuals are born female and later become male, as is the case with the dusky grouper (Epinephelus marginatus), the East Atlantic Peacock Wrasse (Symphodus tinca) and the Mediterranean rainbow wrasse (Coris julis). In addition, this sex change can sometimes involve a change in appearance, as seen in the Mediterranean rainbow wrasse and other species.

Female Coris julis (left) y male (right), Dani San Román (SUBMON).

Regarding species with simultaneous hermaphroditism, it’s worth noting that an individual can undergo a phenomenon known as self-fertilisation, where the male and female reproductive cells or gametes of the same individual cross-fertilise. This strategy is used by certain species when the population density in their habitat is low, making it difficult to find a mate. It’s important not to confuse this with parthenogenesis, which occurs in asexual reproduction and does not involve the fertilisation of sex cells.Finally, across the two articles on marine biodiversity reproduction, we’ve seen that species can reproduce either asexually or sexually. It’s important to note that some groups, like starfish, jellyfish, and corals, have the ability to reproduce in both ways during their life cycle. This flexibility helps them adapt to changing conditions throughout their development.

In summary, the marine environment and the reproduction of its inhabitants are extremely complex. However, there are many other fascinating aspects that we haven’t covered here. If you’re interested in learning more about reproduction, let us know. We’ll be happy to share more information. We also encourage you to let your curiosity lead you to learn more about the fascinating species of flora and fauna that inhabit the seas and oceans. You might be surprised to learn how they reproduce in ways you wouldn’t expect, or even find out that what you thought were two different species are actually the male and female of the same species!