What are the differences in sexual reproduction among vertebrates and invertebrates?

The main difference lies in the complexity of reproductive systems and methods of fertilisation in vertebrates and invertebrates.

In general, vertebrates have more complex reproductive systems compared to invertebrates. Vertebrates, which include mammals, birds, reptiles, amphibians, and fish, typically have internal fertilisation. This means that the sperm is directly transferred to the female during mating, and fertilisation occurs inside the body. For instance, mammals have specialised reproductive organs such as testes for males and ovaries for females. The sperm and eggs are produced in these organs. In many vertebrates, there is also a distinct mating season when the animals are most fertile.

On the other hand, invertebrates, which include insects, spiders, molluscs, and worms, have a wide range of reproductive strategies. Some invertebrates also have internal fertilisation, but many have external fertilisation. In external fertilisation, the female lays eggs and the male then fertilises these eggs outside of the female's body. This is common in many aquatic invertebrates. Invertebrates may also reproduce asexually, through methods such as budding or fragmentation, which is not typically seen in vertebrates.

The complexity of the reproductive system in vertebrates is often associated with their higher level of organisation and development. The internal fertilisation in vertebrates allows for greater protection of the fertilised egg, increasing the chances of survival for the offspring. In contrast, the simpler reproductive systems and methods in invertebrates are often associated with their ability to produce large numbers of offspring, although these may have a lower chance of survival.

In terms of sexual reproduction, vertebrates usually have two distinct sexes, male and female, with each individual producing either sperm or eggs. However, in invertebrates, it is common to find hermaphrodites, individuals that can produce both sperm and eggs. This adds another layer of diversity to the reproductive strategies of invertebrates.

In conclusion, while there are some similarities, the differences in sexual reproduction among vertebrates and invertebrates are significant, reflecting their different levels of complexity and adaptation to their environments.