Whilst the crabs are the competition entry, I had a surge of creativity and so have added this on. There may be more yet to come.

Leviathans of the sea
In our home Earth, many creatures have gone from land to the sea. One cannot contend, however, that the niche is dominated by whales and dolphins, seals and sea lions, turtles, and a variety of birds, with one species of crocodile and several otters also having a go at the niche. With the Insectiverse, a similar pattern took place. However, the insects that entered the water faced stiff competition. The insects and their competition are both attended to in this article.

Filter feeders
With no whales around, many scientists thought that sharks would have taken command of the niche of the filter feeders in this dimension. Whilst they certainly have a significant portion of the niche, in reality only a minority of species of filter feeders are sharks. They have, in fact, faced competition from several other creatures.

Perhaps the most impressive are mayflies, having taken the niche with style. Many mayfly larvae in our world feed on algae, so it is really no surprise that they should adapt to filter feeding in the Insectiverse. Having abandoned the adult stage in favour of a neonatal existence, these creatures grow huge, up to 15m in length. In order to adapt to this existence as huge plankton eaters, they have adapted their legs into an undulating pair of “fins”, which allow them to move gently forward. Stoneflies have also evolved neonatal filter feeders, but these are smaller, being between 5 and 7 metres on average.

Bizarrely, the final group of plankton eaters are the dragonflies. Neonatal species fill the role of baleen whales in our home earth, using a sieve like mask to filter fish, shrimp, and krill out of the water. These sieve dragons drift using a gentle form of jet propulsion and their fin like limbs. These average at around 9 to 10 metres long.

Predators
With the presence of so many large, relatively short lived animals, carnivores have flourished, if not on the large prey but on the vast amount of carrion they leave upon death. Ironically, the creatures that benefited most from this were not invertebrates, but sharks. Almost as ironically, the sharks that benefited were not the classic lamnid sharks which first come to mind when one thinks of a predatory shark but a variety of cat shark and bullhead shark relatives that have no equivalent on home Earth. Both varieties use powerful, crushing jaws to break the neck of the plankton eating invertebrates, and tend to attack from ambush. The largest example is the 12 metre crown shark. In appearance, it is similar in some ways to the home earth horn shark, with a similar head shape, but it’s mouth faces forward, it’s tail is as powerful as a great white’s, and it’s colouration is camouflaged to blend in with the deep sea. Most of these “bug hunters” are between 4 and 7 metres however, choosing smaller game than the filter feeders.

These smaller game are the next on the list of predators. The penguin equivalents of the Insectiverse are mostly dragonflies. As larvae, they catch fish using speedy jet propulsion and the mask that characterises dragonfly larvae today. Most of these penguin niche dragonflies eventually leave the water as adult dragonfly, which are closer to puffin or kittiwake niche. These adults also feed on fish, but the larvae avoid competition with the adults by fishing at greater depths whilst the adults attack surface dwelling fish. Some adults don’t even eat, merely mate, lay eggs, and then die en mass (which provides a great deal of carrion and flotsam for fish to eat).

The seal and sea lion equivalents are, oddly enough, not insects. Neither are they sharks. In fact, they are mantis shrimp. Growing from 2 to even 4 metres in length, most of these predators specialise in taking bottom dwelling prey (which is closer to the niche of some sharks). However, one genus truly acts like ocean going seals, swimming with specialised swimmerets, spearing fish either larger than or at greater depths than what the dragonflies hunt.

Invertebrates also have a claim on the niche of large ocean predator. Whilst the sharks grow the largest, one cannot help but be impressed by the neonatal water beetles. Looking like an insectoid version of a fairy tale sea serpent, swimming with their powerful limbs, these creatures grow up to 9 metres long and hunt all things smaller than they are. Whilst they sometimes get into competition with the bug hunter sharks, these tend to specialise in hunting large soft bodied fish and squid, avoiding competition with the fearsome sharks.