A million years ago, Apterra was terraformed and populated with a minuscule handful of complex species. This led to the planet's ecosystems being dominated by the "big seven" macroscopic life-forms: kiwis, rats, mosquitoes, minnows, geckos, roly-polies, and grasses. While these are the most immediately noticeable of Apterra's inhabitants, Corvolea's payload also included a variety of tiny animals across many phyla, without which the big seven could never have established themselves here. Copepods are one example; certain species are instrumental to keeping algal blooms in check. These are far from the only ones. This entry is a guide to some of the most important "minor" groups of animals that live on Apterra today. Some are already keystone species, while others are destined to someday occupy niches far beyond their original supporting roles in Apterra's food web.
Just below the surf off the coast of southern Loxodia, subalgal copepods in the order Harpacticoida patrol the biofilm. The recently-evolved genus Talitrostrum is particularly effective in this niche, with its upside-down swimming that allows it to easily graze as it moves. The action of these crustaceans ensures that the algae will never return to its Dawn Muricene state of complete domination over all of Apterra's seas. Further offshore, pelagic species filter-feed on free-floating algae as it swirls around under the waves. One of the most common genera in this environment is the half-centimeter-long Cyclictor, a member of the Calanoid clade. It produces powerful currents with its mouthparts, passing water continuously over its body and picking out edible pieces. A single Cyclictor can consume hundreds of thousands of individual phytoplankton in a day, keeping the open ocean clean and providing food for larger organisms. On the seabed, Caenarthrus isopods are joined by the Cyclopoid copepod Lutambulus, a benthic genus that sifts through the dense marine snow for morsels of food. It picks out the various bacteria that grow within this organic sludge, while its woodlouse neighbors prefer to eat the detritus directly.
Water fleas are another cosmopolitan group of aquatic crustaceans with great ecological importance. Small forms like Tolerosterilus (whose ancestors are the freshwater genus Moina) are the first to recover from deadly algal blooms, as they have higher toxin resistance than nearly any other species. They often return to affected areas before even the copepods, though they do little to purify the toxic water, as it poses no danger to this genus of hardy plankton. Further inland, freshwater Daphnia descendants like Saltacephalus dart around in small ponds, evading both piscine and culicid predators with their rapid jumping motion. Lifespans are generally limited to no more than three months even in ideal conditions. During these good times, populations are all female, reproducing via parthenogenesis to create cloned offspring. When water quality declines, males may be produced, helping to increase genetic diversity and, hopefully, improving the odds of the population rebounding. At the same time, non-heritable phenotypic changes can be seen in newly-born individuals. For instance, a Saltacephalus individual born in oxygen-poor water will produce copious amounts of hemoglobin, allowing it to survive the anoxic environment and giving it a deep red color.
Sharing Apterra's waterways but on a still-smaller scale, rotifers are among the most common truly microscopic animals. Sessile forms such as Affigopoda acupes coat the floor of the deep sea, frantically churning the water and ingesting marine snow particles. A very similar species called A. petroprendor clings to jagged rocks closer to shore, with a powerful foot to maintain a firm hold even in stormy seas. A more mobile bottom-dwelling genus, Lutolapsor, lives in bodies of freshwater, slithering through the muck and sifting out algae for food. Common in ephemeral creeks and puddles, Lutolapsor can come back to life after years of complete desiccation. At the surface of oceans and lakes alike, the planktonic Velocimastax floats with the current, vacuuming up smaller organisms in massive quantities. Other than its voracious appetite, this genus is known for its abnormally large size. Some species of Velocimastax may reach nearly half a centimeter in length, more than double the size of the largest earthly rotifers.
Another common class of invertebrates is the Hydrozoa. Ubiquitous on earth, Hydra are massively successful across Apterra's lakes, streams, and rivers. They are nearly unmatched in their niche of sedentary microfaunal ambush hunters, terrorizing other aquatic creatures like copepods, water fleas, mosquito larvae, and even some fish fry. Apterran Hydra are smaller and more venomous than their ancestors, relying on their toxic nematocysts to dispatch prey that may be several times their own size. This trait is also useful for predator deterrence; Hydra are so numerous that they form a significant fraction of certain fish's diets. In the seas, meanwhile, similar-looking species have modified their lifestyle. Tubulomedusa, a small but adaptable cousin of the Hydra, has a two-stage life cycle. Young individuals begin life quite similar to typical hydrozoans, as small, mostly-sessile polyps that feed on microbes and small zooplankton. As they mature, though, they release hydromedusae - small, jellyfish-like offspring genetically identical to their immobile parents. These feed, grow, and breed in the water column, often ending up kilometers from their point of origin. While the polyp stage is biologically immortal, these medusae live only a week or two, reproducing sexually to create thousands of larvae that settle back to the substrate and grow into the next generation of polyps. This is a fairly normal life history for many marine hydrozoans, and it serves as a base for a variety of future adaptations that will one day change the Apterran oceans forever. The only unusual feature of Tubulomedusa medusae is their swimming strength; especially in species that live in the calm waters beneath algal mats, the free-swimming life stage can easily move against prevailing currents at several kilometers per hour.
The land is no stranger to out-of-sight but vital animal groups either. Under every rotting log, moist rock, or pile of leaf litter, springtails dutifully chew away at decaying plant matter. These humble decomposers overlap somewhat with basal Apterran isopods in their niche, but as most woodlice become more and more derived and leave their ancestral lifestyles, these small Hexapods are gradually becoming the dominant terrestrial detritivores. Like many Apterran invertebrates, some members have taken advantage of vacant niches and increased significantly in size. The largest may now clock in at 1.5 cm, though they retain a similar diet to smaller species. As such, giant springtails like Fortifurcula longipes compete directly with pill bugs. Niche partitioning is determined by the two lineages' respective predator-evasion tactics; heavily-armored isopods are common where birds are the dominant insectivores, while quick-jumping springtails fare better in regions where rats or microraptorial mosquitoes are the most prevalent dangers. Meanwhile, trillions of smaller springtails continue to clean Apterra's soils, fertilizing the ground and helping stabilize local carbon cycles.
In all, thousands of species of non-charismatic critters were brought to this planet to help maintain a semi-functional biosphere. Of course, many gaps were left in this ecology, the consequences of which will become clearer in the age after this one. In some cases, the lack of biodiversity has allowed formerly endangered species to thrive, diversifying into forms that would've been impossible in their native habitats. On the other hand, it has also led to instability, mass die-offs, and long-term climatic issues that are already being felt across the globe. These unseen and seemingly unimportant clades are responsible for staving off the total collapse of Apterran environments, and they will be instrumental in saving the planet from future disasters.