At Low Tide
While there is no proof that the sea creatures inhabiting the lower regions of the Edgewater Drain are markedly larger than their sea-going counterparts, there is a difference in color between the Drains' creatures and those of the sea.
The sewers have always existed beneath the city of Freeport. Over time, they have grown to encompass an area that is as large as the city itself, as well as extending beyond the city's borders. One of the natural boundaries of the sewer system is the sea, into which the sewers drain.
At the point where the sewers enter the sea, the salt water from the sea mixes with the effluent flowing through the sewer. While in general the flow is out from the sewer into the sea, when the tides are high, sea water will wash backward quite some distance into the Edgewater Drain. As a result of this tidal flux, several sections of the Drains are underwater.
As the mixture does not affect the gnomish machineries which help facilitate the outward flow of the effluent, there has never been any reason to stop this inward flow of sea water. Inevitably, with the sea water come sea creatures of various types. Over time, a number of creatures have remained locked in the Drains for so long as to develop slightly differently than their ocean-going counterparts.
The main difference, though, is primarily a subtle change in skin color. For example, observe the needleteeth and you will note that they are primarily a muddy brown color. Needleteeth are often found at the junction of fresh or brackish water and salt water. The needleteeth of the Drains are quite large, possibly due to the reduction in the their natural enemies that survive in the Drains.
There are numerous forms of crabs in the Drains. They are no doubt attracted by the feeding opportunities amongst the organic litter. Besides the natural by-products of city dwellers, many merchants dump unsold products directly into the sewers. Also, although we hate to mention this, the unsavory elements also deposit...organic waste...directly in the sewers. As the material decomposes, it forms the perfect food for creatures such as the sludgewalkers and even rockshells.
The organic matter has another effect upon the creatures in the Edgewater Drain. As it decomposes, this matter attracts various fungi and goo which will go through periods called a "bloom" when they are much more abundant that normally. Usually, this occurs when the usual high tides do not occur, allowing waste products to accumulate. The excess fungi and goo will fight for resources with the sludgewalkers, which will drive the sludgewalkers further into the water.
As the sludgewalkers begin to invade the normal territory of the rockshells, these turtle-like creatures will themselves turn to deeper parts of the water, displacing other creatures. If the cycle does not revert, meaning if a high tide does not occur at this point, the creatures will begin to crowd one another, resulting in intense internal fighting for territory.
This will initially benefit the hardiest creatures, such as the piranas and sewerfins, but eventually they will run out of smaller prey. If they turn on each other, only one will survive: the sewerfin, which can swallow a pirana in one gulp. Sewerfins will not generally eat their own kind, and having outlived all other creatures in the depths, would starve to death.
Fortunately, such an event has not occurred, although we are always checking the numbers of the various creatures, starting with the lowest forms of life. If their levels remain stable as they are today, then the larger creatures will also survive.
While mucking around the lowest sections of Edgewater Drains may not be a lifestyle for everyone, consider this: it is the lowest point of the Freeport sewer system. As an item washes downwards through the sewers, provided it is not masticated by gnomish gears, it may be found amongst the litter. Think of time spent in the Drains as a treasure hunt, and the hours will fly past!