If you're strictly asking about the lake itself, the taxonomic diversity is (and always was) fairly low. At least in part, this can be explained by its temperate location, geological history and the fact that it never was a deep lake, allowing it to dry out entirely/mostly in various periods (compare Lake Chad). For example, if you look at the fish fauna, there were only a bit more than 20 species, mostly various cyprinids and mostly widespread species of Euro-Siberian, central Asian or Caspian basin origin. There are and were no strict endemic fish species of the Aral Sea. I guess you're not familiar with freshwater fish zoogeography, but especially the Euro-Siberian region is dominated by a relatively small number of quite widespread species. This includes a large percentage of the freshwater fish that are well-known to Europeans and typically occur as far east as Siberia (if disregarding the unique Lake Baikal with its remarkable sculpin diversity, most Siberian endemic freshwater fish are salmonids). This also explains why the three Siberian mega rivers, the Ob, Lena and Yenisei, are of relatively limited interest from a biological point of view despite their huge sizes.
Of the native fish species that were known from the Aral Sea, only the southern ninespine stickleback still survives in the remnants of the lake. Everything else is either gone or introduced species. The two most famous species in the Aral Sea, the Aral barbel and the fringebarbel sturgeon, are among those that survive elsewhere.
From a biological point of view, the most significant loss has arguably not been from the Aral Sea itself, but the rivers that were the source of its water: Amu Darya and Syr Darya drainages. These have been heavily affected by dams, irrigation and alike. Several subspecies of questionable validity are endemic to these river basins and the Aral Sea, but most have been able to maintain small populations in the rivers (an exception is a brown trout subspecies). Even if the remaining should disappear, they are questionable subspecies of widespread species. A number of other fish species are endemic to the river drainages and still survive there, but these were never found in the Aral Sea itself. The largest tragedy is arguably the rapidly approaching extinction of an entire genus of sturgeon, Pseudoscaphirhynchus. These are the smallest of the sturgeons (<75 cm / 2½ ft), and include two species that are endemic to the Amu Darya drainage and a single endemic from the Syr Darya drainage. Almost all sturgeon species are now kept in aquaculture with good levels of success, allowing them to survive even if they should disappear from the wild. Indeed, several sturgeon populations only remain in the "wild" because of release of captive bred. Pseudoscaphirhynchus is the exception: Starting in the 1980s, there have been attempts of captive breeding of one of the species, P. kaufmanni, but as far as I know without success. P. fedtschenkoi from Syr Darya is probably already gone and it remains to be seen if the two species from Amu Darya will survive.
