The spotted goby, Gobiusculus flavescens, is a small perciform

Perciformes

The Perciformes, also called the Percomorphi or Acanthopteri, is one of the largest orders of vertebrates, containing about 40% of all bony fish. Perciformes means perch-like. They belong to the class of ray-finned fish and comprise over 7,000 species found in almost all aquatic environments...

 fish

Fish

Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

 found in British

British Isles

The British Isles are a group of islands off the northwest coast of continental Europe that include the islands of Great Britain and Ireland and over six thousand smaller isles. There are two sovereign states located on the islands: the United Kingdom of Great Britain and Northern Ireland and...

coastal waters. Its genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 Gobiusculus is monotypic

Monotypic

In biology, a monotypic taxon is a taxonomic group with only one biological type. The term's usage differs slightly between botany and zoology. The term monotypic has a separate use in conservation biology, monotypic habitat, regarding species habitat conversion eliminating biodiversity and...

.

It is a small, slender goby

Goby

The gobies form the family Gobiidae, which is one of the largest families of fish, with more than 2,000 species in more than 200 genera. Most are relatively small, typically less than 10 cm in length...

, reaching up to 6 cm in length. They are distinct from other gobies in that their eyes are on the side of the head (lateral) rather than on top (dorsal).

Colour

There is a black spot at the base of the tail fin, and in the males, there is a second, smaller black spot beneath the pectoral fins. They are reddish to greenish brown in colour, paler on the undersides with dark reticulations dorsally. There are four light 'saddle' marks dorsally from the head to the end of the second dorsal fin. Along the midline of the sides are a series of bluish markings. During the breeding season these markings become extremely bright in the males.

Fins

The two dorsal fins are banded red, with the first dorsal fin bearing 7 rays (instead of the more usual 6 in other gobies). The caudal fin is transversely banded.