April Fish of the Month: Green Goby Microgobius thalassinus

Dr. Geoffrey Smith

NWFSC Biology Instructor

Back in November, I highlighted the Clown Goby Microgobius gulosus. This month will features its sister species the Green Goby M. thalassinus. Similar to the Clown Goby, the Green Goby is a relatively small fish that reaches a maximum size of about 2.5 inches, hence Microgobius (which essentially translates to small goby).

The species name thalassinus roughly translates to “of the sea”.  Its range is slightly smaller than that of the Clown Goby, extending from Maryland southward (excluding southeast Florida) throughout most of the Gulf of Mexico (Figure 1). Both species inhabit similar muddy sand or mud bottoms in brackish to full salinity waters. However, the Green Goby is generally less tolerant of very low salinities. It is also generally much less common, but where it is present, it is often in relatively large numbers. Being a small and relatively uncommon species, there is not much known about their overall biology and behaviors, although they likely use their enlarged mouths to create burrows in which they reside and rear their eggs, similar to the Clown Goby and other species in this genus.

Like all gobies, the Green Goby has its pelvic fins (the paired fin on the bottom of the body toward the front of the fish) modified into a suction cup like structure that allows them to perch on rock, logs, etc. and remain in place in the presence of waves or current. As with most of its sister species and many other gobies, the Green Goby is sexually dimorphic, meaning that there are clear differences in the body form or coloration of males and females. Male Green Gobies have slightly elongated filaments extending off of some of their first dorsal (top) fin spines and females often have a black spot at the back of the first dorsal fin (Figures 2 and 3).

The overall body color of both males and females is light gray to green with an iridescent white to blue patch below eye, on the operculum, and behind the pectoral (on side of body) fins and some yellow and red markings on the dorsal (top), anal (bottom rear), and caudal (tail) fins (Figures 2 and 3). In males, the iridescent patch on the side of the body is often broken into 2 or 3 distinct bars that contrast against the rest of the dark body coloration, and the red markings on the fins are typically much more pronounced (Figures 2 and 3).

The Green Goby is not of any particular economic important and given that it is not as common as the Clown Goby, it is generally less important ecologically as a food source for juvenile sportfish, however, since it does occur in high numbers in localized areas it may be an important link in food chains/webs in those system.