Thermocyclops crassus is a cyclopoid copepod characterized by a short and stout body, with a furca approximately twice as long as it is wide. The ratio between the outer and inner setae attached to the furca is typically 1:3. Females typically measure between 0.8 to 1.1 millimeters, while males are around 0.7 millimeters in length. A distinctive feature is its seminal receptacle, which has a clear “T” shape with relatively short, wide lateral arms that are weakly recurved posteriorly. The length/width ratio of the female genital somite exceeds 1.2, while the furcal length/width ratio is below 2.5. Additionally, the ornamentation on the connecting lamellae of legs 1 and 4 aids in differentiation.