The second gnathopods of caprellid male amphipods are larger than the ones observed in females. The second gnathopods are armed with a propodus or tooth that is associated with a poison producing gland, by which male-male contests lead to fatal outcomes. Here we observed the ultrastructure of the second gnathopods by scanning electron microscopy and compared the relationship between the number of pores in the poison tooth and body length among the males of four caprellid species: Caprella acanthogaster, C. bispinosa, C. scaura, and C. penantis. All species possess pores on the poison tooth, and the number of pores increased with body size. The number of pores also differed among specimens of the four species. Furthermore, in males of three caprellid species, except for C. penantis, we found additional pores at the tip of the dactylus of second gnathopods. The number of pores on the tip of the dactylus did not increase with body length, and the average number of them differed among the three species. Although it is not known if venom is released from the pores of both structures, larger individuals that possess more pores on the second gnathopods could have a competitive advantage in male-male competition.