This relatively small, slender species of linear-denticled Etmopterus can be separated from its closest congeners within the E. lucifer clade by a combination of characteristics: length of its anterior flank markings being much shorter than its posterior branch; black flank markings sharply demarcated by surrounding lighter to whitish lateral markings; prominent white spot on each cheek; a single white stripe extending the length of the midback; 2 parallel rows of prominent dark pores extending between pectoral and pelvic fins; its closest geographic congener E. sculptus has its anterior branch longer than its posterior branch, with a thinner posterior flank marking, 0.2-0.4% (vs. 0.6-0.9% TL in E. alphus), a higher spiral valve count 8-9 (vs. 5-7 in E. alphus); it differs from its closest morphological congener, the Australasian E. molleri by its shorter anterior (6.0-10.6% vs. 8.0-11.5% TL) and posterior (9.3-12.7% vs. 11.0-15.4% TL) flank-marking branches, biogeography and genetics (Ref. 112348).