Plays a role in vesicle-mediated protein trafficking to lysosomal compartments and in membrane docking/fusion reactions of late endosomes/lysosomes (PubMed:25273556). Believed to act as a component of the putative CORVET endosomal tethering complex which is proposed to be involved in the rab-5-to-rab-7 endosome conversion probably implicating sand-1, and via binding SNAREs and SNARE complexes to mediate tethering and docking events during SNARE-mediated membrane fusion (PubMed:25273556). The CORVET complex is proposed to function as a rab-5 effector to mediate early endosome fusion probably in specific endosome subpopulations (PubMed:25273556). Most likely within the CORVET complex, it is involved in the fusion of endocytic compartments (PubMed:25273556). Required for sperm development and function (PubMed:19109425, PubMed:27558849). Probable core component of the class C core vacuole/endosome tethering (CORVET) complex. The common core is composed of the class C Vps proteins vps-11, vps-16 and vps-18, and which further associates with vps-8 and vps-33.2 (PubMed:25273556). Interacts with spe-39 (PubMed:19109425). Broadly expressed in somatic tissues including the pharynx, intestine, spermatheca, and in coelomocytes (PubMed:27558849). Expressed in the lining of the gut lumen (PubMed:25273556). Hermaphrodites exhibit sperm-specific sterility and only lay unfertilized eggs (PubMed:27558849). In contrast to wild-type animals, mutant males produce no or few spermatids in their gonads, but instead their gonads accumulate terminally arrested spermatocytes that do not bud and contain four haploid nuclei (PubMed:27558849). In some instances, the spermatocytes have small buds that do not form spermatids (PubMed:27558849). Oocytes and unfertilized eggs contain some larger endocytic structures tethered to yolk granules (PubMed:27558849). RNAi-mediated knockdown results in defective yolk uptake in oocytes with the appearance of endocytic structures containing yolk tethered to large membranous organelles which could be lysosomes (PubMed:25273556). RNAi-mediated knockdown results in defective endosome maturation with the accumulation of small vesicles near the gut lumen which is possibly indicative of increased endosomal fusion activity, and defective trafficking of proteins such as lmp-1 to lysosomal compartments (PubMed:25273556). RNAi-mediated knockdown in a sand-1 mutant background results in defective endosome fusion with the formation of larger, irregularly-shaped rab-5 positive endosomes in oocytes and intestinal cells, and larger yolk-containing granules than in the sand-1 single mutant (PubMed:25273556). Double RNAi-mediated knockdown together with vsp-33.1 results in defective protein trafficking to lysosomal compartments, and an irregular distribution of vesicles of various sizes throughout the gut cells (PubMed:25273556). Double RNAi-mediated knockdown together with vsp-33.1 in a sand-1 mutant background rescues the large endosome phenotype and the defective protein trafficking to lysosomal compartments in the sand-1 single mutant, but results in lethality (PubMed:25273556). Belongs to the STXBP/unc-18/SEC1 family.