Mass transport induced by group-forced subharmonic waves (infragravity waves) is investigated in the present study. A theoretical solution for subharmonic waves’ kinematic contributions to fourth-order mass transport and drift velocity has been proposed for any depth and bandwidth for the first time. This model is validated using particle-tracking simulations driven by the flow field generated by the SWASH. The subharmonic-induced mass transport solution is a weighted sum of the subharmonic velocity variance spectrum and velocity skewness bispectrum due to the triad-difference interaction among two primary and one subharmonic components. For narrow-banded waves with long wave group relative to depth, the weightings become independent of spectral components, and the solution is recovered in the time domain. Two mechanisms contributing to mass transport were identified: a forward drift resulting from self-interaction similar to Stokes drift, and a depth-decaying backward drift induced by negative subharmonic velocity skewness due to the anti-phase coupling between subharmonics and wave groups. For narrow-banded waves the forward transport surpasses the backward transport for kh< 0.72, where k is the short wave wavenumber and h is the water depth. For other waves, the critical kh for this phenomenon decreases with increasing wave period and bed slope and decreasing bandwidth. At greater depths or steeper bed slopes, near-surface backward transport predominates over forward transport; at shallower depths or gentler slopes, forward transport is dominant throughout the water column. Although smaller than Stokes transport by short waves, the subharmonic wave-induced mass transport can affect the long-term trajectory of a floating and suspended particle. This study provides the first evidence and insight for the influences of group-forced subharmonics on vertically varying mass transport from the ocean surface to seabed in coastal environments.

The superfamily Diplostomoidea Poirier, 1886 is a large, globally distributed group of digeneans characterized by the presence of a unique holdfast organ and parasitic in most major groups of vertebrates (birds, mammals, reptiles, fishes) as definitive hosts. A number of diplostomoideans are associated with diseases in their intermediate and, more rarely, definitive hosts. Prior to this work and upon the recent synonymization of the Brauninidae Wolf, 1903, the Diplostomoidea included 5 families: Bolbocephalodidae Strand, 1935; Cyathocotylidae Mühling, 1896; Diplostomidae Poirier, 1886; Proterodiplostomidae Dubois, 1936; and Strigeidae Railliet, 1919. The separation of these families was based primarily on the structure and shape of prosoma and holdfast organ as well as the presence/absence of cirrus sac and paraprostate. More rarely, distinguishing among families was based on life cycles and types of larval stages, excretory system or even host specificity. However, due to the inconsistent nature of most of morphological and biological characters across the Diplostomoidea and nearly universal lack of agreement on their relative value, the systematic history of the group has been extremely tumultuous, and none of many classification systems proposed over the last 140 years has become broadly accepted or supported by phylogenetic analyses. Extensive molecular phylogenetic studies of the Diplostomoidea in the last 15 years helped to partly improve the classification system and resolve multiple taxonomic questions. Notably, practically all molecular phylogenies have clearly demonstrated non-monophyly of the two largest families, the Diplostomidae and the Strigeidae and indicated it as systematic problem. We provide a brief overview of the history and current state of knowledge of diplostomoidean systematics and re-evaluate the classification system of the Diplostomoidea based on morphological and molecular evidence. We propose changes in the classification system that reconciles the traditional morphological and life cycle data with molecular phylogenies. The major element of the proposed classification system is the synonymization of the families Proterodiplostomidae and Strigeidae with the Diplostomidae as the only feasible way to resolve the problem of consistent non-monophyly of the latter two families and provide stability to the classification system.