Symbiotic relationships are a common phenomenon among marine invertebrates, forming both obligatory and facultative dependencies with their host. Here, we investigate and compare the population structure of two crustacean species associated with both shallow and mesophotic ecosystems: an obligate symbiont barnacle (Ceratoconcha domingensis), of the coral Agaricia lamarcki and a meiobenthic, free-living harpacticoid copepod (Laophontella armata). Molecular analyses of the Cytochrome Oxidase Subunit I (COI) gene revealed no population structure between mesophotic and shallow barnacle populations within south-west Puerto Rico (ΦST = 0.0079, P = 0.33). The absence of population structure was expected due to the pelagic naupliar larvae of the barnacles and the connectivity patterns exhibited by the coral itself within the same region. Laophontella armata exhibited significant structure based on the mitochondrial COI gene between the mesophotic reef ecosystem of El Seco, Puerto Rico and mangrove sediments of Curaçao (ΦST = 0.2804, P = 0.0). The El Seco and Curaçao copepods shared three COI haplotypes despite the obligatory benthic development of harpacticoid copepods and the geographic distance between the two locations. Three other COI haplotypes from El Seco exhibited higher than expected (up to 7%) intra-species variability, potentially representing three new cryptic species of harpacticoid copepods or rare, deeply divergent lineages of L. armata. This result is evidence for the urgent need of a deeper investigation into the meiofauna diversity associated with mesophotic coral ecosystems (MCEs), arguably the most diverse metazoan component of MCEs.

This study investigated whether associated meiobenthic communities, especially harpacticoid copepods, differed amongst habitats. Five pre-defined habitats within and next to the Posidonia oceanica seagrass meadow were sampled: living seagrass canopy leaves (LL), small (SMF) and large (LMF) macrophytodetritus fragment accumulations and sand, bare (BS) and covered (CS). The highest meiofauna abundances were recorded in the BS for the core sampled habitats (BS, CS, SMF and LMF) and in the LMF for seagrass material habitats (SMF, LMF and LL). Harpacticoid copepods were the most abundant taxon in all habitats. The assemblage composition at copepod family level showed two distinct habitats clusters: a leaf (LMF and LL) and a sediment cluster (BS, CS and SMF). Subsequently, stable isotope analyses were conducted to analyse the relationship between copepods and their potential food sources in seagrass material habitats. Based on δ13C isotopic analyses and SIAR mixing model, harpacticoid copepods relied for 70% on epiphytes and for 30% on P. oceanica leaf material in the LMF and LL habitats.

The assemblage structure of harpacticoid copepods inhabiting the soft sediments of littoral and sublittoral areas of Cat Ba archipelago (South China Sea) is analysed. Three basic types of harpacticoid taxocenes were observed in the upper sublittoral. Main dominant species of taxocenes were Amphiascoides sp., Stenhelia latioperculata, Paramphiascella sp., Phyllopodopsyllus sp., Tisbe sp.1 and Amphiascoides sp. In addition, the main characteristics of harpacticoid species composition in littoral rock pools are described. A comparative analysis of shallow-water harpacticoid communities from different biotopes of the tropical region is presented, and shows that harpacticoid diversity and abundance are lowest in muddy sediments. Highest diversity and abundance are found in washed sands and silty sands with detritus. In general, the structure of muddy bottom communities is the most homogeneous throughout the tropics. Mangrove forests have no significant influence on harpacticoid taxocene structure. Harpacticoid associations of washed sands and silty sands with detritus demonstrate greater diversity and are region-specific.

Colonization experiments were conducted in a tropical lagoon (Zanzibar Island, off the coast of Tanzania) to investigate the temporal dynamics and mode of colonization of the harpacticoid copepods community on dead coral fragments. There was fast colonization of the coral fragments attaining a substantial diversity after only two days. The ability to colonize dead coral fragments is thought to be related to the morphology and life style of different harpacticoid species. Phytal taxa (e.g. Tisbidae) were fast colonizers, reaching high abundances during the initial colonization phase. Sediment-associated and eurytopic taxa (e.g. Ameiridae, Miraciidae and Ectinosomatidae) showed lower colonization rates and became the dominant group during the later colonization phase. Most species are able to colonize the coral fragments through the water column. However, colonization along the substrate surface is also considered to be an important colonization mode, especially for sediment-associated taxa, which showed lower colonization rates when migration through the sediment was hindered.

To examine the specific features of trench communities, spatial changes in the assemblages of harpacticoids were investigated at the family level around the Ryukyu Trench (the Ryukyu region) and Kuril Trench (the Kuril region). In the Ryukyu region, there were high average dissimilarities in the harpacticoid assemblages among the trench, trench slope and abyssal plain, indicating that the assemblage structures differ substantially between these topographic settings at the family level. Conversely, in the Kuril region, the average dissimilarities in harpacticoid assemblages between the trench and the trench slope and between the trench and the abyssal plain were lower than that between the slope and the abyssal plain. This result suggests that the hadal assemblage is a transition zone between the slope and the abyssal plain in this region. In addition, the analyses indicate that the composition of harpacticoid assemblages is influenced by the quantity of organic matter in the Ryukyu region, while sediment properties play a key role in the Kuril region. Comparisons of the assemblages between the two regions, however, revealed that the average dissimilarities between the trenches and between the abyssal plains were higher than that between the adjacent slopes. This result suggests that interchange among regions is difficult for deep-sea benthic animals, including harpacticoid copepods, likely due to the presence of physical barriers around trenches.

Three microhabitat types (dead coral fragments, coral gravel and coral sand) were distinguished and sampled at two locations (Matemwe and Makunduchi) in a tropical lagoon (Zanzibar Island, Tanzania), and the community structure, habitat preferences and biodiversity of the associated harpacticoid copepod fauna was investigated. The harpacticoid fauna is affected by sediment granulometry and by the structural differences between coral and both gravel and sediment. The coral fragments contained a specific assemblage composed of typical ‘phytal’ taxa (Tisbe, Paradactylopodia and Dactylopusia) along with other eurytopic and sediment-dwelling forms (Ameira, Ectinosoma and Amphiascus), which may be attracted by the sediment retained between the coral branches. The assemblages of coral gravel and upper sediment layer did not differ significantly from each other and had mostly the same dominant genera. The sediment from Matemwe was dominated by the interstitial Paramesochridae and the sediment from Makunduchi by Tetragonicipitidae. The coral fragments from Makunduchi sustained a more diverse assemblage than gravel and the different sediment layers. It was assumed that coral form and complexity, with implications for habitable space, nutritional resources and level of predation, are important in structuring diversity of the associated assemblage.

The spatial diversity of meiofauna from sea grass beds of Pujada Bay (the Philippines), was studied with special emphasis on harpacticoid copepods. Sediment cores were obtained from areas adjacent to the different species of sea grasses. Meiofauna was enumerated at higher taxon level and harpacticoid copepods were identified to genus level. Diversity indices were calculated corresponding to the hierarchical levels of spatial biodiversity, i.e. alpha, beta and gamma. Nematodes were the most abundant meiofaunal group in all sediment layers and along the entire tidal gradient (37–92%); harpacticoids were second in abundance (3.0–40.6%) but highly diverse (N0: 9.33–15.5) at the uppermost sediment layer (0–1 cm) near all beds of sea grass species. There was a sharp turnover of harpacticoid genera along the tidal gradient, thus suggesting a relatively low proportion of shared genera among benthic communities in different sea grass zones. The families of Tetragonicipitidae and Miraciidae were the dominant harpacticoid groups occurring in all sediment layers of all sea grass species. The presence of the epiphytic genera of Metis at the deepest sediment layers in some sea grass species was striking. Overall, the major contributor to gamma (total) diversity of harpacticoid copepods in Pujada Bay is the high local (alpha) diversity (N0: 80.6%, H′: 94.7% of total diversity); hence, the habitat heterogeneity among sediment layers in sea grass beds is most relevant for the total diversity and richness of harpacticoid copepod genera in the area.