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Amphipod assemblages associated with the biogenic reefs built by the honeycomb worm Sabellaria alveolata were studied at two sites (Praia da Aguda and Belinho) along the northwestern coast of Portugal. A total of 3909 specimens were collected, comprising 14 different amphipod species. A first record from the northeastern Atlantic coast was registered here for the species Caprella santosrosai, which was, up to now, recorded only along the Mediterranean coast of the Iberian Peninsula. A male specimen collected from the Sabellaria-reef located in Belinho allowed an update to the known distribution of C. santosrosai, thus altering its previous status as an endemic Mediterranean species. The most common species collected during the study were Microdeutopus chelifer (n = 1828), Jassa ocia (n = 1426), and Hyale stebbingi (n = 452). Forty-three percent of the total recorded species were encountered in both study sites, whereas the remaining 57 % were restricted to a single site (Belinho). The majority of the collected species (93 %) showed an Atlantic-Mediterranean distribution, confirming the close affinity between eastern Atlantic and Mediterranean amphipod assemblages and the role of the Portuguese coast as a transition zone through which numerous warm-water species, coming from North Africa and the Mediterranean Sea, could enter into the Atlantic and possibly get mixed with species coming from the North Sea and the Arctic, typically having affinity for colder waters.

Most of accumulation curves tend to underestimate species richness, as they do not consider spatial heterogeneity in species distribution, or are structured to provide lower bound estimates and limited extrapolations. The total‐species (T–S) curve allows extrapolations over large areas while taking into account spatial heterogeneity, making this estimator more prone to attempt upper bound estimates of regional species richness. However, the T–S curve may overestimate species richness due to (1) the mismatch among the spatial units used in the accumulation model and the actual units of variation in β‐diversity across the region, (2) small‐scale patchiness, and/or (3) patterns of rarity of species. We propose a new framework allowing the T–S curve to limit overestimation and give an application to a large dataset of marine mollusks spanning over 11 km2 of subtidal bottom (W Mediterranean). As accumulation patterns are closely related across the taxonomic hierarchy up to family level, improvements of the T–S curve leading to more realistic estimates of family richness, that is, not exceeding the maximum number of known families potentially present in the area, can be considered as conducive to more realistic estimates of species richness. Results on real data showed that improvements of the T–S curve to accounts for true variations in β‐diversity within the sampled areas, small‐scale patchiness, and rarity of families led to the most plausible richness when all aspects were considered in the model. Data on simulated communities indicated that in the presence of high heterogeneity, and when the proportion of rare species was not excessive (>2/3), the procedure led to almost unbiased estimates. Our findings highlighted the central role of variations in β‐diversity within the region when attempting to estimate species richness, providing a general framework exploiting the properties of the T–S curve and known family richness to estimate plausible upper bounds in γ‐diversity.