Aim To evaluate the impact of a proposed ancient suture zone in the Gulf of Mexico on the distribution and molecular diversity of dispersal‐limited, sand‐burrowing amphipods of the genera Haustorius and Lepidactylus (Haustoriidae: Amphipoda). Location Gulf of Mexico, USA and Mexico. Methods Phylogenetic and population genetic analyses were performed using mitochondrial (COI and 16S) and nuclear (18S and 28S) data from 93 amphipod individuals from 16 sites across the Gulf of Mexico. Bayesian and ML phylogenies were constructed for all genes, divergence times were estimated using a molecular clock for COI (0.007–0.013 subs./site per Myr), and four species delimitation methods were used to identify operational taxonomic units (OTUs) within each amphipod species. The mitochondrial COI gene was used to construct haplotype networks and estimate population genetic parameters to evaluate historical changes in effective population sizes. Results Deep divergences (most estimated to be >4 Ma) were uncovered between sister clades of both amphipod genera on either side of the Mississippi River as well as within Lepidactylus triarticulatus where each sample site was found to harbour a unique genetic lineage. Two cryptic OTUs of L. triarticulatus were identified living sympatrically at Pass Christian, Mississippi. Two distinct OTUs representing western and eastern Haustorius galvezi clades were identified along the Texas and Mexico coastlines with abutting ranges. Population genetic results show some support for recent population expansions for western Gulf OTUs, while eastern Gulf OTUs may have suffered population bottlenecks in the past. Main conclusions Divergences between sister species of sand‐burrowing amphipods exceeds the timing of previous vicariant hypotheses. The split appears to be consistent with Miocene sedimentation levels from the Mississippi River acting as an east–west barrier to gene flow in the Gulf of Mexico. Given their strong population structure and cryptic diversity, haustoriid amphipods are ideal model organisms for studying open coast biogeography.

Genome sizes vary by orders of magnitude across the Tree of Life and lack any correlation with organismal complexity. Some crustacean orders, such as amphipods, have genome sizes that correlate with body size, temperature, and water depth, indicating that natural selection may constrain genome sizes due to physiological pressures. In this study, we examine the relationship between genome size, repetitive content, and environmental variables on a clade of sand-burrowing amphipods (Haustoriidae) that are distributed across the Gulf of Mexico and the North Atlantic. We uncover a 6-fold genome size variation within a clade that is less than 7 million years old. Unlike previous studies, we find no correlation between genome size and latitude, but do uncover a significant relationship between genome size and body length. Further, we find that the proportion of repetitive content predicts genome size, and that the largest genomes appear to be driven by expansions of LINE elements. Finally, we find evidence of genomic purging and body size reduction in two lineages that have independently colonized warm brackish waters, possibly indicating a strong physiological constraint of transitioning from surf-swept beaches to protected bays.Significance StatementThe evolution of genome size has been a long-standing puzzle in biology. In this work, we find that genome sizes may be driven by different selection regimes following shifts to a new habitat. Dramatic genome size changes can occur rapidly, in only a few million years.Data Availability StatementRaw data sheets have been deposited on Dryad: SUBMITTED. Raw sequence reads are available at from NCBI under Bioproject SUBMITTED.

Dramatic genome size variation exists across the Tree of Life. Some crustacean groups, such as amphipods, have genome sizes that correlate with body size, temperature, and water depth, indicating that genome sizes may be constrained due to physiological pressures. This may indicate that a general “genome-temperature-size” rule exists in ectotherms, in which body-size and latitude may be predictive of genome size. We examined the relationship between genome size, repetitive content, and environmental variables on a clade of sand-burrowing amphipods (Haustoriidae) that are distributed across the Gulf of Mexico and the North Atlantic. We uncovered a 6-fold genome size variation within Haustoriidae, a substantial amount considering this clade is less than 7 million years old. Unlike previous studies, we find no correlation between genome size and latitude, but do uncover a significant relationship between genome size and body length. Further, we find that the proportion of repetitive content predicts genome size, and that the largest genomes appear to be driven by expansions of LINE elements. We also found evidence of genomic purging and body size reduction in two lineages that have independently colonized warm brackish waters, possibly indicating a strong physiological constraint of transitioning from surf-swept beaches to protected bays.

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