Periodic invasions during El Niño events by the predatory lined shore crab (Pachygrapsus crassipes): forecasted effects of its establishment on direct-developing indigenous prey species (Littorina spp.)

Coevolutionary arms races between shelled gastropods and their predators are more escalated near the equator. Therefore, temperate gastropods are predicted to be maladapted to highly specialized tropical shell-crushing crabs. The northern geographical limit of the lined shore crab (Pachygrapsus crassipes J.W. Randall, 1840) does not usually overlap with the southern limit of the Sitka periwinkle (Littorina sitkana Philippi, 1846), which lacks a pelagic larval stage. Large El Niño events increased the winter abundance and poleward transport of P. crassipes larvae from California (USA) in the Davidson Current. Temporary intertidal crab populations that included females with eggs were observed 1–4 years later, >1000 km north of its usual geographical range. Laboratory experiments showed that L. sitkana did not have a size refuge from adult P. crassipes. Moreover, consumption rates of adult L. sitkana by P. crassipes were 10-fold higher than those published for indigenous purple shore crabs (Hemigrapsus nudus (Dana, 1851)) with similar claw sizes. Additionally, the upper intertidal limit of invading P. crassipes was higher than that of H. nudus. Consequently, the invasion of P. crassipes reduced the width of L. sitkana‘s spatial refuge from predation. The permanent presence of this subtropical predator could reduce the intertidal distribution of this temperate gastropod, thereby causing contraction of its southern range limit.

Geographic origin and timing of colonization of the Pacific Coast of North America by the rocky shore gastropod Littorina sitkana

The demographic history of a species can have a lasting impact on its contemporary population genetic structure. Northeastern Pacific (NEP) populations of the rocky shore gastropod Littorina sitkana have very little mitochondrial DNA (mtDNA) sequence diversity and show no significant population structure despite lacking dispersive planktonic larvae. A contrasting pattern of high mtDNA diversity in the northwestern Pacific (NWP) suggests that L. sitkana may have recently colonized the NEP from the NWP via stepping-stone colonization through the Aleutian-Commander Archipelago (ACA) following the end of the last glacial 20,000 years ago. Here, we use multi-locus sequence data to test that hypothesis using a combination of descriptive statistics and population divergence modeling aimed at resolving the timing and the geographic origin of NEP populations. Our results show that NEP populations share a common ancestor with a population of L. sitkana on the Kamchatka Peninsula ∼46,900 years ago and that NEP populations diverged from each other ∼21,400 years ago. A more recent population divergence between Kamchatka and NEP populations, than between Kamchatka and other populations in the NWP, suggests that the ACA was the most probable dispersal route. Taking into account the confidence intervals for the estimates, we conservatively estimate that L. sitkana arrived in the NEP between 107,400 and 4,100 years ago, a range of dates that is compatible with post-glacial colonization of the NEP. Unlike other congeners that are relatively abundant in the Pleistocene fossil record of the NEP, only one report of L. sitkana exists from the NEP fossil record. Although broadly consistent with the molecular data, the biogeographic significance of these fossils is difficult to evaluate, as the shells cannot be distinguished from the closely-related congener L. subrotundata.