Can The Timing And Duration of Planktonic Larval Development Determine Invasion Success? A Case Study Comparing Range Expansion In The European Green Crab, Carcinus Maenas, And The Native Lined Shore Crab, Pachygrapsus Crassipes, In The North-Eastern Pacific.

Major El Niño events and oceanic heat waves are linked to the range expansion of many marine species. For the shores of the northeast Pacific, we compared range expansion in the European green crab, Carcinus maenas, which was introduced to San Francisco Bay prior to 1990, to that of the native lined-shore crab, Pachygrapsus crassipes, which has existed on the coast since at least the end of the last Ice Age (>10,000 years ago). The initial northern range limit of these species was central California and central Oregon, respectively. Both species increased their northern range along the open coast to northern Oregon, Washington and Vancouver Island after strong El Niño events. C. maenas, however, in just a matter of decades, successfully established populations in inlets on the west coast of Vancouver Island, and possibly also in the Salish Sea, while P. crassipes, in thousands of years, never has. We hypothesize that this difference in invasion success is due to the shorter larval duration of C. maenas, < 2 months, compared to that of P. crassipes, 3-4 months and timing of larval release, winter for both species. Because the residency times of water in the inlets of the west coast of Vancouver Island are ~1-2 months, they can act as an incubator for the larvae of C. maenas, while those of P. crassipes are likely flushed out to the open sea before they can complete their development.

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.

Phenotypic variation across the range of the lined shore crab Pachygrapsus crassipes Randall, 1840 (Decapoda, Grapsidae)

Two isolated clades of the lined shore crab, Pachygrapsus crassipes, live on opposite sides of the northern Pacific, presenting an interesting opportunity for studies of range limits and divergence. Prior to this study, P. crassipes’ Asian range was unclear; we confirmed that it is found throughout the main Japanese Archipelago, though sporadic or absent from the Ryukyu Archipelago. We examined phenotypic variation of this species’ chelae, which are conspicuously colored and larger in males, and found positive allometry for both sexes, which was stronger in males, a common feature of sexually selected ornaments and weapons. We also found that Asian and North American clades differ significantly in chela reflectance — in contrast to previous studies, which stated that these clades were phenotypically identical. We conclude that these clades are diverging phenotypically, but that these differences are not yet sufficient to warrant distinction as separate species.