The Pleustidae are a family of marine amphipods, distributed around the world. However, despite their seemingly cosmopolitan distribution and abundance, it has been surprisingly difficult to find information about this family online. Pleustids are one of the many families in the largest of the amphipod suborders, the Gammaridea (gammarideans include the sandhoppers and other sandhopper-looking crustaceans that are what most people think of when they think of an amphipod). Gammaridean interrelationships are a great tangled mess, and in many places fall into a state that we scientists technically refer to as "buggered beyond belief". Only slowly are researchers beginning to draw some sense out of things, and they'll probably be at it for a long time yet. For those who wish to track them down (for the most part, I haven't seen them), the main revisions of Pleustidae were published by Bousfield & Hendrycks (1994 and following) in Amphipacifica*. Stock (1986) (who very tentatively assigns three oddball stygobiotic species from Japan to the Pleustidae) gives the defining characters of the family as "the rudimentary condition of the accessory flagellum of the first antenna, the biramous third uropod (rami lanceolate), the elongate telson, the weak and more or less similar gnathopods 1 and 2, and... the bilobed condition of the labrum". Bousfield & Hendrycks (1994, 1995) divided the family into a number of subfamilies.
*The short-lived journal Amphipacifica ended up with three volumes published between 1994 and 2001 (I haven't found any indications that a planned fourth volume ever made an appearance). While I haven't found a complete contents listing, it appears that most (if not all) articles had the chief editor, Edward Bousfield, as author or co-author, and the majority were on amphipod taxonomy. Perhaps unjustly, the journal is not remembered for its contributions to crustacean systematics as much as it is for Bousfield's perhaps unwise foray into vertebrate taxonomy. Yes, this was the journal that saw the publication of the infamous Bousfield & LeBlond (1995), and the name Cadborosaurus wellsi - a paper so controversial that two of the journal's editors promptly handed in their resignations in protest. For more details, see Darren Naish's 2006 review.
Not having the resources on hand to give you a a decent overview of the family (sorry), I'm just going to give you a couple of the highlights that I have been able to locate - (as it says in the title) the snail mimics and the symbiotic taxa.
Snail mimicry has been recorded for two pleustids, Pleustes platypa and unidentified species of Stenopleustes (Field, 1974). Pleustes platypa lives in kelp beds and mimics the marine gastropod Mitrella carinata. Mimetic Stenopleustes (it is not known how many species are involved) live in beds of Zostera (seagrass) and mimic various species of snails of the genus Lacuna. Different Stenopleustes individuals may have different colour patterns, each matching a different Lacuna species. The amphipod clings tightly to the seagrass, moving slowly to match the speed of a snail. Occassional rocking back and forth mimics the rocking movement of the snails. And while amphipods are perfectly adept swimmers, snail-mimicing Stenopleustes would only swim under extreme provocation, preferring to crawl to the other side of the seagrass blade instead when possible. Obviously, starting to swim would quickly give away that the animal was not a snail!
Other intriguing pleustids are the species that live as symbionts (generally commensals) of larger marine invertebrates. Commensipleustes commensalis is a symbiont of crabs, with enlarged spines on the underside of the forelimbs against which the dactylus (claw) can be folded back, allowing the amphipod to hang onto the host's setae. Members of the genus Dactylopleustes, on the other hand, live on sea urchins, and their legs have notched claws that can be placed around the host's spines. Another species, Mesopleustes abyssorum, clings to the legs of sea spiders. But most remarkable of all is the lifestyle that has been inferred for the species Myzotarsa anaxiphilius (Cadien & Martin, 1999). Like Commensipleustes, Myzotarsa is a symbiont of crabs (in this case, king crabs of the genus Paralithodes), but while most crab-symbiotic amphipods live around the densely setose mouthparts, Myzotarsa lives underneath the crab's recurved abdomen. Without setae to cling on to, the claws on the walking legs of Myzotarsa bear special suckers to allow the animal to latch on. What makes Myzotarsa really remarkable is that not just any crab will make a suitable host - instead, the amphipod shows a strong preference for crabs that are parasitised by rhizocephalans (out of 179 specimens of Myzotarsa referred to by Cadien & Martin, 1999, 167 came from parasitised hosts while only six came from a non-parasitised host [the remainder came from hosts whose parasite status was not recorded). It seems that the diet of the little Myzotarsa is the eggs being incubated underneath the abdomen. While healthy crabs will only be carrying eggs if they're female and if it's the right season, the chemically-castrated, feminised (if male) and mind-controlled infected crabs will be carrying externa filled with yummy rhizocephalan eggs all year round...
Bousfield, E. L., & E. A. Hendrycks. 1994. A revision of the family Pleustidae (Crustacea: Amphipoda: Leucothoidea). Part I. Systematics and biogeography of component subfamilies. Amphipacifica 1: 17-57.
Bousfield, E. L., & E. A. Hendrycks. 1995. The amphipod family Pleustidae on the Pacific coast of North America: Part II. Subfamilies Parapleustinae, Dactylopleustinae, and Pleusirinae. Systematics and distributional ecology. Amphipacifica 2: 65-133.
Bousfield, E. L., & P. H. LeBlond. 1995. An account of Cadborosaurus willsi, new genus, new species, a large aquatic reptile from the Pacific coast of North America. Amphipacifica 1 (Supplement 1): 3-25.
Cadien, D. B., & J. W. Martin. 1999. Myzotarsa anaxiphilius, new genus, new species, an atylopsine amphipod (Gammaridea: Pleustidae) commensal with lithodid crabs in California. Journal of Crustacean Biology 19 (3): 593-611.
Field, L. H. 1974. A description and experimental analysis of Batesian mimicry between a marine gastropod and an amphipod. Pacific Science 28 (4): 439-447.
Stock, J. H. 1986. Amphipoda: Pleustidae. In Stygofauna Mundi: A Faunistic, Distributional, and Ecological Synthesis of the World Fauna inhabiting Subterranean Waters (including the Marine Interstitial) (L. Botosaneanu, ed.) pp. 560-561. E. J. Brill / Dr. W. Backhuys: Leiden.