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Alvinocaris muricola - Williams, 1988
Alvinocaris muricola (from the Latin “muriacola" meaning "brine inhabiting"[2]) is a species of caridean shrimp first described by Williams in 1988 using the DSV Alvin[2]. A. muricola belongs to the family Alvinocarididae, first described by Christoffersen in 1986[3]. The family currently consists of 36 species over 9 genera[4],which inhabit deep sea vents, cold seeps[3], and organic falls in some cases[1]. Within the genus Alvinocaris, there are 17 species currently described[4].

Habitat & Distribution
Alvinocaris muricola lives amongst mussel beds in Atlantic cold seeps[5][8] between depths of 1125-3300 m[11]. Within mussel beds the shrimp has been observed on vestimentiferan tubes, suggesting a wide range of tolerance to chemical conditions[12]. Recently, the species has also been observed at organic falls[1] which may enhance A. muricola’s genetic connectivity and capability of tolerating a range of environments.

Haplotype network analysis of the COI gene marker suggests that populations of A. muricola in the Southwest Atlantic are recent founder events with genetic connectivity to Equatorial Atlantic populations[1][9]. This conclusion is substantiated by findings suggesting that a westward equatorial current system is responsible for dispersion of A. muricola[9][27]. Coupled with an extended Planktonic Larval Duration, this enables A. muricola to disperse and colonise a variety of regions around the Atlantic[1][8][16][26].

Microsatellite analysis has demonstrated that Alvinocaris markensis, a species found at hydrothermal vents along the Mid-Atlantic Ridge, is likely conspecific with A. muricola[8][9]. The findings conclude that A. muricola is a morphologically plastic species[8], able to occupy both hydrothermal vent and cold seep habitats[4][5][9] across distinct biogeographic regions.

The wide distribution of A. muricola across the Atlantic may be attributed to their larval development[5], during which time larvae are released into the water column allowing them to be dispersed over larger distances than non-planktonic larvae[5]. This increased dispersal distance facilitates genetic connectivity and gives the opportunity to colonise new regions [1], as shown between the Equatorial Atlantic and Brazilian populations of A. muricola[1][5]. Furthermore, the presence of A. muricola at whalefalls lends some credibility to the hypothesis that organic falls serve as ecological stepping-stones [14]. The ecological hypothesis is as yet unproven due to the lack of available data regarding whale falls, though it is an area of ongoing investigation[15].

Trophic Ecology & Adaptations
Research suggests that Alvinocaris species’ are opportunistic generalist feeders[17], taking advantage of both seep and surface derived material. They likely feed on free living bacteria as well as meiofauna[12]. When the species was first described, Williams[2] suggested a feeding method relying primarily on bacteria coupled with predation and necrophagy as secondary methods[2].

Stable isotope analysis of nitrogen (δN15) and carbon (δC13) in Alvinocaris muricola tissue samples showed that individuals within the same environment had varying isotopic compositions, indicating diverse feeding behaviour within populations[12]. Some individuals were enriched in heavier isotopes of δN15, suggesting a diet of small predatory meiofauna and surface derived material, which are at a higher trophic level than A. muricola[12]. Other individuals had lower values of δN15 implying that the shrimp also grazes on symbiont bacteria, which can be found on their mouthparts[17], and on free-living bacteria both within the mussel bed and amongst vestimentiferan tubeworms[12].

A. muricola may also be a predatory species. Observations of feeding on mussels with fractured shells highlight the opportunistic and necrophagous nature of the Alvinocaris species[19]. Similarly, predation behaviour has been observed in the relative species A. markensis at the Mid-Atlantic Ridge, further backing this hypothesis[5]. To facilitate this feeding strategy, A. muricola has enlarged chelae lined with fine teeth to shred larger food material[2][19]. The chelae are spoon shaped suggesting an adaptation to scoop bacteria towards the mouthparts[2].

Life-history ecology & adaptations
Alvinocaris muricola is a gonochoric species, with research suggesting that the species exhibits protandric hermaphroditism[5], wherein males develop female sexual organs with age. Observed sex ratios of A. muricola have been both 1:1 and female skewed[5] across different studied populations. The skewed ratio may be explained by a difference in spatial distributions between males and females within mussel beds, as well as the exhibition of protandric hermaphroditism[5]. Females are larger than male specimens, with the largest recorded male having a carapace length of 16.7mm, whilst the largest recorded female has a carapace length of 21.5mm[11].

Reproduction occurs through copulation wherein the male delivers aflagellate sperm to fertilise the female’s oocytes[5]. Females can carry broods ranging from 1432 to 5798 eggs, with total fecundity being correlated to larger body size[5]. Brooding females carry eggs under their pleopods during embryonic development[7][8], avoiding the anoxic and sulphurous extremes of the cold seep environment by remaining around the periphery of mussel beds to protect the developing embryos[5]. Researchers have identified three embryonic stages before the hatching[5]. During early development, embryos are featureless and are around 0.59mm in diameter, reaching 0.75mm by late development by which time clear features are observable.

Debate surrounds the type of larval stage of A. muricola. The small size of eggs and larvae supports a pelagic planktotrophic stage[5][18], during which larvae may migrate upwards in the water column to feed on organic matter within the photic zone[5][18]. However, recent evidence found the first larval stage of A. muricola does not exhibit mouth parts, implying that during early stages larvae are primarily lecithotrophic[8], building on earlier findings[5][7][18]. Lecithotrophic larvae are common amongst decapod species[25] and Alvinocaris sp. larvae have been observed to survive for 74 days at 4.5°C in laboratory conditions[26]. This method of larval development may prolong the time larvae spend in the water column, consequently enhancing the wide distribution of A. muricola observed today[1][8][16].

It is currently unknown whether reproduction is seasonal, due to limited data available specific to A. muricola[5][24]. However, studies of a relative cold seep species, Alvinocaris stactophila found that they exhibit seasonal cycles of reproduction, with females brooding eggs from autumn before spawning in early spring[7]. The spawning event is believed to be triggered by increased phytodetrital fluxes, which act as a zeitgeber to release larvae during peak food availability[7]. A. muricola may exhibit similar reproductive seasonality[5], however A. stactophila is the only Alvinocarid shrimp that has been found to exhibit such seasonal behaviour[24], and further investigation is required.

Other adaptations & ecological features
Specimens of a nematode species, Chromadorita regabi, have been found among developing embryos of Alvinocaris muricola[10]. It appears that this nematode predates on the embryos carried by female A. muricola individuals. Observations of this predatory behaviour include a specimen of C. regabi partially embedded within an embryo[5], as well as darkly coloured embryos where development of A. muricola larvae has been disrupted. It is thought that the high density of A. muricola found at cold seep mussel beds make embryos an accessible food source for C. regabi[10]. Research has also suggested that A. muricola may be predated upon by the deep-sea fish Pachycara caribbaeum which has been observed alongside A. muricola at hydrothermal vents, cold seeps and mussel beds[13][20].

The conditions of cold seeps can be extremely hypoxic meaning fauna have to optimize oxygen uptake. A. muricola has a large scaphognathite appendage to pump water through 10 pairs of gills which maintain a near constant surface area to body weight ratio throughout the shrimp’s life. The enlarged scaphognathite allows for increased water flow over the gills which increases oxygen uptake efficiency of A. muricola[21].

Whilst A. muricola itself is not directly threatened, cold seep environments are threatened by human activity, namely through the exploration for mineral and hydrocarbon resources to meet modern demands[22]. This creates a potentially catastrophic scenario wherein deep sea environments such as cold seeps face direct impacts resulting from human activity before they are fully understood[23], which will have dire impacts for cold seep fauna such as Alvinocaris muricola.