Rimicaris exoculata

Life-History

Apr14_clam_shells

Fig. 4 – Dead empty shells of the vent clam Calyptogena magnifica at the pacific Hanging garden site. Discontinuation of vent activity is responsible for the death of the organisms

Hydrothermal vents are extremely unstable and ephemeral habitats. Changes in the volcanic activity fuelling the vents can cause the flow of vent fluid to slow and finally stop (Van Dover, 2000). Such a scenario removes the energy source of the chemosynthetic bacteria resulting in a complete collapse of the vent ecosystem (fig, 4). The finite lifespans of hydrothermal vents makes the dispersal of larvae and the colonization of new habitats of premium importance for the organisms that live there. Given the frequently large distances between vent sites and potential hydrological and topographical barriers to larval dispersal, the mechanisms of larval dispersal and recruitment are of great interest to researchers (Tyler & Young, 2003).

Although Rimicaris is found on the more geologically stable MAR, dispersion is still of critical importance to this organism in avoiding local extinction. Genetic studies on Rimicaris have revealed a surprisingly low amount of genetic diversity between sites (Teixeira et al, 2012) suggesting extensive dispersal capabilities. How this occurs is at present unknown however this page will discuss what is currently known about the reproductive strategy and life history of Rimicaris.

Fig. 5 – Rimicaris eggs. The deep orange colour indicates a high lipid content.

In comparison with closely related Acanthephyra shrimps, Rimicaris produces more eggs per unit of body weight (Llodra et al, 2000). The eggs (fig. 5) were found to be small (0.5-0.8 mm) suggesting a short period of embryonic development with planktotrophic (free swimming and able to feed) larvae hatching from the eggs. Typical of other decapods, the eggs brooded under the abdomen prior to hatching (Dixon et al, 1998). Rimicaris samples taken over a period of 10 years has failed to find any evidence of a seasonal reproductive cycle (Copley et al, 2007) indicating that R. exoculata reproduces year round (asynchronous reproduction). Continuous reproduction has also been inferred for other well studied vent invertebrates (e.g. Bathymodiolus thermophilius and Riftia pachyptila, Tyler & Young, 1999) suggesting that asynchronous reproduction may be a common strategy for vent organisms.

larave

Fig. 6 – Recently hatched larvae of Rimicaris

The larvae of hydrothermal vent invertebrates are poorly known and Rimicaris larvae are no exception (Adams et al, 2012). Extensive searching of the literature can reveal only 2 papers discussing Rimicaris larvae. Larvae sampled by Pond et al, 1997 were found to be rich in lipids absent in the adults. Pond et al postulated that these lipid reserves were an adaptation to prolong the larval life thus providing support for the theory that Rimicaris produced larvae with a long planktonic phase.

More recently, larvae were observed hatching from mature, lipid rich eggs by Guri et al, 2012. The larvae resembled typical Zoea larvae (fig. 6) however they did posses stalked eyes, not present in the adults. Given that no further developed larvae have yet been examined we are as yet unsure how Rimicaris locates new vent sites with Chemical cues and temperature anomalies postulated as possible cues for larval settlement.

Juvenile Rimicaris differ from adults in both their morphology and colour. Known as “small, orange shrimp” to researchers, they were initially classified as two seperate species; Rimicaris aurantiaca (Martin et al, 1997) and Iorania concordia (Vercheska, 1996) and it was not until genetic analysis was conducted that their true identity was established (Shank et al, 1998). The morphology of juvenile R. exoculata development is discussed in detail by Komai & Segonzac, 2008 but it is interesting to note that the juvenile’s have lost the stalked eyes characteristic of the larvae.

Indirect methods have revealed that the lipid reserves of juveniles (responsible for the characteristic orange colour) are of photosynthetic origin suggesting an extensive larval phase in the upper water column, however the feeding mechanism is as yet unknown (Gebruk et al, 2000). Research from Gebruk et al has also suggested that as the juvenile grows it becomes more dependant on episymbiotic bacteria for food, episymbionts which are acquired horizontally (from the environment) (Guri et al, 2011)

Given the paucity of information regarding the life history of Rimicaris at present we can only speculate about possible dispersion mechanisms that would explain the low levels of genetic diversity found by Teixera et al, 2012. However, from what we do know we can be confident in proposing that the larvae of Rimicaris are capable of dispersing over wide areas and can survive extended periods without feeding due to their substatial lipid reserves. Whilst in the plankton, the larvae feed on food of photosynthetic origin although what this is and how they do so remains unknown. The vital question of what cues the larvae use to detect suitable habitats remains unknown and presents one of the biggest questions regarding Rimicaris. Readers still interested in learning more about vent shrimps are directed towards the next section on the feeding strategy of Rimicaris.

 

 

 

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