Snails in the genus Smaragdia may be small (no known species has a shell length longer than a centimetre!) but they pack a punch when it comes to influencing seagrass dynamics. At present, there are 10 species in the genus, all of which are found amongst seagrass blades in shallow marine environments of tropical and sub-tropical oceans all over the globe. Very few of them have been studied in detail, but the few that have reveal a secret life full of interesting facts and ecological potential.
I have chosen Smaragdia for my first Spotlight on the spineless because they are one of my favourite genus; a love inspired by my friend Jose L. Rueda (a Smaragdia expert) that drove (and still drives) me to spend many hours over the last two years crawling around the seagrass beds of Moreton Bay resulting in a recently submitted paper we co-authored about one of Australia’s representatives Smaragdia souverbiana. I will feature a Spotlight on the spineless each month, and with each post giving you the TOP 5 reasons I think each spotlight group deserves your attention! We recently published a paper on the feeding ecology of the most common Indo-pacific species (Smaragdia souverbiana) in J. Molluscan Studies.
But I bet you’ve never seen one? Smaragdia are found in shallow seagrass environments all over the world (for a focus on our region Figure 1.), in each place seeming to associate with a different set of seagrasses [1,2,3]. The Indo-Pacfic hosts almost ALL of the known species, except for Smaragdia viridis which has an intriguing distribution split across the Atlantic (one population in the Mediterranean  and one in the Caribbean ) and Smaragdia bryanae which, so far, has only been found in Hawaii . The few studies that have looked at density find they represent a large component of the snail assemblage – some finding almost 100 per square meter  or that they account for almost 50% of the snails found in each sample .
2. They are TINY Good things come in small packages right, and you cant get much smaller than these guys and still be able to see them with your naked eye. In order to find them alive and doing their thing, expect to spend a lot of time crawling around on all fours in the seagrass with your face no more than a foot from the ground (Figure 2.). But don’t fret, this is actually a great way to experience a seagrass ecosystem! Once you look up close you will see a whole suite of other tiny seagrass critters that would have gone unnoticed if you kept your bipedal non-mud-grubbing stance. Being small has its advantages in the seagrass, Smaragdia feed directly on seagrass cells therefore they need to remain small so their tiny radula can burst open individual seagrass cells and suck out their insides . Being small also means you can hide yourself from predators – some Smaragdia nestle right down at the base of seagrass blades during the day to avoid being eaten and emerge at night when most of their fish predators are asleep or cannot see them [J.L.Rueda, unpub. data]. 3. They represent a very special part of the seagrass food-web Our current model of seagrass food webs splits the herbivores in the system into two factions : one which consists of the BIG things like Green Turtles and Dugongs which eat large amounts of seagrass and avoid a lot of the predation in the system because they are so huge, and another which consists of all the LITTLE guys like snails, crabs, shrimps and sea urchins, who mostly feed on epiphytes (algae that parasitise seagrass) offering a helping hand to seagrass by keeping it clean and being a tasty food source for all those meso-predators that we like to eat (like predatory crabs and fishes)(Figure 3.). However, Smaragdia sit somewhere in the middle – all of those studied to date EAT seagrass directly (Figure 4.) meaning that unlike their small invertebrate kin they are having a negative effect on seagrass, but they are also different to the other organisms that eat seagrass in this system because they are small and relatively limited in mobility. Studies of Smaragdia are helping bring this specialised role to the attention of seagrass biologists, a role that is also played by a number of other equally ill-studied species .
4. They were one of the first marine species to be studied in a biogeographical context Studies of how marine populations connect across surprising scales is common practice now-a-days, employing cutting edge techniques to link populations via their genetic similarity, but back in the 1970’s you had to do it the HARD way. Back in 1971 Rudolf Scheltema  intercepted plankton caught in the oceanic currents that cross the Atlantic, giving one of the first studies that demonstrated that disjunct populations across oceans are connected by planktonic dispersal. Using Smaragdia viridis as one of his focal species he showed that larval gastropods were traversing on trans-Atlantic currents, possibly upon floating debris (like the assemblages documented on floating pumice by my mate Lucy Hurrey & her friends ) keeping the genetic exchange between Mediterranean and Central American populations open irrespective of what we perceive as an unfathomable distance. 5. They are beautiful Whilst basing arguments of why a species deserves your attention purely on their looks is something I theoretically shy away from (see other posts on environmental ethics) Smaragdia are a stunning species, with a huge range of shell patterns and colours, all of which often go unnoticed because to really appreciate them, you need to take a peek through your microscope. To end my ode to Smaragdia, here are some of my favourite images from the internet, ending with a little sketch of them by me.
For more information on our ongoing Smaragdia research and the research of Jose L. Rueda see http://www.researchgate.net/profile/Jose_Rueda2/ I would like to acknowledge the students who volunteered their time during their E.A.P. program at U.Q. BIOL for without them my work on Smaragdia would have been impossible.
1. Numerous publications by Jose Luis Rueda such as: Rueda J.L. and Salas C. 2007 ‘Trophic dependance of the emerald neritid Smaragdia viridis on two seagrasses from European coasts’ Journal of Molluscan Studies 73 p211-214 accessible here http://mollus.oxfordjournals.org/content/73/2/211.full, and Rueda J.L., Salas C., Urra J. and Marina P. 2009 ‘Herbivory on Zostera marina by the gastropod Smaragdia viridis’ Aquatic Botany 90 p253-260 accessible here amongst many others.
2. Holzer K., Rueda J.L. & McGlathery K.J. 2011 ‘Differences in the feeding ecology of two seagrass-associated snails’, Estuaries and Coasts 34:6 p1140-1149 accessible here http://link.springer.com/article/10.1007%2Fs12237-011-9406-6
3. Unabia C.R.C. 2011 ‘The snail Smaragdia bryanae (Neritopsina, Neritidae) is a specialist herbivore of the seagrass Halophila hawaiiana (Alismatidae, Hydrocharitaceae)’, Invertebrate Biology 130:2 p110-114 accessible here http://onlinelibrary.wiley.com/doi/10.1111/j.1744-7410.2011.00225.x/abstract
4. Rueda J.L. and Salas C. 2008 ‘Molluscs associated with a subtidal Zostera marina L. bed in southern Spain: Linking seasonal changes in fauna and environmental variables’, Estuarine, Coastal and Shelf Science 79 p157-167 accessible here http://www.sciencedirect.com/science/article/pii/S027277140800139X
5. Valentine J.F. and Duffy J.E. 2006 ‘The central role of grazing in seagrass ecology’ in ‘Seagrasses: Biology, Ecology and Conservation’ accessible here http://books.google.com.au/books?id=emkDvfazY2UC&pg=PA463&lpg=PA463&dq=the+central+role+of+grazing+in+seagrasses&source=bl&ots=K6sF6-l8pO&sig=NuMjSFxDahYl1V9w7wftrIizXVI&hl=en&sa=X&ei=06vBUeqYOsnkyQHC0IGYDA&ved=0CDMQ6AEwAA#v=onepage&q=the%20 central%20role%20of%20grazing%20in%20seagrasses&f=false
6. For some examples of other seagrass grazing invertebrates see Zimmerman, R.C., Kohrs, D.G. & Alberte, R.S. 1996 ‘Top-down impact through a bottom-up mechanism: The effect of limpet grazing on growth, productivity and carbon allocation of Zostera marina L.(eelgrass)’ Oecologia, 107 p560-567 or van Tussenbroek, B.I. & Brearley, A. 1998 ‘Isopod burrowing in leaves of turtle grass, Thalassia testudinum, in a Mexican Caribbean reef lagoon’ Marine and Freshwater Research, 49 p525-531 or Reynolds, L.K., Carr, L.A. & Boyer, K.E. 2012 ‘A non-native amphipod consumes eelgrass inflorescences in San Francisco Bay’ Marine Ecology Progress Series, 451 p107-118
7. Scheltema R.S. 1971 ‘Larval dispersal as a means of genetic exchange between geographically separated populations of shallow-water benthic gastropods’, Biological Bulletin 140 p284-322
8. Bryan S., Cook A.G., Evans J.P., Hebden K., Hurrey L., Colls P., Jell J.S., Weatherley D. and Firn J. 2012 ‘Rapid, Long-distance dispersal by pumice rafting’, PLOS ONE 7:7 e40583 accessible here http://www.plosone.org/article/info:doi/10.1371/journal.pone.0040583