TAFKAMI Walks

Various individuals of the amoeboflagellate Breviata anathema. Note particularly the extended anterior and posterior pseudopodia on numbers 13, 14 and 18. Figure from Walker et al. (2006).

There's been a couple of really interesting things come through the pipeline lately. For this post, I'm not going to talk (yet) about yesterday's publication of an analysis of acanthodians that suggests that they are not a monophyletic grouping. If you want to know what that's all about, ask Adam Yates (and if you don't know what an acanthodian is, I've briefly discussed them previously). Today, I'm going to discuss another recent publication.

One of the first posts I wrote for this site (in fact, the sixth) was on an organism that I dubbed TAFKAMI, or The Amoeboid Formerly Known As Mastigamoeba invertens. This organism had been originally identified as 'Mastigamoeba invertens' when isolated in 1992, and was eventually properly described by Walker et al. (2006) as Breviata anathema* (the real Mastigamoeba invertens is known only from an undiagnostic description published in 1892, and, short of someone inventing a time machine so that they can look over its original describer's shoulder, will probably never be identifiable). Breviata is an amitochondriate, microaerobic amoeboid or amoeboflagellate (depending on life cycle stage). As explained in the previous post, Breviata has proven to be an obscenely difficult organism to place phylogenetically. Its position in phylogenetic analyses has been very unstable, and it jumps wildly about depending on the analysis parameters. The earliest division in eukaryotes appears to be between unikonts (animals, fungi and amoebozoans, which have a single flagellum with a single basal body) and bikonts (including plants, algae and excavates, with flagella in doublets or with double basal bodies), and it has not even been conclusively established whether Breviata is a unikont or a bikont. It has a single flagellum like a unikont, but two basal bodies attached to that flagellum like a bikont**, and sturdy branching filose pseudopodia like nothing else. Whatever its position, it seems likely that the divergence of Breviata from other eukaryotes happened not long after the the origin of crown eukaryotes in total.

*Tragically, Walker et al. (2006) gave no etymology for the new name. I have always wondered what exactly is so anathematic about Breviata anathema.

**Just to confuse matters, there are unikonts with double basal bodies, and bikonts with single flagella. However, bikonts with single flagella always retain two basal bodies. The anterior basal body in bikonts is always the younger of the two, and when the posterior basal body dies off the anterior body moves to the back and a new basal body grows in front of it. Those unikonts with two basal bodies still lack this distinctive growth pattern. Unfortunately, the flagellar growth pattern has not yet been studied for Breviata.

A new paper published by Minge et al. (2009) presents a new phylogenetic analysis incorporating Breviata anathema that draws on 17,283 nucleotide sites from no less than 78 genes (for contrast, the analysis of Breviata by Walker et al. used 1274 sites). The results of this analysis place Breviata with the amoebozoans, the clade including the majority of amoeboids with lobose pseudopodia. The support for this result is actually not too bad for this high a level of evolutionary divergence. Under certain analytical parameters, Breviata fell within other amoebozoans as sister to the other amitochondriate amoeboids Entamoeba and Mastigamoeba proper, but in the majority of cases it was the sister group to all other amoebozoans. This seems the more likely option as Breviata lacks certain sequence signatures (including a small insertion) characteristic of other Amoebozoa.

Sadly, as interesting as this result is, and as impressive as the amount of data used is, the analysis of Minge et al. (2009) suffers a fatal flaw. Though the analysis by Walker et al. did not give a conclusive result, the position they suggested to be most likely for Breviata was as sister to the Apusozoa. Apusozoans are a small group of flagellates with doubled flagella, and have been suggested to represent the basalmost divergence in the bikont lineage. As well as the double basal bodies, Apusozoa also produce filose pseudopodia like Breviata. Unfortunately, due to lack of data, the analysis by Minge et al. (2009) doesn't include a single apusozoan. While I'm personally sceptical of an apusozoan-Breviata relationship, I do think that without their inclusion the results of Minge et al. can't really be taken as conclusive.

Even if the phylogenetic results can't be entirely trusted, Minge et al. (2009) do have some interesting things to say. One of the interesting results from Walker et al. (2006) was the identification in Breviata of what appeared to be a hydrogenosome. Hydrogenosomes are hydrogen-processing organelles found in a number of anaerobic eukaryotes that have been shown to be altered mitochondria (Akhmanova et al., 1998). If Breviata did have a hydrogenosome, that would add to an increasing amount of evidence that all of the various 'amitochondriate' eukaryotes living today actually descend from ancestors that once had mitochondria (in contrast to previous opinions that they diverged from other eukaryotes prior to the origin of mitochondria). Among the genes possessed by Breviata, Minge et al. identify a number of genes derived from the pre-mitochondrial endosymbiont, confirming that Breviata's lack of mitochondria is a secondary feature.

Finally, there is the way Breviata moves. Amoeboids move, of course, by the extension of pseudopodia, but the exact method by which pseudopodia are produced can differ significantly between taxa. Indeed, in organisms with few permanent morphological features, the mode of pseudopodium formation has turned out to have a fair amount of phylogenetic significance (Smirnov et al., 2005). With its unique phylogenetic position, it seems only fitting that Breviata should have a unique mode of movement - it walks. A pseudopodium is protruded from the front of the cell and attached to the substrate. The rest of the cell body then rolls forward over the attached pseudopodium (like a tractor on treads, is Minge et al.'s analogy), until the pseudopodium is left trailing behind before being retracted and another pseudopod is extended from the front to repeat the process. No other organism has a mode of movement like Breviata - always twirling, twirling, twirling towards the future!

REFERENCES

Akhmanova, A., F. Voncken, T. van Alen, A. van Hoek, B. Boxma, G. Vogels, M. Veenhuis & J. H. P. Hackstein. 1998. A hydrogenosome with a genome. Nature 396: 527-528.

Minge, M. A., J. D. Silberman, R. J. S. Orr, T. Cavalier-Smith, K. Shalchian-Tabrizi, F. Burki, Å. Skjæveland & K. S. Jakobsen. 2009. Evolutionary position of breviate amoebae and the primary eukaryote divergence. Proceedings of the Royal Society of London B 276: 597-604.

Smirnov, A., E. Nassonova, C. Berney, J. Fahrni, I. Bolivar & J. Pawlowski. 2005. Molecular phylogeny and classification of the lobose amoebae. Protist 156: 129-142.

Walker, G., J. B. Dacks & T. M. Embley. 2006. Ultrastructural description of Breviata anathema, n. gen., n. sp., the organism previously studied as "Mastigamoeba invertens". Journal of Eukaryotic Microbiology 53 (2): 65-78.

The Beautiful Angel of Death

If you're a tiny zooplanktic animal, that is.

This video was recently posted by Kevin Zelnio at Deep Sea News:



The animal shown in the video (swimming to music from Hayao Miyazaki's great ecofable Kaze no Tani no Naushikā [Nausicaa in the Valley of the Wind]) is the pelagic gastropod Clione limacina. Aquatic gastropods are usually members of the benthos, and relatively few groups have made the change to a pelagic lifestyle. Those that have are invariably freakishly bizarre:


Two species of the planktic nudibranch Glaucus (G. atlanticus on the left and G. marginatus on the right). Glaucus is a member of the Aeolidioidea, and like other aeolids it feeds on cnidarians (siphonophores and chondrophores in the case of Glaucus) and sequesters their stinging cells for its own defense. Photo by Gary Cobb.

Even the superficially more normal-looking (albeit attractively coloured) violet snails of the genus Janthina (also predators of planktic hydrozoans) make up for their unassuming appearance with their bohemian lifestyle, hanging upside down from a floating raft constructed from bubbles held together with mucus:


Janthina feeding on a Portuguese man-of-war, Physalia. Photo by Bill Rudman.

The sea angel Clione belongs to one of the larger groups of pelagic gastropods, the Gymnosomata. Together with another group, the Thecosomata, they have been classified into the Pteropoda, characterised by the adaptation of the foot into a pair of 'wings' for swimming. Recent authors have disagreed as to whether or not the pteropods form a monophyletic group (with probably the majority favouring "not"), or whether gymnosomates and thecosomates took to the seas independently, but pteropod monophyly was supported by Klussmann-Kolb & Dinapoli (2006). Pteropods have an interesting taxonomic history. When Cuvier first established the group in 1804, he regarded them not as gastropods but as a separate order in their own right. Pteropods seem to have been regarded as a link between gastropods and cephalopods, with some believing them closer to the latter than the former. Even after the modern pteropods were well-established to be gastropods, and 'Pteropoda' often abandoned as a formal category, the name maintained a strange shadow existence in palaeontology, with many Palaeozoic fossil problematica such as tentaculitoids, hyoliths or even conulariids (almost meaninglessly) regarded as or compared with "pteropods".

Within the pteropods, the most obvious difference between Gymnosomata and Thecosomata is that gymnosomates such as Clione completely lack a shell, while thecosomates retain one. And what incredible shells they are - the ultimate in lightweight, translucent construction, some thecosomates are ethereal constructions in blown glass:


Limacina helicina, photographed by Russ Hopcroft.

REFERENCES

Klussmann-Kolb, A., & A. Dinapoli. 2006. Systematic position of the pelagic Thecosomata and Gymnosomata within Opisthobranchia (Mollusca, Gastropoda) – revival of the Pteropoda. Journal of Zoological Systematics and Evolutionary Research 44 (2): 118-129.

In Which I am Defeated by Shells

The tiny Mediterranean marine snail Fossarus ambiguus. Photo by A. Piras.

I knew this day was coming. Every Monday I assign myself a random taxon to write a post on, and so far I've generally been pretty successful. But I always knew that some day I'd assign myself a target that would prove hopeless. That day has come.

The Fossarinae are a group of marine gastropods belonging to the superfamily Cerithioidea that also includes the famous gastropod radiation of Lake Tanganyika in Africa (which, now I think about it, would have probably been a much more promising post subject). As a rule, fossarines are pretty tiny - the best-known species, the Mediterranean Fossarus ambiguus, is only three to five millimetres in size - and they are usually short, squat shells with fairly prominent ribbing. Older references place the fossarines in their own family, Fossaridae, but they were placed in the Planaxidae as a subfamily by Kowalke (1998) (which I haven't read). Fossarines and other Planaxidae (Planaxinae) are fairly distinct in appearance (planaxines are longer and more turret-shaped) but they have similar protoconch morphologies and are both larval brooders. After fertilisation, the female does not lay eggs but incubates her embryos in a pouch behind the head, eventually releasing them as free veliger larvae. One individual of the possible (see later) fossarine Larinopsis turbinata held as many as 400 embryos in its brood pouch.

And that is pretty much it, as far as I've been able to find. And to be honest, most of that I lifted off one webpage. Research-wise, Fossarinae seem to have been almost criminally ignored. They don't seem to be particularly rare, and seem to be found pretty much worldwide, so I can only ascribe this to their small size and unassuming natures. Also, as a result of this lack of study, there seems to be an underlying implication that the subfamily is poorly defined and many of the taxa currently regarded as "fossarines" may not be so.


One of these doubtful fossarines is this rather neat little shell, Larinopsis ostensus (photo from here). The shell in the photo is the holotype of this species, collected off Jervis Bay in New South Wales, and still the only known specimen. Larinopsis is big for a fossarine (more than three times the size of Fossarus ambiguus) and lacks the prominent ribbing of most fossarines. It also has that very neat loosely coiled shell, so thin that you can see right through it.

Completely unrelated postscript: While looking stuff up for this, I made the mistake of looking into Finlay (1927). Why are old taxonomic works on molluscs always so horrendously painful to read? There seems to have been this great conspiracy among malacologist prior to, say, 1950 to only ever present the most turbid of prose, to never be explicit when they could possibly be obscure, and to never present explanatory details. For instance, they might state that "Species A is so obviously distinct from species B that I am creating a new genus C" without (a) giving any actual detailed description of these "obvious" differences, or even (b) stating whether it is species A or B that is to belong to this new genus. Take this all-too-typical example from Finlay (1927):

Phasianella huttoni Pilsbry, 1888

As already noted, Thiele includes Prisogaster in his subfamily Phasianellinae (which is better regarded as a family), with two other genera, Phasianella and Tricolia. For the latter, Humphrey's name Eutropia must be used, and the family name would become Eutropiidae, but the Neozelanic species does not belong to the genus Eutropia. Pilsbry has pointed out that the small Australian species have a radula of the Phasianellid style, not of the Tricolia (=Eutropia) form. Consequently one may propose for the Neozelanic species the new generic name, Pellax, associating with it the Australian rosea Angas, virgo, Angas, etc.

And that is all Finlay has to say about that particular genus. He continues in this manner for 159 pages (excluding references), establishing no less than 177 new taxa in the process. In some places, even a single page is enough to inspire thoughts of suicide in the suffering reader.

REFERENCES

Finlay, H. J. (1927). A further commentary on New Zealand molluscan systematics. Transactions and Proceedings of the New Zealand Institute 57: 320-485.

Kowalke, T. 1998. Bewertung protoconchmorphologischer Daten basaler Caenogastropoda (Cerithiimorpha und Littorinimorpha) hinsichtlich ihrer Systematik und Evolution von der Kreide bis rezent. Berliner geowiss. Abh. E, 27: 1–121, 11 Taf., 13 Abb.

Why Animals are not Plants

Orchids, from Ernst Haeckel's Kunstformen der Natur.

Before I start this post, I should note that this is simply a line of speculation I've had running through my head recently. I have no idea how accurate this is, or whether I'm just spouting a load of hooey. Either way, I think the question is an interesting one.

Why are there so many more species of animals than plants? More than 1.2 million species of animals have been described on this planet to date, as opposed to only about 300,000 species of plant. Even if one allows for differing species concepts, as yet undescribed species*, etc. there is no question of measuring error - animal species outnumber plant species more than four to one. Also, while most animal species tend to have clear distinctions from their closest relatives, plant species have a greater tendency to bleed into each other, with less clear boundaries (The same is also true of other non-animal eukaryotes and prokaryotes. Animals are the wierd ones in this regard). A comparison between animal and plant fossil records shows significant differences as well - even when they first arrived on land, plants never underwent an equivalent of the animals' Cambrian explosion. Plant evolution has been a far more sedate affair, with the divergences of the major modern taxa more spread out in time.

*Of which there are probably a higher proportion of animals than plants, anyway. Plants are generally easier to study in terms of biodiversity because they don't usually run away.

These points all suggest that speciation tends to happen differently for animals and plants. Why should this be? At least one major factor, I suspect, is that unlike most plants, most animals engage in active behaviour. Mobility is the key to the animals' evolutionary diversity. In most animals, reproduction happens through more or less direct copulation. As such, animals have a direct choice as regards whose gametes they fertilise or are fertilised by. Plants, in contrast, use more indirect means - wind pollination, or intermediary pollinators. As a result, their control over their fertilisation is more limited. Even in those species pollinated by insects and other animals, pollinator specialisation seems to play little part in speciation, and most pollinators are not hugely discriminating in their visits (Waser, 1998).


A diversity of cichlids at Dallas Aquarium. Photo by Tanya Dewey.

Speciation is the process of isolating gene pools. The differences between controlled fertilisation in animals versus more uncontrolled fertilisation in plants means that speciation in animals tends to be an active process, while that of plants tends to be a passive process. Changes in mate choice can lead to rapid speciation - for instance, they were probably a major factor in the evolution of the cichlid species flocks of the African Great Lakes*. For those organisms in which mate choice is not generally a factor, speciation is more likely to occur as a result of stochastic processes such as genetic drift. Isolating factors between species will develop with less frequency, and those barriers that do develop are likely to be less resilient.

*Lake Victoria is home to over three hundred endemic cichlid species, but sedimentary data indicate that the lake itself has only been there in its present incarnation for the last 12,000 years or so (Johnson et al., 1996). That implies the divergence of, on average, one new cichlid species every forty years.

As a test of this idea, one can look to those plants and animals that provide the exceptions proving the rules. In animals, many marine animals such as corals are broadcast spawners - they release gametes into the water and fertilisation occurs after the gametes leave the parent. Despite their worldwide distribution, corals are not a hugely speciose group of animals, with a little over two thousand known species. Or compare the diversity of the mostly broadcast-spawning bivalves (30,000 species) with the more often directly-fertilising gastropods (40,000 species).

Perhaps the most dramatic support comes from the orchids. While other flowering plants entice their fertilisers with food rewards such as nectar and pollen, orchids have developed more nefarious methods such as pseudocopulation or providing their pollinators with fragrant chemicals that the pollinators can use in their own mating displays. As a result, orchids are one group of plants for which direct mate choice is a significant factor, and speciation in orchids has boomed. Orchids include about 22,000 species, while their sister group, the remainder of the Asparagales, contains only about four thousand species all up.

REFERENCES

Johnson, T. C., C. A. Scholz, M. R. Talbot, K. Kelts, R. D. Ricketts, G. Ngobi, K. Beuning, I. Ssemmanda & J. W. McGill. 1996. Late Pleistocene desiccation of Lake Victoria and rapid evolution of cichlid fishes. Science 273: 1091-1093.

Waser, N. M. 1998. Pollination, angiosperm speciation, and the nature of species boundaries. Oikos 82 (1): 198-201.

More Giant Larvae

Just a brief post - the air-conditioning in our office hasn't been working these past two days, I'm currently sitting in thirty degree-plus heat, and consequently I simply haven't the mental strength to compose anything more. Weather in Perth is evil.

I just thought that I'd show you something that I alluded to briefly nearly a year and a half ago in my post on Planctosphaera. This is the giant phoronid larva described by Temereva et al. (2006 - phoronids are small filter-feeding worms related to brachiopods), as illustrated in a figure from that paper:



For comparison, the animals to its right are more normal phoronid larvae (Actinotrocha is a form genus for such larvae, as it is not generally possible to identify a particular larva with its mature adult form). Phoronids are not the only marine animals for which such giant larvae have been found. If you've read the other post, you may recall that Planctosphaera was such an example. There's also the famed giant leptocephalus larvae, similar to the ten-centimetre (at most) leptocephalus larvae of eels or tarpons but reaching lengths of over six feet. Findings of giant larvae have lead to speculations about the existence of truly gigantic adults (particularly, it hardly needs saying, in the case of the leptocephali), but these adults remain as yet undiscovered. Many researchers suspect that giant larvae are not spawned from giant adults, but instead are pathological larvae of more normal-sized species that have failed to mature in the proper manner.

Even if the majority of giant larvae are merely abortive freaks, they are not without interesting implications for our understanding of evolution. Temereva et al.'s giant phoronid larva differed from other phoronid larvae in more than mere size. It also possessed a more fully developed circulatory system, as well as rudimentary gonads (which normally don't appear in phoronids until maturity). It takes little imagination to see the next step leading to a phoronid larva attaining full maturity while maintaining its larval form. It would not be the first known case - in 1928, Heath described Graffizoon lobatum, an animal very similar to the larva of a polyclad flatworm except for its possession of fully-developed gonads (as a reminder of our lack of familiarity with marine life, Graffizoon does not seem to have been recorded since).

For comparison, this is what adult phoronids look like (Phoronopsis viridis, from UCMP):



How difficult would it be to recognise the relationship between animals potentially only separated by a single generation?

REFERENCES

Heath, H. 1928. A sexually mature turbellarian resembling Müller's larva. Journal of Morphology and Physiology 45 (1): 187-207.

Temereva, E. N., V. V. Malakhov & A. N. Chernyshev. 2006. Giant actinotroch, a larva of Phoronida from the South China Sea: the giant larva phenomenon. Doklady Akademii Nauk 410 (5): 712-715 (transl. Doklady Biological Sciences 410: 410-413).

Linnaeus' Legacy # 15 - The Legacy Goes to the Congo and Gets Eaten

The latest edition of Linnaeus' Legacy is up at Greg Laden's Blog. This month's keywords: Not, a third of it is in Latin, now you get it for free, dahlias, something about the way, littlest sauropodomorph, martini, can of worms, Jocko, finches on mescaline, wench, tricks, oriole, parrots, bucket full of gasoline, extinction.

Next month's edition will be at Seeds Aside.

Kneel before the Shrimp Queen

A pair of commensal snapping shrimps Synalpheus on their host crinoid. Photo from here.

As a child, snapping shrimp were one of my favourite things to find under rocks at the beach. The characteristic bang made by their enlarged pincer snapping shut never fails to fascinate. This 'snap' can often be heard for some distance, and the explosive force generated by it can be strong enough to stun small animals that get too close. Snapping shrimp form the family Alpheidae, and Synalpheus, with well over a hundred described species and counting, is one of the larger genera in that family.

Synapheus has a pantropically-centred distribution. Though it seems to be more abundant in the east Pacific and Atlantic Oceans than in the Indian, I'd be a little suspicious of the role collection bias has played in this. The various species of Synalpheus are retiring animals by nature, and sequester themselves in cryptic habitats, all the better to defend themselves against would-be intruders with that impressive claw. The best-known species of Synalpheus live within the body cavity of other animals such as sponges or corals, and a few species live on the underside of crinoids (VandenSpiegel et al., 1998). It is debatable to what extent the relationship between Synalpheus and their host should be regarded as commensal (with the shrimp feeding on food particles brought in by the host) or parasitic (with the shrimp feeding directly on the host tissue), as evidence exists for Synalpheus individuals doing both. Dardeau (1984) suggested that Synalpheus species could be divided into three broad levels of host association, from group I (generally free-living or opportunistically commensal species with very low or no host specificity) to group III (invariably commensal species with high host specificity). Many commensal-living individuals will do so as male-female pairs, aggressively excluding any conspecific competitors that attempt to settle in their home. Other species may be more tolerant, with numerous individuals in a single host.


Colonial Synalpheus on a sponge. Photo from Biology-Blog - this would appear to be a laboratory colony, with dabs of identifying paint on the individuals.

The most remarkable association of all, though, is found in certain species of what is called the Gambarelloides species group (after the species Synalpheus gambarelloides). The Gambarelloides group is a morphologically distinct association of species (most notably, they have a dense brush of setae on the smaller pincer) that was separated by Ríos & Duffy (2007) from the remainder of Synalpheus as their new genus Zuzalpheus. This separation was debated by Anker & De Grave (2008), but the complaint does not seem to concern the integrity of 'Zuzalpheus' itself, but that of the remainder of Synalpheus if the Gambarelloides group species are not included. Some of the group III sponge-dwelling species in this group (using Dardeau's grouping) form large colonies with hundreds of individuals in a single sponge. It was only recognised as recently as 1996 (Duffy et al., 2000, 2002) that these Synalpheus colonies actually qualify as eusocial, in the manner of bees and ants, representing the only known occurrence of eusociality outside insects other than mole rats. Reproduction within the colony is conducted by a single queen, though it remains unknown how the queen of a colony is established, and how she prevents other members of the colony becoming reproductive. The sexual ratio of the remainder of the colony remains unknown, as males are indistinguishable from non-egg-bearing females (and gender may be environmentally-determined rather than genetic), but the colony does include a number of larger individuals (called "males" by Duffy et al., 2002) that seem to be primarily responsible for the colony's defense, moving about the sponge more than the smaller juveniles seemingly on the lookout for intruders. The queen plays little part in defending the colony, and in one eusocial species, Synalpheus filidigitus, she lacks the large snapping pincer of the other individuals (Duffy et al., 2002). It is not yet established how fertilisation of the queen occurs, but allozyme analysis suggests that there may be only a single reproductive male in the colony (Duffy et al., 2000).

Phylogenetic analysis of the Gambarelloides group by Duffy et al. (2000) found that eusociality has evolved at least three times within the group. They suggested that it may have evolved as a response to severe competition for habitat. Where eusocial shrimps are found, almost all suitable hosts are home to a colony, so unoccupied homes are few and far between (offhand, how new colonies do become established is yet another unknown factor - eusocial Synalpheus lack a planktonic larval stage, so hatching offspring remain in the parent colony). A colonial group may be more effective at defending their host against would-be usurpers than a solitary individual or pair would be. With the large "soldiers" defending her, the queen is able to spend more time feeding and reproducing, safely hidden within the sponge.

REFERENCES

Anker, A., & S. De Grave. 2008. Zuzalpheus Ríos and Duffy, 2007: a junior synonym of Synalpheus Bate, 1888 (Decapoda: Alpheidae). Journal of Crustacean Biology 28 (4): 735-740.

Dardeau, M. R. 1984. Synalpheus shrimps (Crustacea: Decapoda: Alpheidae). I. The Gambarelloides group, with a description of a new species. Memoirs of the Hourglass Cruises 7 (2): 1-125.

Duffy, J. E., C. L. Morrison & K. S. Macdonald. 2002. Colony defense and behavioral differentiation in the eusocial shrimp Synalpheus regalis. Behav. Ecol. Sociobiol. 51: 488-495.

Duffy, J. E., C. L. Morrison & R. Ríos. 2000. Multiple origins of eusociality among sponge-dwelling shrimps (Synalpheus). Evolution 54 (2): 503-516.

Ríos, R., & J. E. Duffy. 2007. A review of the sponge-dwelling snapping shrimp from Carrie Bow Cay, Belize, with description of Zuzalpheus, new genus, and six new species (Crustacea: Decapoda: Alpheidae). Zootaxa 1602: 1-89.

VandenSpiegel, D., I. Eeckhaut & M. Jangoux. 1998. Host selection by Synalpheus stimpsoni (De Man), an ectosymbiotic shrimp of comatulid crinoids, inferred by a field survey and laboratory experiments. Journal of Experimental Marine Biology and Ecology 225 (2): 185-196.