The “Thorny Long-stalked Sea Slater” (Macrostylis spinifera G.O. Sars, 1864) is the type species of the genus Macrostylis and of the family Macrostylidae. It occurs along the coasts of Norway and Iceland, even at relatively shallow depths of 200 m and deeper. This picture was taken with a confocal laser scanning microscope and shows an adult female carrying eggs in her marsupium. Scale bar = 500 µm.

Biogeography, systematics and evolution of Macrostylidae (Crustacea: Isopoda)

Did you know that isopod crustaceans, which also include woodlice, rolly pollies and sea slaters, are one of the most remarkable and striking groups of animals in the deep sea?

Isopods are highly diverse and very common in the deep but the deep-sea isopods are only distantly related to those ones that you may find in the woods or even in your backyard. Their last common ancestor likely crawled on Earth much more than 250 million years ago (Lins et al. 2012) — that is long before the dinosaurs even first appeared. One of the isopod groups that are common in deep-sea sediments is the family Macrostylidae that is rather intensively studied at the Senckenberg Institute Frankfurt.

Macrostylids are small (~ 1 mm – 1 cm), enigmatic isopod crustaceans about which only little is known. Feeding preferences, mating strategy, longevity, lifecycle and ecological interactions are all still largely unstudied. Currently, around 90 species are known and described worldwide but many more await description.

Macrostylidae (Crustacea: Isopoda) have a worldwide distribution in the cold water masses of the deep sea. In cold-water Boreal and Austral regions species have been also recorded in the upper bathyal and even at shelf depths, for instance in Norwegian Fjords or on the Antarctic continental shelf (Riehl and Kaiser 2012; Riehl and Brandt 2013). Macrostylids frequently occur in trenches and Macrostylis mariana Mezhov, 1993 from the deep Mariana Trench is one of the deepest ever recorded isopod species. Dr. Torben Riehl works on this group continuously since 2008 which makes him currently the world’s leading expert for Macrostylidae. He has described (or was involved in describing) multiple species of the genus Macrostylis G. O. Sars, 1864 as well as of the sister group of Macrostylidae, the family Urstylidae.

This long-term project includes, besides taxonomic work (descriptions, revisions), anatomical, phylogenetic, biogeographic, population-genetic and ethological aspects. Current studies on Macrostylidae comprise species descriptions from the Clarion-Clipperton Fracture Zone in the Pacific, from the Australian slope, and the deep Atlantic. At the first glance all of these look more or less alike. Despite, or probably even because they are so enigmatic, they are fascinating to study.

Why study Macrostylidae?

  • Because of their commonness in deep-sea sediments, one of the most widely distributed habitat types in the world, many general ideas and theories about deep-sea biodiversity, ecology and evolution can be studied using Macrostylidae as a model taxon.
  • Because of their locomotory abilities, habitat preferences, and brooding mode of reproduction, macrostylids can be considered relatively poor dispersers. This helps uncovering patterns of colonization, connectivity, and biogeography.
  • Macrostylids are such a common and diverse element of abyssal communities that a vital role of this group in benthic communities and for abyssal ecosystem functioning can be assumed. In this regard, every new aspect discovered about their ecology, evolution or general natural history means getting closer to understanding this ecosystem.
  • Macrostylidae occur in all oceans and almost across the entire available depth of the sea. This allows inference of depth-colonization patterns.


What do we know about Macrostylidae reproduction? 

Caring for offspring requires energy, so offspring numbers have to be small. 

Like all peracarid crustaceans, macrostylids are brooders. Ovigerous females (females bearing eggs) have a brood sack called marsupium on the ventral side of their body which is formed of leave-shaped and overlapping extensions of the female’s front legs. They carry the offspring beyond the moment of hatching. The larval development, which is typical for crustaceans, is shortened and happening while still within the eggs. After hatching, the post-embryonic macrostylids remain within the marsupium until they are fully functional and almost look like smaller copies of their mother. This life phase is called manca stage. The released babies still lack a seventh pair of pereopods and the seventh pereonite is underdeveloped or still absent. Over a few molting cycles these body parts successively develop. The advantage of brooding is that as the released offspring is larger and further developed than typically released crustacean larvae, the chances of survival and successful reproductions for each individual are increased. The disadvantage is that the number of offspring is limited to very few (maybe 5-25, depending on species) by the size of the marsupium. This mode of reproduction may be particularly advantageous in the abyss because it is generally food-limited. Especially the water column, the habitat of many free-swimming crustacean larvae, is oligotrophic (poor in nutrients) and the little food available accumulates at the seafloor, where the macrostylid mancae thrive.

Evolutionary history

Dated phylogenetic inference suggests Macrostylidae belong to the oldest isopod lineages in the deep sea with a date of origin around the Permo-Triassic boundary 250 million years ago (Raupach et al. 2009; Lins et al. 2012).

Because of a rather high degree of similarity amongst each other but at the same time distinct differences to potentially related isopod families, the evolutionary history of Macrostylidae is still unresolved. As potential relatives the families Desmosomatidae (e.g., Wägele 1989) and the Urstylidae (Riehl et al. 2014) have been discussed. Also within the family relationships are cryptic which is reflected in a monogeneric (having only one recognized genus) status of the family: Macrostylis Sars, 1864.

Macrostylid behavior 

Macrostylids are shy, it seems!  

Direct behavioral observations of macrostylids are rare. The single behavioral report published (Hessler and Strömberg 1989) described their behavior rather briefly. Specimens put into an aquarium could be observed sinking to the sediment where they immediately dug themselves in and never returned to the surface again. This single observation in conjunction with sampling evidence suggests that macrostylids are sediment dwellers and burrowers. This behavior suggests macrostylids do not tend to swim regularly or actively.

This seems to agree with the interpretation of their morphological adaptations (see above). All other information we have about macrostylid behavior are circumstantial and indirect clues. Sampling evidence also supports that macrostylids live within the sediments because in corer samples that dug up relatively undisturbed deep-sea sediment surfaces, macrostylids have been found deep within rather than on top of the sediment – hence we can conclude they are part of the endobenthos (the organismal community living primarily below the sediment surface).

How do macrostylids look like? 

The macrostylid body plan is all about digging! 

The habitus of macrostylids is elongate and narrow, almost worm-like. Similar to some tunnel-digging insects, such as termites, their head is prognathous (biting parts oriented forward). Their head is wedge-shaped and together with the elongate, cylindrical, and compact body they distantly resemble a jackhammer chisel, indicating a face-forward “head into the sediment” digging locomotion. A compact and strong anterior tagma (functional body unit) contributes to this impression. The three anterior pereonites (body segments) are inflexibly articulated providing very strong muscular attachments. The first three pairs of pereopods (walking legs) attached to this tagma are particularly strongly armored with setae on their proximal and middle articles and while the pereopod pairs one and two are facing anteriorly the third pair can be moved to the side and back as if to be used for shoveling sediment from side and back of the body or crawling through under-water tunnels. Other morphological peculiarities, apomorphies (derived traits or evolutionary novelties distinct to a certain species) distinguishing macrostylids from other (potentially) related isopods, point in the same direction (Riehl et al. 2014).

The other four pairs of pereopods are positioned closely to the body underside and are differently armored with setae. Their long and stick-like articles feature “crowns” of setae emerging from all around the articulations at their distal ends. These may function sort of like a ski-pole basket (the little disc near the tip of a ski pole) preventing the legs from sinking into soft ooze at the seafloor when pushing the body forward. If you are interested in reading more about macrostylid morphology, check out these papers published by our group (Riehl 2014; Riehl et al. 2014; Bober et al. 2018a).

Macrostylid sexual dimorphism 

Macrostylid females and males may differ so much morphologically that these appear as completely different species 

Most macrostylids look rather similar, at least at the first glance (Riehl and Brandt 2010). Interestingly, however, in some macrostylid species the males look rather extreme — diverging in various aspects from the general macrostylid Bauplan. The phenomenon that males and females of the same species have different appearances is called sexual dimorphism. It is rather common in the animal kingdom. However, in some macrostylids this dimorphisms is rather extreme — in these cases, conspecific males and females may differ more from each other than two species (Riehl et al. 2012; Bober et al. 2018c).

Such differences in morphology must also be linked to behavioral differences. We could show that in some species the male body is more slender and generally reduced in overall size. Their hind legs are strongly increased in length and the first pair of antennae (the antennulae) are increased in relative size and carry and increased numbers of aesthetascs, which are chemo-sensory setae used for smelling. This suggests that once they became adult the males change from an endobenthic mode of life to a more epibenthic lifestyle (from digging within to crawling on top of the sediment). What could be the advantage and evolutionary driving force behind this? While the typical females remain locally restricted in their range and passively ‘wait’ for mating partners, males may be the roaming and searching party. With such a behavior populations may be connected across larger distances and males would be independent from local female abundances (Bober et al. 2018c).

In a most fascinating observation the behavioral differences between conspecific males and females could be indirectly confirmed by sex-specific infestation patterns with protist epibionts (single-celled eukaryotic organism that is not an animal, plant, or fungus and which lives on the body surface of a host) which only occurred on adult males. Adult females and juveniles of both sexes did not show infestation indicating their endobenthic lifestyle is disadvantageous for the epibionts (Kniesz et al. 2018).

Biogeographic patterns 

Macrostylids are everywhere in deep-sea sediments! 

Most macrostylids are known from single of few records. But the impression that they are locally restricted or even endemic may actually be rather a result of a bias from very limited sampling rather than a real pattern. Interestingly, despite their poor dispersability macrostylids may have distributions spanning hundreds of kilometers, in some cases even 2000 km (Bober et al. 2018c, b; Riehl et al. 2018; Riehl and Kühn 2020). How they achieve such wide distributions is currently unknown. However, as geographic structuring of genetic diversity shows, long-distance dispersal only infrequently occurs in these isopods (Riehl et al. 2018; Riehl and De Smet 2020).

Further reading 

Bober S, Riehl T, Brandt A (2018a) An organ of equilibrium in deep-sea isopods revealed: the statocyst of Macrostylidae (Crustacea, Peracarida, Janiroidea). Zoomorphology 137:71–82. doi: 10.1007/s00435-017-0376-5

Bober S, Brix S, Riehl T, Schwentner M, Brandt A (2018b) Does the Mid-Atlantic Ridge affect the distribution of abyssal benthic crustaceans across the Atlantic Ocean? Deep-Sea Res Part II Top Stud Oceanogr 148:91–104. doi:

Bober S, Riehl T, Henne S, Brandt A (2018c) New Macrostylidae (Isopoda) from the Northwest Pacific Basin described by means of integrative taxonomy with reference to geographical barriers in the abyss. Zool J Linn Soc 182:549–603. doi: 10.1093/zoolinnean/zlx042

Hessler RR, Strömberg JO (1989) Behavior of janiroidean isopods (Asellota), with special reference to deep sea genera. Sarsia 74:145–159.

Kniesz K, Brandt A, Riehl T (2018) Peritrich ciliate epibionts on the new hadal isopod species Macrostylis marionae from the Puerto Rico Trench as an indicator for sex-specific behaviour. Deep-Sea Res Part II Top Stud Oceanogr 148:105–129. doi: 10.1016/j.dsr2.2017.10.007

Lins LSF, Ho SYW, Wilson GDF, Lo N (2012) Evidence for Permo-Triassic colonization of the deep sea by isopods. Biol Lett 8:979–982. doi: 10.1098/rsbl.2012.0774

Raupach MJ, Mayer C, Malyutina MV, Wägele J-W (2009) Multiple origins of deep-sea Asellota (Crustacea: Isopoda) from shallow waters revealed by molecular data. Proc R Soc B Biol Sci 276:799–808. doi: 10.1098/rspb.2008.1063

Riehl T (2014) A phylogenetic approach to the classification of macrostylid isopods and faunal linkages between the deep sea and shallow-water environments. Dissertation, University of Hamburg

Riehl T, Brandt A (2010) Descriptions of two new species in the genus Macrostylis Sars, 1864 (Isopoda, Asellota, Macrostylidae) from the Weddell Sea (Southern Ocean), with a synonymisation of the genus Desmostylis Brandt, 1992 with Macrostylis. Zookeys 57:9–49. doi: 10.3897/zookeys.57.310

Riehl T, Brandt A (2013) Southern Ocean Macrostylidae reviewed with a key to the species and new descriptions from Maud Rise. Zootaxa 3692:160–203. doi:

Riehl T, De Smet B (2020) Macrostylis metallicola spec. nov. — An isopod with geographically clustered genetic variability from a polymetallic-nodule area in the Clarion-Clipperton Fracture Zone. PeerJ 8:1–44. doi: 10.7717/peerj.8621

Riehl T, Kaiser S (2012) Conquered from the deep sea? A new deep-sea isopod species from the Antarctic shelf shows pattern of recent colonization. PLoS ONE 7:e49354. doi: 10.1371/journal.pone.0049354

Riehl T, Kühn MAL (2020) Uniting what belongs together — reevaluation of the isopod species Macrostylis grandis and M. ovata using ontogenetic, morphological and genetic evidence. Prog Oceanogr 181:102238. doi: 10.1016/j.pocean.2019.102238

Riehl T, Wilson GDF, Hessler RR (2012) New Macrostylidae Hansen, 1916 (Crustacea: Isopoda) from the Gay Head-Bermuda transect with special consideration of sexual dimorphism. Zootaxa 3277:1–26. doi: 10.11646/zootaxa.3277.1.1

Riehl T, Wilson GDF, Malyutina MV (2014) Urstylidae – A new family of deep-sea isopods and its phylogenetic implications. Zool J Linn Soc 170:245–296. doi: 10.1111/zoj.12104

Riehl T, Lins L, Brandt A (2018) The effects of depth, distance, and the Mid-Atlantic Ridge on genetic differentiation of abyssal and hadal isopods (Macrostylidae). Deep Sea Res Part II Top Stud Oceanogr 148:74–90. doi: 10.1016/j.dsr2.2017.10.005

Wägele J-W (1989) Evolution und phylogenetisches System der Isopoda: Stand der Forschung und neue Erkenntnisse [Evolution and phylogeny of isopods. New data and the state of affairs]. E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart