Classification of Benthic Fauna – Muck Special

Within the realms of biology lies an underlying skeleton that allows the very heart of the science to thrive, this being the concept of classification. The organisation and categorisation of different organisms can be based on type, size, and any number of variables and characteristics, each with its own crucial importance to research to the most detailed level, and to the largest biological questions of all. This skeleton is comprised of many parts, and at its very backbone, lies the concept of taxonomy, used as the most common form of classification. Taxonomy is the classification system by which organisms are divided into phyla, families, species, etc, and rely strongly on the variation in the internal anatomy of organisms and in modern analysis, differences in the genetic makeup between organisms, i.e, DNA. You can learn more about a taxonomic approach to classification in our introduction to key invertebrate species of Koh Tao and the Gulf, in our learning resources section here.

However, classification can come in many forms. A number of classification systems, based on behaviour, habitat use and other characteristics, are used in both terrestrial and marine biology. Here we explore some broad categories used regularly in science to describe the wide variety of organisms, focusing on those we class as ‘benthic’, i.e living on the sea floor. This is as opposed to planktonic (plankton, most jellyfish, salps etc), among a wide variety of other ‘categories’ (even classification needs classification to classify its categories…). And what better benthic ecosystem to explore than our very own Muck habitat (which you can read more about here).

So here is a brief intro into some commonly used terms to describe benthic fauna (animals of the sea floor) that sometimes are overlooked, terms that create barriers between those looking to learn more, and those looking to sound smarter (scientists). We apply, where possible, organisms from our own muck explorations as part of our research as examples to illustrate some of these creatures.



1)      Infaunal –

Firstly we cover those organisms that spend most of their lives beneath the sea floor, among the silt and the sand. They are buried and rarely come to the ‘surface’ of their deep homes. A wide variety of organisms are classed as infaunal, such as those seen here, the Golden Mantis Shrimp (Lysiosquilloides mapia), the bautiful nudibranch Cerberilla ambonensis, and the rarely observed moon snail, Naticarius onca, that all burrow beneath the sand. Organisms that live ‘below’ the substrate layer, in a completely hidden part of an already hidden world, are also sometimes referred to as Endo-benthic organisms. Though hidden, this in no way means they are always cryptic or camouflaged, in many cases they are quite the opposite, as seen here. These three infaunal organisms of the muck, exhibit very different strategies and behaviour but share a common habitat.

2)      Epifaunal –

These are organisms that live on the surface of the substrate, never intentionally burying themselves into the sand itself, and are known as epifaunal organisms. They tend to rely less on nocturnal behaviours, though this is certainly not a rule, and will therefore likewise feed on other epifaunal organisms. Though the large majority of sea slugs are epifaunal, several have evolved to be largely infaunal. Many solitary corals such as mushroom corals are an ideal example of non-sessile epifaunal organisms, in the muck of Koh Tao, these are more typically in the form of unrelated solitary heteropsammia corals. However, a number of azooxanthellate (non photosynthetic) solitary hard corals can also be found in the muck of Koh Tao, that are infaunal, leaving only their mouth and tentacles exposed above the sand. Here we have examples of some common epifaunal critters to be found in the muck, including the Pseudobiceros bedfordi flatworm, one of the largest invertebtrates on the island the Broadclub Cuttlefish (Sepia latimanus) and one of our most recent first time discoveries for the island, the Elysia tomentosa sacoglossan sea slug. You can learn more about our sea slug discoveries here and more about sea slugs themselves and their ecology here.



1)      Meiofaunal –

A number of benthic species can broadly be defined by their size opposed to simply surface or sub-surface dwelling. Meiofauna are organisms that have less strictly defined limits but are broadly considered to be smaller than 1mm in max size and larger than 0.05mm. These are organisms that are just barely visible to the naked eye and typically require serious magnification to observe thoroughly. Meiofauna can be either epifaunal or infaunal depending on species etc. Though we typically do not come across much meiofauna in our exploits in the muck, they are arguably the most diverse and abundant organisms we can see in the muck, if we looked hard enough. Additionally juvenile stages and eggs of a number of species may count as meiofaunal aggregations, such as egg masses of a variety of organisms, though these aren’t true examples of the classification.

2)      Macrofaunal –

Perhaps the most popular and well known benthic organisms fit within what we call ‘macrofaunal’ definitions. These are organisms that live on the sea floor that are larger than 1mm (or to some academics, 0.5mm) in max length and typically include most commonly observed benthic organisms. As much of our research here at the program focusses on the the macrofaunal communities in the muck, most of the diversity seen in our transects are either infaunal or epifaunal examples of macrofauna. The Helmet Conch Cassis cornuta is certainly a macrofaunal organism reaching expansive sizes of up to and over 30cm (or in macrofaunal definitions 300mm) in length. However, what few people recognise is than an organism even as big as this, often spends its time beneath the benthos, making it an infaunal, macrofaunal organism

2.5) Muck Vertebrates

The vast majority of vertebrate species (those classed in the taxonomic phylum Chordata) would be classified as Macrofauna and even rarely Megafauna. Benthic vertebrates also commonly referred to as Demersal animals, as they are organisms that live in the ‘Demersal zone‘ of the water collumn. The Demersal zone is the region of the water collumn closest to, and entirely influenced by, the dynamics of the sea floor (benthos). As with all such classificiations discussed here, the limits to the definition are broad and variable. Where in our own muck habitat some might suggest the demersal region to be the bottom meter (ish..) of the water collumn, regions of the deep ocean can have signficiantly larger demersal zones, as the slow moving dynamics and shifts of the deep sea floor are on an entirely different scale to the vibrant imitation we see in the muck. Regardless, within the Benthic vertebrates of the Demersal zone are animals that are either typically classed as True Benthic, or Bentho-Pelagic organisms.

True benthic organisms are those that live partially, or rarely completely, buried beneath the sediment. This allows them to at least partially be considered ‘infaunal’ but not really, however, these macrofaunal fish (as they almost entirely are fish) typically utilise this trait more often as a predatory tactic than solely one of camouflage or hiding. Some examples of true benthic vertebrates of the muck are the  burrowing snake eels, the kuhls ray (Neotrygon kuhlii), and Devil’s Stinger Stonefish (Inimicus didactylus) which is one of the most venemous and brilliant ambush predators in the region, burying itself almost entirely awaiting prey to come close enough for a strike.

Bentho-pelagic vertebrates are what might be considered as the ‘epipelagic’ equivalent of vertebrates and include those fish that live on the surface of the sea floor, but have the capability to swim into the water column, often for extended periods of time, before succumbing to the forces of gravity and returning to the sea floor. The key difference here is that these organisms are rarely observed beneath the sea-floor, yet spend the majority of their lives on top of it. Syngnathids such as pipefish and seahorses (which you can learn more about here) are ideal examples of such fauna, but also include organisms such as the rounded porcupinefish and the brilliant lionfish seen below.


3)      Megafauna –

The last and largest of the size classes defined within the benthic community are the megafauna. This term in particular has multiple roots and multiple meanings and therefore a wide variety of limitations. For example, in the deep sea, a number of researchers class organisms larger than 1cm to be megafauna. Historic uses in palaeontology defined megafauna as those larger than 2 meters, and some placing a minimum limit to much larger, based on weight. Modern biologists, on the rare ocasssion that they use the term, define it by those organisms that are more massive than 45 kilograms. How this arbitrary figure came to be, is another enquiry for another mind-numbing exploration at a later date. In general however, benthic megafauna in the realms of the muck of Koh Tao are not particularly present or diverse, as megafauna tend to be more pelagic such as marine mammals and large sharks. The only organism (thus far) that could broadly be recognised in the muck of Koh Tao as megafaunal, is the Jenkins Whipray (Himantura jenkinsiI). This species grows up to 1.5m in diameter and twice that in length including the tail, however those seen around the island are most commonly observed approximately two thirds that size. The ray, though a frequent dweller of the reef edge, is also a common occupant of muck habitats and lives exclusively on soft sediment habitats.

Hopefully exploration of some of these terms and classifications will allow more people to indulge in the sometimes unnecessarily complex world of scientific literature but may even encourage use of these terms yourself. After all, ‘one small step from here, is one giant leap in a muck dive’….(sorry…).

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