16B. Organism otherwise, go to 17.

17A. Organism basically clam, bean, or seed shaped. Body covered by a pair of shells in a manner similar to clams. Very small, less than 1/25 in (1mm) long. Legs only briefly visible when moving, often capable of rapid locomotion. Ostracodes (also called - incorrectly - ostracods). Phylum Arthropoda, Subphylum Crustacea, Class Ostracoda. These live on and in the sediments and eat detritus and algae; they are small, generally less than ½ mm (1/50th in) long. Good fish food, favourite food of Mandarin dragonets.

17B. Organism otherwise go to 18.

18A. Organism flattened from top to bottom. Scurry rapidly over the bottom or swim with great vigour. Often quite small, but some get very large. Legs are all basically similar. Isopods. Phylum Arthropoda, Subphylum Crustacea, Class Malacostraca, Order Isopoda. Isopods are common in all marine ecosystems, they occupy all ecological niches; the most common ones in reef system sediments are small (less than 2 mm (1/10th in.) long and are probably detritus feeding animals.  Some others are found moving along the sand surface and are detritivores.  Still others, with large and evident eyes and a "torpedo-shape" when viewed from above, are Cirolanids and are fish parasites and predators.  These animals are dangerous to fish and will even bite aquarists.

18B. Organism flattened from side-to-side; body often curled on itself to form a "C" shape. Legs of a couple of different types. Amphipods. Phylum Arthropoda, Subphylum Crustacea, Class Malacostraca, Order Amphipoda. Amphipods are common in all marine ecosystems, they occupy all ecological niches; the most common ones in reef system sediments typically are small (less than 6 mm (1/4th in.) long and are probably herbivores. They are good fish food.

Diagram of a typical amphipod of the kind found in aquaria.

19A. Organism is a worm, may or may not be segmented or have tufts of bristles. go to 20.

19B. Organism otherwise, go to 25.

20A. Organism's body divided into a series of rings or segments, generally (not always) tufts of bristles visible on each side of each segment - Annelid worms.

20B. Organism otherwise, go to 21.

21A. Organism body bulbous with a long "Y" shaped, not retractable appendage - Echiuran worms. Phylum Echiura.  Bonellid echiurans are relatively commonly found in aquaria. The animal lives in a burrow and the long proboscis extends out of the burrow collecting particulate food which is brought back to the mouth at the junction of the long proboscis and the body.

21B. Organism otherwise, go to 22.

22A. Organism body an elongate "tear-drop" shape, has a long retractable proboscis at one end, often has a small point on the opposite end - sipunculan worm. Phylum Sipuncula. Sipunculans may be found in either sediments or in rocks. If the latter, they can create their own burrow by abrading the rock. They are typically considered to be bottom "daubers" using their tentacles to collect some sediments to feed upon.

This illustration of a sipunculan, contributed by Jeanne Smith, shows many of the features characteristic of the group.
Note there are no segments.

22B. Organism otherwise, go to 23.

23A. Organism body round, tapered to a point at both ends, moves by coiling or flexing into a circle or whipping back and forth. No visible appendages - round worms. Phylum Nematoda. Roundworms are often parasitic, but many free-living species are found living in sediments. They may be detritus feeders, herbivores or carnivores. Common in aquarium sediments, and part of the normal aquarium benthos, probably most are scavengers.

23B. Organism otherwise, go to 24.

24A. Organism body flat, very thin, moves by "gliding" or muscular undulation. - flatworm. Phylum Platyhelminthes, Class Turbellaria. Numerous species are found in aquaria, where some are pests (such as the red planarian). Most are beneficial, living in the sediments eating diatoms or detritus. A few are also predatory on soft corals and hydroids.

24B. Organism body, rounded not flat, very "stretchy", moves by gliding and muscular movement. - ribbon worm. Phylum Nemertea. Ribbon worms are rarely reported from reef aquaria. Most are predatory, but a few parasitize crabs and clams. They may be huge (I have seen some well over 20 m (66 ft) long). Those reported from aquaria are generally less than about 25 mm (1 in) long and probably eat amphipods and other crustaceans or small worms.

25A. Organism a polyp (consisting of a cylindrical body with a fringe of tentacles around the mouth). go to 26.

25B. Organism not a polyp. Go to 27.

26A. Organism without hard internal structures - sea anemone

26B. Organism with hard internal structures - coral

27A. Organism with visible hard shells (these may be very cryptic and hard to see)- go to 31.

27B. Organism without visible hard shell (may have internal shell) - go to 28.

28A. Organism with smooth rounded top, grey, brown or black, two tentacles at front end - bubble snail. Phylum Mollusca, Class Gastropoda, Subclass Opisthobranchia, Order Cephalaspidea. The bubble shells found in aquaria look like flatworms with a lump in the middle. They are typically herbivorous and eat algae.

Diagram of a bubble shell showing some of the distinguishing features.

28B. Organism otherwise, go to 29.

29A. Organism with frills on the back  - aeolid nudibranchs if the frills arranged in rows, but this may be difficult to see;  Phylum Mollusca, Class Gastropoda, Subclass Opisthobranchia, Order Nudibranchia, Sub-order Aeolidacea. -dendronotid nudibranchs if the frills are on the edges. Phylum Mollusca, Class Gastropoda, Subclass Opisthobranchia, Order Nudibranchia, Sub-order Dendronotacea.

Both of these types are generally predatory on cnidarians. Most found in aquaria come in on live rock and eat either corals, soft corals, hydroids or sea anemones.

Left: Diagram of an aeolid nudibranch showing many of the diagnostic features; Right: Diagram of a dendronotacean.

29B. Organism otherwise, go to 30.

30A. Organism with a plume of feather-duster-like gills in the middle of the hind part of the top of the animal, two tentacles near the front - dorid nudibranchs. Phylum Mollusca, Class Gastropoda, Subclass Opisthobranchia, Order Nudibranchia, Sub-order Doridacea. Dorids are generally predatory on sponges or bryozoans. Most found in aquaria come in on live rock.

Diagram of a dorid nudibranch showing the diagnostic features.

30B. Organism with two large tentacles near the front; has a bulbous body with a large flap on each side - sea hares. Phylum Mollusca, Class Gastropoda, Subclass Opisthobranchia, Order Anaspidea. Sea hares are herbivorous. Most found in aquaria come in on live rock and eat either red or green algae.

31A. Organism with a single coiled shell - go to 33.

31B. Organism with more than one shell - go to 32.

32A. Organism with two shells; shells basically similar, symmetrical or mirror-image shells, animal may be free living or held on to substrate by a fibrous material (byssus), animal may or may not have tubes bringing water into it - clams - bivalves. Phylum Mollusca, Class Bivalvia. Other than Tridacna, most bivalves found in aquaria are incidental inhabitants that come in with either live sand or on live rock. Most are suspension-feeders which eat phytoplankton or suspended particulate organic material.

32B. Organisms with 8 plate-like shells on their upper surface - chitons. Phylum Mollusca, Class Polyplacophora. Generally found on hard substrata, these animals may occasionally be found in sediments, particularly if there are some coarse sediment particles. They eat diatoms and other algae off the rocks.

33A. Shell flat, without holes - Stomatella varia. Phylum Mollusca, Class Gastropoda, Subclass Prosobranchia, Family Trochidae. Trochids such as Stomatella or Astraea, graze on micro algae such as diatoms.

33B. Shell otherwise - go to 34.

34A. Shell top or turban shaped, aperture basically round; trochid snails, Phylum Mollusca, Class Gastropoda, Subclass Prosobranchia, Family Trochidae; and turban snails, Family Turbinidae. Trochids such as Tectus, or Trochus, and turbinids such as Turbo and Astraea graze on microalgae such as diatoms.

Shell features of these snails are shown.  Trochids have a proteinaceous operculum,
Turbinids have a calcareous operculum

34B. Shell otherwise - go to 35.

35A. Shell globose, close to spherical, aperture "D" shaped, with flanges or folds on inside (body-side) edge - nerites. Phylum Mollusca, Class Gastropoda, Subclass Prosobranchia, Family Neritidae. Nerites graze on micro algae such as diatoms.

Diagrams of a nerite and a moon snail (from #36A below) showing shell characters.

36A. Shell globose to slightly flattened, aperture without visible flanges or folds on the body side, foot completely covers the body when extended - moon snails, Phylum Mollusca, Class Gastropoda, Subclass Prosobranchia, Family Naticidae. Naticids are wholly predatory eating only other mollusks, commonly bivalves, but also other gastropods.

36B. Shell otherwise - go to 37.

37A. Shell elongate, (long and narrow); aperture rounded, without spout-like projection of the shell - turritellids. Phylum Mollusca, Class Gastropoda, Subclass Prosobranchia, Family Turritellidae. Turritellids plough through and ingest sediments eating the bacteria and detritus in the sediments.

Diagrams of a Turritellid (left) and a Cerith (right - #38A below) showing shell characters

37B. Shell elongate (may be "stubby" to very long and narrow), aperture not round or if round possessing a spout - go to 38.

38A. Shell elongate, aperture rounded with a short spout - cerith. Phylum Mollusca, Class Gastropoda, Subclass Prosobranchia, Family Cerithidae. Ceriths may plough through and ingest sediments eating the bacteria and detritus in the sediments; others may graze on algae.

38B. Shell otherwise, go to 39.

39A. Shell with pronounced spout (at front end of the shell as the animal is moving) - go to 40.

39B. Shell without pronounced spout - go to 41.

40A. Shell with a noticeable groove found around the bottom of the body, formed by the growth of the small curved spout. - Mud Snails, Nassarius. Phylum Mollusca, Class Gastropoda, Subclass Prosobranchia, Family Nassaridae. Nassarius eat carrion and buries in the upper centimetre of the sediments. Good scavengers.

Diagram of a Nassarius (left) and a more standard whelk (right)

40B. Shell without a noticeable groove found around the bottom of the body. - whelks. Phylum Mollusca, Class Gastropoda, Subclass Prosobranchia, Superfamily Stenoglossa (mostly). Whelks are generally predatory and are not good reef animals - not that Nassarius, a whelk, is an exception.

41A. Shell polished, oval, flattened on bottom, aperture a slit on bottom side running the complete length of the shell - cowrie; Phylum Mollusca, Class Gastropoda, Subclass Prosobranchia, Family Cypraeidae. There are over 500 species of cowries. Generalizations about their aquarium suitability and diet are impossible.

41B. Shell may or may not be polished, conical with basically a narrow elongate aperture, spout small - go to 42.

42A. Shell aperture with folds on body side, shell polished; animal burrows in sediments, rapidly moving. - olive or marginellids. Phylum Mollusca, Class Gastropoda, Subclass Prosobranchia, Families Oividae and Marginellidae. Both types burrow through sediments and are predators of sand-dwelling animals.

42B. Shell aperture without folds, shell not highly polished; may burrow in sediments, but slow moving - cone shell. Phylum Mollusca, Class Gastropoda, Subclass Prosobranchia, Family Conidae. All are venomous, injecting their prey with a toxin. Those that eat fish, around 10-20 species, are dangerous to humans and are responsible for numerous fatalities.

Diagrams of two cone snails.  The one on the left with a narrow aperture is not likely to be dangerous to humans.
The one on the right with a flaring aperture may be dangerous. When in doubt remove the animal with tongs and dispose of it.

Annelid Worm Key.

Annelids are typically "THE" dominant and diverse group in marine sediments.

These worms are in the Phylum Annelida which has 3-5 classes (depending on which taxonomic expert you believe). Representatives of all classes may be found in marine aquarium systems. The major characteristic of all annelid worms is that the body is divided into a series of segments or compartments by transverse or cross-wise septa. This division is manifested on the outside of the animal, by numerous grooves surrounding the animal. No other worm group has these septal grooves.

The three basic types of marine worms are those related to terrestrial earthworms (called "Oligochaetes"), "bristle worms" (called "Polychaetes,") and the leeches (called "Hirudineans"). Marine leeches and marine oligochaetes are occasionally found in reef aquaria. The former come in on fishes, the latter with live sand. Both are rare in aquaria, and I won't discuss either further.

The polychaetes (bristle worms) are the dominant live sediment organisms found in our systems - and in the real world. Biologists assign the various types of polychaetes to taxonomic groups called families. Generally the polychaete families are considered to be reasonably discrete; there are very few that are obviously related to one another. These families are the basis for the discussion of the bristle worms.

There are between 75 and 100 polychaete families, and I have found representatives of 12 of these in my aquaria. Undoubtedly others are found. However, the key to the families given below just works for these few that I have seen. If you find a worm, and attempt to key it out, and it doesn't fit, it probably means you have a species from a family that I haven't included in the key. I would like to know of such non-keyable worms; please contact me at: rshimek@imt.net if you find an oddball.

More and Different Worms Are Found Living On and In the Rock; 

This Key Is Not Applicable to Them

1A. Body segmented but relatively smooth, no bristles, suckers at both ends; animal muscular and capable of swimming. - Leeches, Class Hirudinea.

1B. Body with bristles (sometimes these may be very hard to see); appendages are often visible along the sides of the body, no suckers - go to 2.

2A. Animal with evident (visible without magnification) white tufts of bristles, along each side of the body - go to 3.

2B. Animal may have bristles, but these are difficult to see and will need magnification to be noticeable - go to 4.

3A. Animal pink, grey, or orange; gills visible as small pink or reddish tufts found above the bristles near the front of the worm; animals live in sediments or rocks and emerge to scavenge food after feeding or at night; may reach lengths of 45 cm (18 in). Eurythoe or Pareurythoe (scavenging fireworms). Family Amphinomidae; very common in reef aquaria; good scavengers. Harmless to most aquarium animals, but bristles cause irritation if touched.

Eurythoe complanata, a beneficial fireworm common in reef aquaria
 

3B. Animal red, or orange; gills prominent as large bright reddish tufts found above the bristles near the front of the worm; animals live in sediments or rocks and emerge to scavenge food after feeding or at night; will also eat gorgonians, possibly other soft corals; may reach lengths of 45 cm (18 in). Hermodice carunculata, predatory/scavenging fireworms). Family Amphinomidae; rare in reef aquaria. Harmless to most aquarium animals, but bristles cause irritation if touched.

4A. Animal found in a more-or-less permanent tube or burrow - go to 5.

4B. Animal found free-living in sediments - go to 9.

5A. Tentacles or string-like structures extend from the tube; worm body never visible - go to 6.

5B. Tentacles or string-like structures not typically extended from the tube. Worm may emerge from tube (partially or wholly), but retreats rapidly into tube if bothered - go to 8.

6A. Two tentacles extend from tube; these are pale gray or white, thin and wave in the current or move over the substrate - Chaetopterid or Spionid polychaetes. Families Chaetopteridae, and Spionidae.

6B. More than two tentacles - go to 7.

7A. Tentacles feather-like and formed into a plume looking like a feather duster which extends from the tube to project up into the water - Feather-duster worms, Family Sabellidae.

7B. Tentacles are string-like and lay on the substrate looking like very thin spaghetti - Spaghetti worms, Families Terebellidae or (rarely) Trichobranchidae.

8A. Head of animal with 3-5 tentacles extending from each side of the head; no tentacle or process arising from the center of the head, 2 pairs of eyes visible on the head; lateral appendages (parapodia) prominent, about ½ as wide as the body; color typically green, black, blue; worms long (may exceed 30 cm (1 ft) but typically narrow (width seldom is greater than 5 mm (1/4 th inch); worms possess jaws that look like pinchers, most use these jaws to manipulate algae and are herbivorous - Nereid worms; Family Nereidae.

8B. Head of animal 2 or fewer tentacles extending from each side of the head; prominent tentacle or process arising from the center of the head, 0-2 pairs of eyes visible on the head; lateral appendages (parapodia) evident, about 1/3 as wide as the body; color typically tan, gold, yellow. Worms may be huge, 75 cm (30 inches) long, up to about 1.5 cm (3/5 in) in diameter; worms possess jaws that look like scythes or linear knives; may be predatory on small fishes or scavengers - Eunicid worms; Family Eunicidae. Very common in natural reef situations and appear in our systems with live rock.

A large (length = 3ft+) Eunicid from one of the authors aquaria.

9A. Worms without long (½ body length or longer) tentacles or processes from the head; worms active, small (typically visible only with magnification) - go to 10.

9B. Worms with long tentacles or processes from the body. Worms basically inactive. Typically worms are visible without magnification, lengths may exceed 1 cm (2/5 inch) - Cirratulid worms; Family Cirratulidae. Common in aquarium sediments. Worms brown to yellow, feed on detritus or sediments. They are a good part of the clean-up crew.

A cirratulid common in aquaria.  Often the tentacles are blood red and look like fine red hairs in the aquarium sand

10A. Appendages visible and prominent along the sides of each segment - go to 13.

10B. Appendages not visible or reduced (looking like small bumps) along the sides of each segment, bristles visible, but only with magnifications of 20x or more - go to 11.

11A. Worms uniform black or dark brown, not reddish; look like a "stubby" earthworm, both ends of the worm rounded - Ctenodrillids. Family Ctenodrillidae. Mud eaters; harmless and sometimes common; typically less than ½ mm (1/50th inch) in length.

11B. Worms pink, light brown, red, or striped; not uniform black or dark brown. If shaped like an earthworm, they are long and slender and reflect light with an iridescent sheen - go to 12.

12A. Worms earthworm-like; uniform pink or light brown, often have iridescent sheen in reflected light - Lumbrinerids. Family Lumbrineridae. Debris and detritus feeders. Harmless and a good component of the clean up crew; may be quite large, but the biggest I have seen in aquaria are about 1 cm (2/5 inch) long.

12B. Worms not earthworm-like; either one or both ends of the worm modified into funnel or plate or shield shapes. Typically the animal is striped around the segments; may have mottled surface patterns (like small black freckles)- Maldanids. Family Maldanidae. Burrowing sand eaters. They are a valuable component to the sediment infauna, eating bacteria and clearing spaces on the sediment to allow for bacterial growth.

13A. Head appendages (tentacles), or lateral appendages (parapodia), or both appearing as made of string of clear beads (Visible only under magnification of 20x or greater) - Syllids; Family Syllidae. Predatory on other infauna, normal component of marine sands. They may be quite large in nature, but I have not seen any over 2 mm (about 1/10 in) long in aquaria. They are a normal component of the benthic ecosystem.