Marine Shell Identification Part 2 — Tips for Recognising Shell Families

Shells should be regarded as museum specimens, worthy of careful exploration.

Shells are far more fascinating than we imagine.

Some fire venomous darts to catch fish; some live as commensals inside shark stomachs.

Some armour themselves with pebbles; others wander between land and sea.

Right there beneath that blue ocean, they wait to be discovered.

Identifying Characteristics of Shell Families

Last time we introduced five types of shells:

Abalone, Top Shells, Cowries, Cone Shells, Nerites

(Premium new-year dish, triangular stones, money cowrie, inverted triangle, little pebble)

This time we start from those five and branch outward.

To help the features stand out, the author has sketched the silhouettes of each family,

so let's build on the previous instalment and get acquainted with each family one by one!

(When you see the * symbol in the text, refer to the glossary at the end of the article.)

Shells: Abalone (Haliotidae), Limpets (Patellidae), Keyhole Limpets (Fissurellidae)

All three share the same key features: flat shell shape, *broad foot

Whenever you encounter a shell with these characteristics, these three families should be your first consideration.

(The cap-shaped gastropods can be further divided into: Patellidae, Nacellidae, Lepetidae, Lottiidae … we'll group them all under "limpets" for now.)

Interestingly, a study once published in the shell enthusiasts' journal (Issue 22) surveyed limpets in abalone pools versus wild limpet populations, comparing shell-width-to-shell-height ratios. It found that the higher the waves, the taller the average limpet shell (more internal muscle); in calmer water, shells were flatter (less muscle). The lesson: if you don't want to be swept away by waves, you'd better build some muscle first!

Here are common species names for reference and comparison:

．Haliotidae: Small Abalone　Haliotis diversicolor (Reeve, 1846)

．Patellidae: Star Limpet　Scutellastra flexuosa (Quoy & Gaimard, 1834)

．Nacellidae: Mosaic Limpet　Cellana toreuma (Reeve, 1854)

．Fissurellidae: Keyhole Limpet　Diodora suprapunicea (Otuka, 1937)

※ Keyhole limpets (Fissurellidae) are also called "perforated shells." Many species in the family have an apical hole on the shell's dorsal plate. The image below shows a species from the family without an apical hole (the shield shell, Scutus).

Shells: Top Shells (Trochidae), Turbans (Turbinidae), Delphinula (Angariidae)

Fellow members of the superfamily Trochoidea, these three share: oval aperture, *generally low spire

Many shells have oval apertures, but when you find one that can almost be drawn as a triangle, consider these three families.

Worth noting is the Turbinidae *operculum — it is among the thickest in the shell world. Seen on its own, you might almost wonder whether it's some entirely different organism.

As a child I occasionally found them on the beach without knowing what they were. Only when I looked them up in a book and discovered they were opercula did I exclaim, "Oh! So that's what it is!" — a reaction I'm sure many readers will share.

Here are common species names for reference and comparison:

．Trochidae: Textile Top Shell　Tectus conus (Gmelin, 1791)

．Turbinidae: Silver-mouthed Turban　Turbo argyrostomus (Linnaeus, 1758)

．Angariidae: Dolphin Shell　Angaria delphinus (Linnaeus, 1758)

※ Both Turbinidae and Angariidae include quite a few spiny species. If you're interested, look up two in particular: the Hairy Triton and Angaria hochenwarthi.

Shells: Cowries (Cypraeidae), Egg Shells (Ovulidae), Trivia (Triviidae)

All three belong to the superfamily Cypraeoidea. Key features: spire almost invisible, ventral slit

(Cowries with a visible spire will be introduced later.)

Compared to other families, the cowrie group has such distinctive features that identification is quite straightforward.

All cowrie-family members have a prominent *mantle, which means that photographing a live animal underwater and examining an empty shell on shore look completely different — like seeing someone in a coat and sunglasses versus without: you simply can't recognise them.

A special mention goes to the Ovulidae. Their shell shapes vary the most among the three families, including lance-like Volva habei, the alien-esque Rotaovula hirohitoi, and the Jenneria pustulata, which is frequently mistaken for a true cowrie. Their rebelliously diverse appearances make them an adorably intriguing "villain" for newcomers.

Here are common species names for reference and comparison:

．Cypraeidae: Serpent's-head Cowrie　Monetaria caputserpentis (Linnaeus, 1758)

．Ovulidae: Adamson's Egg Shell　Pseudocypraea adamsonii (Gray, 1832)

．Triviidae: Rice Trivia　Trivirostra oryza (Lamarck, 1810)

※ When the dorsal surface of the Serpent's-head Cowrie is eroded and worn smooth, the underlying purple layer shows through — it is not a different species!

Shells: Cone Shells (Conidae), Strombidae (Conchs), Fig Shells (Ficidae)

These three are quite distantly related, but all share: narrow elongated aperture, lateral aperture

Cone shells use a venomous dart to prey on small fish — never touch one with bare hands if you encounter it on the shore!

There is much more to say about cone shells, but in terms of shell features, the Strombidae (conchs) deserve special attention.

Conchs are not hard to identify: look closely at the *outer lip alongside the *siphonal canal, and you will usually find what is known as the "stromboid notch" — the indentation through which the conch extends its eye. It peers out at you through its shell, and staring back at it directly can be quite startling.

Within Strombidae, four names warrant extra attention because they look nothing like a typical conch:

"Spider Conch, Scorpion Conch, Camel Conch, Water-character Conch" (and similarly named XX Spider Conch variants)

Additionally, the Terebellum (nose shell) and Tibia (shinbone shell), which were previously classified under Strombidae, have each been placed in their own families, though they remain within the superfamily Stromboidea.

Here are common species names for reference and comparison:

．Conidae: Textile Cone　Conus textile (Linnaeus, 1758)

．Strombidae: Little Vase Conch　Strombus microurceus (Kira, 1959)

．Ficidae: Common Fig Shell　Ficus ficus (Linnaeus, 1758)

※ In some species the stromboid notch is less pronounced, but a slight indentation is still present, unless the aperture is excessively worn.

Shells: Nerites (Neritidae), Moon Snails (Naticidae)

These two shells are also not easily confused with others. Key features: round shell, semicircular aperture

Nerites and moon snails are actually easy to tell apart: nerites are always harder than moon snails, and the *callus area of nerites is broader.

Nerites are fascinating creatures — they hide under sand or rocks during the day, then swarm onto rocks at night to bask in moonlight (actually to forage).

Relatives of the Neritidae range across a wide variety of habitats; some even live in freshwater streams, such as freshwater nerites and wall nerites.

(Wall nerites bear a passing resemblance to limpets, but the former live in freshwater.)

It is also worth noting that Taiwan's nerite diversity is exceptionally rich by global standards — a richness now under threat from habitat destruction caused by recent land development. A great shame.

Here are common species names for reference and comparison:

．Neritidae: Ox Palate Nerite　Nerita albicilla (Linnaeus, 1758)

．Naticidae: Gualteri's Moon Snail　Natica gualteriana (Recluz, 1844)

※ Many nerite species exhibit highly variable shell colouration; identification based on the callus is more reliable.

Four Slightly More Complex Shell Groups:

Tonnoidea (Tonnidae, Bursidae, Ranellidae, Cassidae)

Muricoidea (Muricidae)

Buccinoidea (Buccinidae, Fasciolariidae, Melongenidae)

Mitroidea group (Olividae, Mitridae, Volutidae, Volutidae)

We will start introducing these four slightly more complex groups in the next instalment.

Shell Glossary:

Foot: The muscular organ used for locomotion by gastropod-class species. Think of it as a beer belly that crawls forward.

Spire: Most gastropods grow in a spiral fashion, secreting shell material as they grow to form a tower-like structure called the spire. Many spires develop spines, axial ribs, and spiral cords.

Callus: Using nerites as an example, there is a smooth patch near the inner side of the aperture — occasionally with granules — which is one of the key diagnostic features for identifying nerites.

(Cover image: Rose Bubble Shell) (Image source: Apple Daily)

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Further reading:

• Marine Shell Identification Part 1 — Starting from Scratch

• Marine Shell Identification Part 3 — A Key to the Shell World: 26 Family Classifications

• Marine Shell Identification Part 4 — Identifying Common Shells Along Taiwan's Coast: Make Your Own Shell Field Guide

• Marine Shell Identification Part 5 — What Are Shells? Do You Know Enough About Them? Did You Know There Is Even a Grading System for Shell Collecting?