Everything about crustaceans – the armored acrobats of the reef!

Crustacean definition:
Crustaceans are among the most diverse and fascinating invertebrates found underwater. From tiny shrimp hiding in coral to large lobsters roaming the reef, they belong to the phylum Arthropoda — the same group as insects and spiders! What sets them apart is their hard exoskeleton, jointed legs, and often elaborate antennae.

Habitat:
Crustaceans inhabit nearly every underwater habitat — from tide pools to the deepest trenches. In coral reefs, you’ll find them in cracks, crevices, under rubble, or hidden in symbiotic relationships with other animals like anemones or sea cucumbers. Some are nocturnal and only come out at night, making night dives a great time to spot them.

Diet:
Crustaceans have a wide range of feeding strategies. Some are scavengers that clean up detritus and organic matter; others are predators, feeding on worms, mollusks, or small fish. Some, like certain shrimp, are cleaners that pick parasites and dead tissue from other fish. There are also filter feeders, like barnacles, that extract particles from the water.

Identification:
Crustaceans can be recognized by their segmented bodies, exoskeleton, and articulated limbs. While their forms vary greatly, common features include:
• Antennae (often long and sensitive)
• Exoskeleton that must be shed to grow (molting)
• Paired limbs (used for walking, swimming, or feeding)
• Two main body segments: the cephalothorax and abdomen (though this can vary)

How to tell them apart: Crabs, shrimps, lobsters, mantis shrimp & more

Shrimps

  • Body shape: Long and slender, laterally compressed (flattened side to side)
  • Antennae: Very long, often longer than the body
  • Movement: Swim by flicking their abdomen; some walk on the reef
  • Tail (abdomen): Well developed and used for fast backward escapes
  • Behavior: Many live in symbiosis (e.g. cleaner shrimp, anemone shrimp)
  • Examples: Cleaner shrimp (Lysmata), glass shrimp (Periclimenes)

Crabs

  • Body shape: Broad and flattened, often round or oval
  • Tail: Reduced and tucked under the body (not visible)
  • Legs: 10 legs — first pair modified into pincers (chelae)
  • Movement: Walk sideways; fast and agile
  • Behavior: Often hide in crevices or bury in sand
  • Examples: Porcelain crab, boxer crab

Lobsters & Reef Lobsters

  • Body shape: Elongated, somewhat like large shrimps
  • Antennae: Very long and thick (spiny lobsters); claws only in some species
  • Tail: Strong and muscular
  • Movement: Walk with legs, swim backward when threatened
  • Behavior: Hide during the day, come out at night
  • Examples: Spiny lobster (Panulirus), reef lobster (Enoplometopus)

Mantis Shrimps (Stomatopods)

  • Body shape: Flattened but robust, alien-looking
  • Eyes: Very large and independently moving, often on stalks
  • Front limbs: Modified into powerful smashers or spear-like appendages
  • Behavior: Live in burrows or holes; extremely fast strikes
  • Fun fact: Some species can punch faster than a bullet and see polarized light!
  • Examples: Peacock mantis shrimp (Odontodactylus)

Isopods & Amphipods

  • Size: Tiny to small
  • Body shape:
    • Isopods: Flattened top to bottom (like pill bugs)
    • Amphipods: Flattened side to side, often curled
  • Behavior: Often scavengers, parasites, or found clinging to other animals
  • Examples: Fish lice, skeleton shrimp

Reproduction:
Most crustaceans reproduce sexually, with separate male and female individuals. Fertilization is usually internal, and many species carry eggs on their bodies until they hatch. In shrimps and crabs, you might see berried females (with clusters of eggs under their abdomen). The offspring may hatch as free-swimming larvae that go through several planktonic stages before settling down.

Classification / Taxonomy:

  • Kingdom: Animalia
  • Phylum: Arthropoda (joint-legged animals)
  • Subphylum: Crustacea
  • Class: There are several classes, but most of the marine species divers see belong to:
    • Malacostraca (includes crabs, shrimps, lobsters, mantis shrimp)
    • Maxillopoda (barnacles)
  • Orders & Families:
    • Decapoda (ten-legged crustaceans like shrimps, crabs, lobsters)
    • Stomatopoda (mantis shrimps)
    • Isopoda & Amphipoda (small scavengers or parasites)

We will organize the species we observe on this site primarily by families and genera, where possible.

Size:
Crustaceans range dramatically in size. Tiny reef shrimps may be just a few millimeters long, while lobsters or mantis shrimp can reach over 30 cm. Deep-sea species can grow even larger.

Lifespan:
Their lifespan varies widely. Small shrimp may live 1–2 years, while some lobsters can live decades in the wild if undisturbed. Growth occurs through molting, which also makes them vulnerable for a short time after shedding their exoskeleton.

 

To keep in mind:

• Crustaceans are not just reef cleaners — they play critical roles in marine ecosystems.
• Some live alone, others in pairs or colonies, and many form symbiotic relationships.
• Their eyesight and behavior can be surprisingly complex — some shrimp can even see polarized light!

Now... Everything about Seashells!

Definition:
Seashells are the hard, protective outer coverings created by marine mollusks. These shells are made primarily of calcium carbonate and serve as armor against predators, physical damage, and desiccation. The living animal — such as a snail, clam, or scallop — builds and maintains the shell throughout its life. When the animal dies, the shell may wash ashore and become what we commonly call a “seashell.”

Not all marine animals with shells are the same — the term “seashell” usually refers to the exoskeletons of marine gastropods and bivalves, but includes other mollusks like scaphopods and chitons.

Habitat:
Seashell-producing mollusks are found in all marine environments, from intertidal zones and mangroves to coral reefs and deep-sea plains. Shells often reflect their environment — thick and heavy in rough surf zones, lightweight or elongated in calm lagoons or deep waters.

Diet:
The diet depends on the type of mollusk:

  • Gastropods (e.g. snails, cowries): feed on algae, detritus, sponges, or other invertebrates. Some are carnivores or scavengers.

  • Bivalves (e.g. clams, mussels): filter feeders, they extract plankton and organic particles from the water.

  • Scaphopods & chitons: less commonly known, they feed on microscopic organisms in the sediment or graze on algae.

Identification:
Shell shape, size, sculpture (ridges, spines, knobs), and coloration are key identifiers. You can often tell:

  • Gastropods: Spiral-shaped shells, with one opening.

  • Bivalves: Two hinged shells (like clams).

  • Scaphopods: Tusk-shaped, tubular shells.

  • Chitons: Flat, oval bodies covered by eight overlapping plates.

Note: In live animals, the soft parts like the foot, siphon, and tentacles also help in identification.

Reproduction:
Most seashell-producing mollusks are gonochoric (separate sexes), though some are hermaphroditic. Fertilization may be internal or external, depending on the species. They usually lay gelatinous egg masses or release free-floating eggs into the sea. The larvae (often trochophore or veliger) drift in the plankton before settling and metamorphosing into adults.

Lifespan:
Ranges widely depending on the species — from 1 year in small or fast-growing gastropods to 20–100 years in large bivalves like giant clams.

Ecological Role:

  • Filter feeders help clean the water.

  • Grazers control algal growth.

  • Shells provide microhabitats for other marine life after the animal dies.

  • Prey for fish, crabs, sea stars, and humans.

Fun Fact:
The patterns and colors on shells are formed by pigment-secreting cells in the mantle. In some species, these patterns follow a mathematical logic and are used as case studies in morphogenesis!

To Keep in Mind:

  • Empty shells may be reused by other animals (e.g. hermit crabs).

  • Shell collecting can be fun, but removing living animals or too many shells from beaches can disturb local ecosystems.

  • Not all mollusks have external shells — octopuses, for instance, evolved to lose theirs!