Subphylum Crustacea - BEDIM · Subphylum Crustacea with seven articles. In many species the first one or two pairs of pereopods are variously modified as chelae. The forms of the

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Crustaceans are of particular interest for humans because

they efficiently convert living plants or dead organic

matter into animal biomass, represent important food

for free-living fishes, or are delicious fisheries resources

themselves. On the downside, many crustaceans are

highly infectious parasites of aquaculture or fisheries

resources, and many species are ravenous consumers of

commercially important algae. To the marine biologist

they are of great interest because of their diversity of

life styles and evolutionary innovations. The general

public finds pleasure in looking at crustaceans for their

fascinating colours, shapes, and behaviours.

General MorphologyThe morphological diversity of crustaceans beats that

of most other higher taxa. There are tiny copepods,

smaller than a pinhead, that navigate the waters next

to large king crabs clumsily walking the sea-floor on

their wide-spanning legs. All crustacean species have

a chitinous exoskeleton, which gives them stability and

also provides protection to the interior organs. The shape

of the exoskeleton has been

variously modified within

the different classes and

orders of crustaceans, but

the general Bauplan is

maintained throughout all

crustacean taxa. In some

of the highly modified

parasites, the typical

crustacean characters only

appear during the larval

stages.

Crustaceans are segmen-

ted animals and in many

species the body segments

can be easily recognized.

There are different degrees

of fusion of segments,

mostly in the larger

decapod species. In these

the anterior (thoracic) body

Subphylum CrustaceaMartin Thiel

General Introduction Crustaceans are ubiquitous organisms throughout the

fjord region. A water sample can contain thousands of

tiny copepods darting from one corner of the jar to the

other. An alga washed up on the shore will quickly be

covered by hundreds of beachhoppers. Dredge samples

from the bottom of the fjords can produce mountains of

shrimp and squat lobsters. Cool rivers of the fjord region

are home to nimble aeglid crabs that roam between river

cobbles. In the humid forests of southern Chile large

families of pill bugs aggregate under rotten logs.

Why is it that crustaceans have conquered such a

diverse range of environments? They have a hard

exoskeleton protecting the vital interior organs, and

they possess articulated legs that are modified to fulfil

numerous tasks including walking, swimming, grasping,

digging, cutting, among others. Furthermore, crustacean

appendages have numerous setae, which complement

the morphological adaptations of the body, or even offer

new functional innovations. Finally, many crustacean

parents provide some degree of protection to their brood,

at least during the initial

stages of development.

Consequently their

offspring have a head start

and this allows them to

colonize habitats difficult

to colonize by species with

more delicate life history

stages. All these morpho-

functional adaptations

enable crustaceans to

inhabit a wide range of

environments, encom-

passing forest soils,

river beds and marine

ecosystems.

Fig. 1. Principal body regi-

ons of a peracarid amphipod

and of a decapod shrimp.

Fig. 1

Fig. 2

Fig. 4

Fig. 3

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segments are fused to form the carapace. All vital

organs are contained within the carapace, which often

is hardened by the incorporation of calcium-carbonate.

In most smaller crustaceans (e.g. the peracarids) only the

head and the immediately following segment are fused

to form the cephalon, but some taxa (e.g. the cumaceans

and the tanaids) also have a carapace covering some or

most of the thoracic segments.

The segmented body of crustaceans is subdivided in three

main regions, the head, the thorax and the abdomen

(Fig. 1). The head (cephalon) bears the antennae, the

mouthparts and the eyes. Crustaceans have two pairs

of antennae, which can be short and robust as in some

crabs or long and slender as in many shrimp species.

The antennae bear numerous sensory receptors, which

receive and transmit tactile and chemical information.

Several pairs of specialized mouthparts work in concert to

process the food before ingestion. The mandibles cut and

grind larger food items, and the maxillae and maxillipeds

function as movable bib, holding and pushing the food

particles into the mouth. Mandibles (M), maxillae and

maxillipeds are generally similar among the diverse

crustaceans (Fig. 2), but subtle modifications of the general

design are frequently used as taxonomic characters, in

particular among the peracarid crustaceans. The eyes

are typical compound eyes as in other arthropods (e.g.

the spiders and insects). In many decapods the eyes

are on movable stalks (Fig. 3). Crustacean eyes may be

colourless or of diverse colours, typically black, red or

yellow, a character also used in taxonomy.

The head (or cephalon) is either separated from the

remaining body segments (as in the amphipods or isopods)

or it is inserted into the carapace, which comes in very

diverse shapes. The carapace surface often is sculptured

and together with the colour pattern this character

is also used to identify species (Fig. 4). The thoracic

segments (the thoracomers) bear

the walking legs (the pereopods).

These are articulated, typically

Fig. 2. General design of Mandible

(M), Maxillae (X1 and X2) and

Maxilliped in typical malacostracan

crustaceans.

Fig. 3. Many decapod crustaceans

(here brachyuran crab, caridean

shrimp and anomuran hermit crab)

have stalked eyes.

Fig. 4. Diversity in carapace shape

and coloration in porcelain crabs.

Fig. 6

Fig. 7

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with seven articles. In many species the first one or two

pairs of pereopods are variously modified as chelae.

The forms of the chelae are as diverse as is their use in

food acquisition or intraspecific combat (Fig. 5). The

remaining pereopods usually are relatively simple and

in most species are used for walking. In alga-dwelling

species, the pereopods often have a sharp and pointed

last article (the dactylus), which allows a firm grip onto

algal branches (Fig. 6 insert). In contrast, burrowing and

swimming species commonly have flattened pereopods

which allow efficient digging and swimming (Fig. 6).

The abdomen, which includes the pleon and the last

body segment (the telson), can be tiny as in many crabs

or it may be large and powerful as in the lobsters and

Fig. 5. Diversity in chela shape and size in various decapod

crustaceans.

Fig. 6. Swimming crab with the flattened dactylus used as

swimming paddle, and algal-dwelling crab with the hook-

like dactylus used for clinging to the algae.

Fig. 7. In brachyuran crabs the female abdomen is subs-

tantially wider than the male abdomen because the female

incubates the egg mass under the adomen.

shrimp. In the crabs, the abdomen is not visible at first

glance, because they carry it tucked underneath their

carapace (Fig. 7). In contrast, in the lobsters and shrimp

the muscular abdomen is easily visible – anybody who

likes to eat shrimp or lobsters knows the abdomen. The

abdomen bears the pleopods, the number of which

varies among species depending on the degree of fusion

of abdominal segments. In species with a muscular

abdomen the pleopods have swimming function and

are shaped like paddles. Many of the larger crustaceans

live on the bottom, but using their pleopods they can

swim efficiently, at least over short distances.

In most decapod crustaceans (crabs, lobsters and

shrimp), the female pleopods hold the eggs during

embryo development. Their pleopods are highly

Fig. 8

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ramified, feather-like appendages, enabling them to

hold hundreds or thousands of eggs. This is also why the

female abdomen, which has the egg-bearing pleopods,

is often wider than the male abdomen (Fig. 7). In the

shrimp and lobsters the mass of developing embryos

in their egg cases can be easily seen underneath the

female’s abdomen, but in the crabs the laterally flattened

abdomen of the female covers the entire egg mass, and in

order to see whether the female is ovigerous (incubating

embryos) one has to lift the abdomen carefully (Fig. 8).

Not all crustaceans incubate their embryos under the

abdomen. Barnacles incubate their embryos in their

mantle cavity, and peracarid crustaceans have a special

brood pouch (the marsupium) in which the embryos

develop into small juveniles. Males are not involved in

these parental tasks and consequently they either do

not possess these structures or their pleopods are not

modified for incubating the brood.

In addition to these sex-specific differences in brooding

structures, in many crustaceans the sexes differ in other

subtle or easily visible features. Males are often larger

than females, which bears testimony to intense male-

male fights for access to reproductive females. One

of the main characters shaped by the intense sexual

selection are the claws. Males use these claws during

fights, and the males with the largest claws often are

the most successful in mating with the females. In other

species, the males do not fight directly for the females,

but instead engage in races to be the first in

finding a female that is ready to mate. This type

of selection has lead to the evolution of males

with huge eyes and enormous antennae (which

bear many chemoreceptors), which enable them

to pick up signals emitted by the receptive female.

In many of these species the males look very

different from the females, and in some cases it

is not easy to match the two sexes. However, in

other species, where sexual selection is weak or

absent, the differences between the sexes are very

subtle and males and females look alike.

Many crustaceans have gills or gill-like structures.

In small species these gills might be unprotected,

but in the large decapods they are covered by

the carapace. Very small crustaceans have no

specialized respiratory structures. Crustaceans

have an open circulatory system and a simple heart,

which distributes the hemolymph throughout the

body. In most larger crustaceans the excretory system

leads into the bladder, and the urine is released through

the nephropore in the head region. The digestive system

is relatively simple with a gut that is subdivided into the

foregut, midgut and hindgut. The foregut has specialized

chitinous filters and mills, which grind and sort the food

slurry. Digestion occurs in the midgut gland where

the absorption of the digested food also takes place.

Absorption of water and formation of the feces occurs

in the hindgut. The paired testes or ovaries are located

dorsally. The testes lead into the vas deferens where the

ripe sperm are accumulated for mating. In the females

of some larger decapods, the oviducts have special

pockets, the “spermathecae”, in which sperm can be

stored for long time periods.

General BiologyCrustaceans can be found in all marine and also in many

freshwater and terrestrial habitats. They dwell in soft-

bottoms where some of the larger species excavate deep

galleries, many species roam through rocky habitats,

cling onto algae or other epibenthic substrata. The

diverse morphological adaptations found in crustaceans

reflect the wide range of environments inhabited and

food sources exploited by crustaceans.

Feeding Biology: Crustaceans as a group are generalist

consumers, and they feed on any kind of organic food

Fig. 8. Ovigerous females of two species of brachy-

uran crabs with egg masses.

Fig. 9

Fig. 10

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source, whether alive or dead. There are grazers that

consume large macroalgae or microscopic algal turfs

growing on rocky surfaces. Suspension-feeders have

some of their appendages (antennae or pereopods)

converted into highly efficient sieves, which they use to

filter food particles out of natural or self-induced currents

(Fig. 9). Others feed on deposits, which they scrape

from surface sediments or excavate in burrows that can

reach up to a meter in depth. Many of the larger crabs

are voracious predators, which crush even the thickest

bivalve or snail shells with their powerful chelae. Ecto-

or endoparasitic crustaceans exploit a wide variety of

hosts, including fish or other crustaceans. For example,

the so-called fish lice, which are a major nuisance in

salmon aquaculture, are small crustaceans belonging to

the class Copepoda.

Sensory Biology: Crustaceans use all three main senses,

visual, tactile and chemical. The most developed

is the chemical sense, which is employed in food

finding and also in intra- and interspecific interactions.

Dense batteries of chemoreceptors are located on the

antennae, which allow crustaceans to pick up dissolved

chemical cues. The mouthparts also contain many

chemoreceptors, which most likely are used to taste

the food. Rheoreceptors that receive tactile stimuli are

distributed over the entire body surface, but also occur

in higher concentrations in the head region and on the

antennae. Most crustaceans have paired eyes, but the

question of what they really can see is still a matter of

intense investigation. Information received by all these

sensory organs is processed in the well-developed

central nervous system with a prominent brain.

Reproductive Biology: Most crustacean species have

separate sexes. A few shrimp and many tanaid species

can change sex during their life time. Many barnacles

are simultaneous hermaphrodites, which means that

they can act as male and female at the same time.

Different from many other marine invertebrates, very

few crustaceans release their sexual products freely

into the water column. In most species, males and

females meet for mating, and many of the larger species

even have internal fertilization. The sexual biology of

crustaceans has received a lot of research attention

during the past decades. Usually one sex searches for

the other. While in many species the males search for

females there also exist many examples where the roles

are reversed, i.e. the females search for males. When a

male and female meet, they evaluate each other (using

primarily chemical signals), and if all conditions are met

they proceed with the mating. Courtship is usually brief,

but in many species the males may guard the female until

the actual mating takes place. This occurs in many crabs,

but also in copepods and in peracarid crustaceans. In

these species, the male carries the typically smaller

female around with him (Fig. 10). The female achieves

receptivity after the reproductive molt. The males

protect her during the molting period (basically they

Fig. 9. Examples for suspension-feeding crustaceans are

barnacles (upper left) and porcelain crabs.

Fig. 10. Amphipod male carrying the female during

precopulatory mate-guarding.

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defend her against other males). Shortly after the female

has finished the reproductive molt, the male transfers

one or more sperm packages into her reproductive tract

or onto her ventral body surface, where fertilization takes

place. In many crab species the female stores sperm in

the spermathecae and later use them to fertilize broods

without the need to mate again. After fertilization, the

female incubates the developing embryos either under

her abdomen or in other specialized structures.

After incubation periods of variable duration, many

crustacean females liberate planktonic larvae (e.g. the

Cirripedia, Copepoda, Decapoda, Stomatopoda). These

larvae then live and grow for several weeks or months in

the plankton. During this time they might be dispersed

over long distances with the prevailing currents. The

larvae can migrate up and down in the water column,

but they can not swim against the currents. Depending

on the water depths in which they are, the larvae might

be moved in and out of the fjords and channels. The

larvae of many species spend the first half of their larval

life in or near the upper water column and then move

to deeper water layers to return to their bottom habitats.

While marine biologists have a general appreciation

of the larval life, this phase of the crustacean life cycle

still holds many unresolved questions. For example, the

question of where a larva that hatched from a female

in a particular fjord might be settling at the end of its

larval voyages continues to motivate extensive research

programs.

Many smaller crustaceans have no planktonic larval

stages. Females brood their embryos until early juvenile

stages, which are identical in their morphology to the

adults. These small juveniles can immediately colonize

the adult habitats.

Life History: Crustacean life histories are as diverse as

their reproductive patterns. In bottom-living species that

produce planktonic larvae, juvenile and subadult stages

grow up in benthic habitats. At reaching sexual maturity,

the adults mate and after incubating the developing

embryos the females release usually hundreds and

thousands of planktonic larvae. The larvae develop

and feed in the water column and may travel extensive

distances during this time period. Larval life is risky

and during their planktonic journeys many larvae may

fall victim to predators, starve to death or be carried to

unsuitable environments. Few larvae return to benthic

habitats. Small juveniles also face a high risk of predation

and often lead a cryptic life style during the first months

or years of their life. Many of the larger crustaceans live

for several years and participate in several subsequent

reproductive seasons. In the fjord region, reproductive

periods are highly seasonal. Mating may take place

during fall or winter, larvae are released during early

spring, develop during the spring months when food in

the plankton is abundant and return to the bottom early

in the summer.

The species with direct development (Peracarida)

produce small broods, often with only tens of relatively

large eggs. Juveniles emerge from the female’s brood

pouch and directly colonize the natal habitat. Females

of species that live in warm waters can produce several

subsequent broods within a few weeks. Survival of their

offspring generally is high, and if the conditions are

suitable these species can build up large populations

over relatively short time periods. Growth is fast, and

during summer the juveniles reach sexual maturity within

several weeks, further contributing to population growth.

Life times of these small crustaceans are relatively short,

rarely exceeding a year. Species inhabiting very cold

waters, e.g. in the southern fjord region, may live for

several years.

In order to grow all crustaceans have to shed their old

exoskeleton. During this time period their body surface

is soft and they are very vulnerable to predators. Many

crustaceans hide shortly before and after their molt.

Small juveniles, which grow very fast, can molt every

week, while large crabs or lobsters might only molt

every other year or even stop molting (and growing)

altogether.

EcologyCrustaceans are important secondary consumers in all

major marine habitats of the fjord region. In many rocky

environments crabs are among the most ubiquitous

predators. Similarly, in algal-dominated habitats,

peracarid crustaceans are gluttonous grazers, consuming

large quantities of algal primary production. Myriads of

copepods and krill are the main consumers of planktonic

microalgae. In soft-sediments, crustaceans are some of

the most significant consumers of particulate organic

matter that is continuously deposited in these sediments.

Most crustaceans themselves are food to tertiary

consumers. Crabs, shrimps, and peracarid crustaceans

are eagerly devoured by predatory fish. Whales feed on

the abundant krill swarms in the outer fjord region. Thus,

crustaceans play an important role in the conversion of

organic matter and in the transfer of primary products to

higher trophic levels.

Fig. 11

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The Crustaceans of the Fjord Region

Crustaceans form one of the most species-rich taxon in

the fjord region. One of the best known species is the

southern king crab, the “centolla” Lithodes santolla. This

species is intensively fished throughout the entire fjord

region. Other species that are commonly fished are crabs

from the genus Cancer and the stone crab Homalaspis

plana. Thalassinid shrimp from the genus Callichirus

tunnel under soft sediments in the low intertidal zone.

The visitor to tidal flats usually can only see their burrow

openings and mounds of expelled sediment (see arrow

in Fig. 11). Suction pumps need to be employed to

pull them out of their burrows. Many other crustacean

species can be found during visits to the fjord region (see

following chapters).

Few of the presently known crustacean species are

restricted to the Chilean Fjord Region. Many of the

species found in the northern fjord region have the centre

of their distribution along the continental coast of central

or northern Chile. The highly diverse crustacean fauna

from the southern fjord region shares many connections

with the crustacean fauna from the Antarctic Peninsula.

Collection, Preservation and StudyCrustaceans can be easily collected and preserved (for

details see following chapters). Decapods can be identified

based on their carapace, colour and chelae, without the

need of species dissections. However, the smaller peracarid

species usually require dissection and microscopic

examination, especially in the less known taxa.

Many crustaceans are easily maintained alive if the

conditions are adequate (clean water and air supply).

The observation of the living animals can be most

rewarding for the upcoming marine biologist or

interested layperson. Knowledge about the biology of

the crustaceans from the fjord region is very scarce. For

most species we barely can put a name on them, but

we know next to nothing about their feeding habits or

reproductive behaviours. This information, though, is

very important if we aspire to better understand their

role within the fjord ecosystem.

SystematicsCrustacean systematics is still a matter of debate. One

of the most recent proposals has been brought forward

by Martin and Davis (2001). They distinguished six

classes of crustaceans, the Branchipoda, Remipedia,

Cephalocarida, Maxillopoda, Ostracoda, and the

Malacostraca. The Maxillopoda contain among others

the well known barnacles (Cirripedia) and the planktonic

Copepoda, and the Malacostraca contain the species-

rich decapod and peracarid crustaceans, which are

presented in more detail in the following chapters.

Fig. 11. Collecting ghost

shrimp with self-made

suction pump.