Amphipod computer model
Crustaceans
Above: a 3D computer (Pov-Ray) model of an amphipod of the hyperiid type such as
Phronima.

Crustaceans are extremely diverse animals of the arthropod ('jointed limb') type. In this regard
they have a hard exoskeleton and employ a very diverse array of jointed appendages. We
begin with an example and take a look at amphipods. Amphipods belong to the crustacean
class called Malocostraca, which includes many familiar crustaceans such as crabs, lobsters,
krill, shrimp, isopods such as woodlice, amphipods and others. Amphipods like a carapace, the
hard shield-like shell so characteristic of crabs, and are laterally compressed (i.e. they have
narrow bodies) in contrast to isopods which have dorsoventrally compressed (i.e. flattened
from front to back) bodies.

There are at least six recognised classes of crustacean at present:

  1. The branchiopods ('gill limbs') such as the water flea and brine shrimp, mostly found in
    freshwater and have limbs equipped with flattened plates functioning as gills
  2. The ostracods or seed shrimps, with a two-valved carapace, found in fresh and salt
    water
  3. The maxillipods, including barnacles and copepods (at one time place din two separate
    classes)
  4. The remipedes, which live in saline aquifers and superficially resemble centipedes which
    swim on their backs and which inject venom into their prey via a pair of fangs
  5. The cephalocarids, a small group of a dozen or so species of tiny horseshoe shrimp
  6. The malocostracans, more than half of known crustaceans, many of large size, including
    decapods (crustaceans with 10 limbs such as crabs, shrimps and lobsters), amphipods,
    isopods (including the terrestrial woodlouse, but mostly marine) and various other
    groups.
Amphipods range in size from about 1 mm to 34 cm (just under 62 inches) in length. Almost 10
000 species of amphipod have been described!  Most are marine, but some are freshwater
and terrestrial. A familiar example is the sandhopper,
Talitrus saltator, found on beaches. our
model more closely resembled
Phronima.

The amphipods can be divided into four basic types: the Gammarids are small shrimp-like
crustaceans; the Caprellids include unusual forms such as skeleton shrimps and whale lice; the
Ingolfiellids are worm-like, often inhabiting bottom deposits; and the Hyperiids are planktonic,
exclusively marine with large eyes.
Phronima is a Hyperiid. Most Hyperiids spend at least part
of their life living symbiotically, whether as harmless commensals or parasites, with
salps,
jellyfish, ctenophores or siphonophores. Phronima females occupy salps which they convert
into floating barrel-like gelatinous homes (!) which they steer through the water and use as
brood chambers for their young (in some species the male also occupies the barrel).

The body of a typical amphipod is divided into the following segments:
1. The head (H1 to H5):

The head consists of five fused segments (H1 to H5):

  • H1 bearing one pair of sensory antennae (antennules, primary antennae)
  • H2 bearing one pair of sensory antennae (secondary antennae)

Thus, there are two pairs of antennae in total,
although the first pair (antennules) may be
reduced
and short;

  • H3, H4 and H5 bear the mouthparts consisting of the following 3 pairs of short
    appendages: mandibles (1 pair on H3, each member with three processes) for cutting
    up and crushing food; maxillules (1 pair on H4, first pair of maxillae) and maxillae (1
    pair on H5); each of the two maxillules (maxillae 1) and maxillae (maxillae 2) consist of
    an inner and an outer lobe with bristles or setae, often arranged in brushlike arrays at
    the ends of each lobe and are involved in tasting and manipulating food; an upper lip
    plate (labrum) is also present

The head bears at least one pair of stalkless
compound eyes which are very large in
Hyperiids and being compound consist of many optical units or facets. In
Phronima, in addition
to the two large compound eyes on top of the head, there are two smaller lateral compound
eyes on the sides of the bottom of the head (these are visible in our model).

2.The first thoracic segment (T1):

This is fused to the head, bears one pair of small limbs called maxillipeds, which are
functionally part of the mouthparts and look like tiny legs

3. The mesosome (T2 to T8):

This consists of
the remaining thoracic segments (thoracic segments 2 to 8) as follows:

  • T2 bearing one pair of leg-like gnathopods
  • T3 bearing one pair of gnathopods
  • T4 to T8 each bearing one pair of pereiopods (walking legs)

The gnathopods assist in feeding, along with the maxillipeds. In
Phronima, one pair of
pereiopods (on T6) is modified as a pair of large clawed appendages.

4. The
metasome (first three abdominal segments, A1 to A3):

These three abdominal segments each bear one pair of
pleopods. The pleopods are
biramous (branched into two, each branch consisting of a series of segments joined end-to-
end) and covered in long setae. Pleopods are generally used in swimming.

5. The
urosome (comprised of the next three abdomenal segments, U1 to U3):

Each of the three urosome segments contains one pair of
uropods, which may be biramous
(branched or forked into two) or uniramous (not branched like an insect's leg or antenna). The
uropods are fanlike and assist in swimming.

6. The
telson

This is not a true segment (somite) and bears the anus. In some crustaceans the telson forms
part of the tail fan and may bear one pair of sensory processes, the cerci.
Amphipod computer model
Amphipod computer model
Amphipod computer model
Amphipod computer model
Respiration and Circulation

On the inside base of most of the pereiopods there is a flattened plate (lamella) or vesicle
which acts as a gill and is attached to the basal-most leg segment (coxa) on the inside. The
pleopods generate a ventilating current of water across the gills. The 'heart' consists of a
tubular dorsal vessel situated in the thorax above these
coxal gills. Blood (haemolymph)
enters the heart via 1 to 3 pairs of pores (one per segment) in the side of the dorsal vessel
and is expelled from the front end into the haemocoel cavity where it bathes the tissues as
haemolymph. The haemolymph cntains the blue copper-based pigment haemocyanin which
binds oxygen for transport of oxygen from the gills to the tissues.
Locomotion and Camouflage

Phronima
is pelagic and by inhabiting a 'barrel' constructed from a salp it improves its
buoyancy. However, it must expend energy steering the barrel but data suggest that it has
more watery and weaker muscles than free-living forms even though the total energy
expended in locomotion is the same (what is gained by improved bouyancy is lost by steering
the barrel).

Gammarids are mainly bottom dwellers, walking on their pereiopods, but can swim at intervals
using their pleopods and uropods. Skeleton shrimp climb with the help of claws and may live on
the surface of other invertebrates. Some amphipods burrow and may secrete tubes or
construct tubes out of mud. Vermiform (worm-like) can live in the substrate and the tinniest
amphipods can live between sand/sediment particles as interstitial animals. Beach fleas and
sand hoppers (
Talitrus and relatives) are semi-terrestrial, occurring in sand at the high water
mark, surfacing from the moist sand at night to feed. Their behaviour is controlled by an
endogenous circadian (about 24h) clock which is reset by celestial cues, such as the angle of
the Sun in the sky. Leaf-hoppers live further inland in the leaf litter of coastal forests and
require moist habitats and still use their gills for respiration (see below).

The hard cuticle forms an
exoskeleton to which muscles attach. This exoskeleton may extend
inwards as skeletal processes called apodemes, to which muscles attach. Anyone who has
ever examined the exoskeleton of a crab claw will have realised that pulling on the
apodeme
operates the claw. The tough exoskeleton of decapods like crabs and lobsters is formed of an
outer epicuticle, a middle exocuticle and an inner endocuticle resting upon an inner membrane
with a layer of epidermal cells underneath, called the hypodermis. All the three outer layers are
calcified, strengthened by calcium carbonate, some of it in the form of calcite crystals and are
highly durable biological nano-composite materials. The epicuticle also contains lipids and
protein, the exo- and endocuticles also contain fibres of protein and chitin.

In hyperiid amphipods, translucency can confer camouflage as they float in the water column
and the cuticles of these animals forms a layer of minute protruding spherules (200 nm high)
on the surface of the animal. These spheres form a
gradient refractive-index material, that
is a material across which the refractive index slowly changes. This reduces reflections of light,
which occurs at sudden changes in refractive index, improving the translucency and the
camouflage! (External link:
Bagge et al. 2016).

Nervous System and Senses

The nervous system follows the basic arthropodan plan as found in
insects and millipedes, in
which a double ventral nerve cord runs the length of the animal to the  brain (suproesophageal
ganglion, a fused mass of ganglia above the oesophagus) with paired ganglia in each body
segment. However, this basic plan has been heavily modified in some crustaceans. For
example, in crabs all the ganglia are fused into a large central mass or 'brain'.

Sensory bristles or hairs (sensilla) sre located at various positions on the body surface. Some
are chemoreceptive (called aesthetascs) such as the pheromone sensors on the antennae of
male amphipods. Others are mechanosensitive, detecting touch or vibration, whilst others are
proprioceptive, detecting movements of body parts relative to one-another. The compound
eyes of many crustaceans are stalked, but are stalkless in amphipods. Those of the lobster
may contain about 14 000 light-sensitive units (ommatidia, see
compound eyes in insects
which are similar in principle). Most decapods, such as crabs, have one pair of gravity-sensing
statocysts, one on the basal segment of each of the antennules (first antennae). This consists
of an invaginated pouch of body wall epidermis open to the outside via a pore or slit, with a
massive structure or statolith inside, which is often formed of sand granules cemented together
by secretion. Hairlike projections (cilia) from sensory cells in the wall of the statocyst embed in
the statolith and sense deflections as gravity pulls the statolith from side to side as the animal
moves and tilts. This way the animal can sense which is the right way up!
Feeding

Most Amphipods are primarily detritus feeders and scavengers, feeding off organic remains
and algae on sand. Gammarids may scrape away bottom sediment or sand with their
gnathopods and/or their antennae (especially the secondary antennae). Many tube-dwelling
Gammarids are filter-feeders and extend their bristly antennae, and sometimes also the first
few pairs of legs, into the water current to act as a net, sieving the water or waiting for organic
remains to fall down as marine snow.

Some forms (such as some Caprellids) are, at least occasionally, predaceous (raptorial) and
may launch themselves into the water to catch prey. We have already discussed how hyperiids
like Phronima predate or parasitise salps. A few Amphipods are parasites of fish with suctorial
mouths. Whale lice cling to the hide of whales with the help of specially modified clinging legs
with large claws and probably feed on algae and other detritus on the whale's skin.
Excretion and Osmoregulation

Crustacean excretory organs are located in the head and consist of an end sac, an excretory
canal and a short duct which opens via an excretory pore. In Amphipods the excretory organs
are
antennal glands and open via a pair of pores, one on the underside of each secondary
antenna. The end sac consists of a layer of epithelial cells perforated by slits (they resemble
the podocytes of the vertebrate kidney) across which excess fluid and waste materials are
filtered from the haemolymph. Since the gills vent off nitrogenous waste as ammonia, it is likely
that the antennal glands are primarily for regulation of water volume. Ion regulation occurs
across the gills, which may absorb salts in freshwater forms.


Reproduction and Development

Most crustaceans have separate sexes (they are dioecious). The testes and ovaries are
paired tubular organs lying in the dorsal part of the trunk. The male locates the female, often
with the help of pheromones as in Amphipods and may have modified limbs to clasp the female
and often some form of organ or modified appendage for sperm transmission. Crustacean
sperm may be delivered in packets (spermatophores) and may be immotile (lacking flagella).

In Amphipods, the female has a ventral brood chamber (
marsupium) and during copulation
the male releases sperm through the pair of gonopores, situated on the ends of a pair of penis
papillae on the sternum (breastplate) of the last thoracic segment. The male brings his
uropods in contact with the female marsupium and emits the sperm which are swept into the
marsupium by the current of water generated by the female's pleopods to ventilate her gills.
The female's oviducts open, via the gonopores, on the coxae (basal-most leg segment) of the
sixth thoracic segment (T6). After the male and female separate, the female releases her eggs
through her gonopores into the marsupium where they are fertilised. The eggs are brooded in
the marsupium, ventilated by the respiratory current.

Development is
direct, that is the hatchlings resemble miniature adults. However, in most
marine crustaceans there is a free-swimming planktonic larva, such as the
nauplius which has
three pairs of appendages: the two pairs of antennae and a pair of limb-like mandibles. The
second pair of antennae and mandibles have long bristles and are used in swimming. The
body of the nauplius appears unsegmented and there is a single 'naupliar' eye on the centre
of the head which consists of 3 or 4 optical units or ocelli. As the nauplius undergoes a series
of molts, additional segments and appendages are added (at the anterior face of the larval
telson). The nauplius may give rise to a zoea larva with 8 pairs of appendages, swimming by
means of the rearmost appendages. Crustaceans must molt their hard cuticles as they
continue to grow in size.
Article created: 22/6/2017