Shrimplike Crustacean Zooplankton Larger Eyes And Limbs

The shrimp-like or elongate zooplankton include the larvae of commercial Crustacea, which are familiar to us as prawns (the commercial penaeids), shrimps (everything else similar), lobsters, hermit crabs and crabs (the decapods). The various species and larval stages sometimes have specialised names, but all crustacean larvae begin as a nauplius (Figure 8.4A). To identify the major groups, two key traits to look for are the presence or absence of a carapace and the presence or absence of stalked eyes. The first task with this group is to be able to recognise the small adult shrimps - the krill (euphausids) and the mysids.

Adult krill are recognised initially by their size and abundance, and are typically found in night-time tows. They may have bio luminescent dots along each segment of their abdomen, stalked eyes, a loosely fitting carapace around the abdomen and their long and setose feeding limbs (Figure 8.5A). The larval stages with no swimming limbs are more difficult to identify, and the juveniles may be recognised after eliminating other candidates (below).

Mysids also have stalked eyes, but are nearly translucent (when alive) and more slender, with a looser fitting carapace than the krill (Figure 8.5B). Their remarkable translucence allows one to admire the tubular heart, the gut peristalsis and the many beating limbs (including eight thoracic pairs). Nearly all mysids have a pair of balance organs (statocysts) in the tail fan limbs (the uropods), which appear initially like a pair of translucent

Figure 8.5 Larger crustacean zooplankton line drawings showing (A1-A2) euphausids and various life stages, (B1-B2) mysids including detail of statolith within uropod, (C1-C2) larval penaeids and sergestid shrimps, (D1-D3) anomuran zoea, (E) cumacean, (F) lobster larva, (G1-G2) amphipods, (H1-H2) isopods, (I1-I3) stomatopod zoea, (J1-J4) crab zoea and megalopa (Sources: Dakin and Colefax 1940; Wickstead 1965).

Figure 8.5 Larger crustacean zooplankton line drawings showing (A1-A2) euphausids and various life stages, (B1-B2) mysids including detail of statolith within uropod, (C1-C2) larval penaeids and sergestid shrimps, (D1-D3) anomuran zoea, (E) cumacean, (F) lobster larva, (G1-G2) amphipods, (H1-H2) isopods, (I1-I3) stomatopod zoea, (J1-J4) crab zoea and megalopa (Sources: Dakin and Colefax 1940; Wickstead 1965).

bulls-eyes from a dartboard. Statocysts (unlike the fishes' otoliths) are usually composed of calcium fluorite, with the consistency of toothpaste. Mysids (700 species) are particularly abundant in riverine estuaries, and are the prey ofjuvenile fish and prawns.

The third group of elongate crustaceans are the many decapod shrimps and their larvae - which also have a carapace and stalked eyes. However, remember that these are larvae, so these can often be identified by the possible lack of swimming limbs (Figure 8.5C, D), or the lack of uropods (just a spade-like telson). This group contains a wide variety of shapes and species:

• Prawns and sergestid shrimps fertilise their eggs externally, which hatch into a nauplius (Figure 8.4A) and thence moult into a zoea or mysis. A distinctive member of this group is the holoplanktonic Lucifer, a sergestid shrimp with a stalked head and eyes (so called ghost shrimp, Figure 8.5C2).

• The remaining decapods retain their eggs on the swimming limbs, and the nauplius stage is completed in the egg, hatching into a zoea. There is a wide variety of larval carid shrimps (including Alpheidae, Pandalidae, Hippolytidae and Palaemonidae) that, like many groups in this section, may have distinctive features, but are only useful for recognising genera or species rather than the group as a whole (Figure 8.5D).

• Thalassinids are an under-appreciated group of prawn-like crustaceans, often used as bait and sometimes known as yabbies or mud-shrimps. Their larvae also appear as elongate zooplankton (examples are Jaxea and Callianassa, Figure 8.5D1-D3).

The remaining decapods are those that as adults have a heavy exoskel-eton, such as lobsters (Palinuridae, Scylaridae), hermit crabs (Anomura) and crabs (Brachyura). Lobster larvae are outstanding, hatching into large and distinctive zoeae, known as a phyllosoma, which range in size from a few mm across up to 20 mm (Figure 8.5F). Crab zoeae have relatively large globular heads and thoracic bodies, often with large and distinctive dorsal and lateral spines, and quite small limbless abdomens (Figure 8.5J). Crab zoea then moult into a megalopa larva, taking on the appearance of a small crab (Figure 8.5J3).

As a group, crustaceans could be viewed as rather benign. However, adult stomatopods are the jaguars of the crustacean world - often called 'prawn killers' or 'shell smashers'. Adults are relatively intelligent and beautiful, and sometimes quite colourful. Taxonomically they are quite separate from the above decapods. Stomatopod zoea have a large flared loose carapace (like a translucent cloak or wing) with spines at the corners, and their distinctive spearing limb of the second maxilliped is apparent even in the early larvae (Figure 8.5I).

Only a few crustaceans are found on land and the most successful are the amphipods (beach hoppers) and isopods (pill bugs), which are familiar to us from damp areas in the garden. In the sea, members of these groups are usually benthic, and the females retain their eggs and larvae in a mar-supium between their legs, later releasing miniature adults. At night they may swim up into the plankton - as may another related group: the tanaids. These groups have no carapace and a sessile compound eye, and are usually greater than 3 mm in length. The amphipods tend to be compressed laterally (Figure 8.5G) while the isopods are dorso-ventrally flattened (Figure 8.5H). The hyperid amphipods are holoplanktonic, and are characterised by very large eyes (Figure 8.5G1). At night the normally benthic living cumaceans can swarm into the water column to mate and moult. They look superficially like a large calanoid copepod, but with a bulbous head and thorax, and a slender abdomen (Figure 8.5E).

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