Other Zooplankton Worms And Snails

Holoplanktonic snails include the heteropods and pteropods (literally 'winged foot', because the foot is divided into two flaps for swimming, Figure 8.7D4). Pteropods may be shelled (thecosomate) or naked (gymno-somate) and are related to the often beautiful sea slugs or nudibranchs (Figure 8.7G). Shelled pteropods appear as coiled shells or simple cones, or resemble seeds when the snail has completely withdrawn into its shell (Figure 8.7D). Naked pteropods may initially appear as an amorphous lump, but closer inspection will reveal the foot, a proboscis and palps or tentacles (Figure 8.7E). Pteropods are all predatory, capturing prey and eating it with a rasp-like tongue (radula).

Glaucus (Figure 8.7G) is a planktonic nudibranch, and may be washed ashore with its prey, which includes the harmless Velella (a colonial pelagic hydrozoan related to jellyfish) or the related, but far more potent, blue-bottle (Physalia). Remarkably, the stinging cells of Physalia seem to be grazed undischarged, which Glaucus incorporates into its lateral extensions for its own protection.

Heteropods are ecologically similar to pteropods, but are highly modified snails (prosobranch gastropods, Figure 8.7C, Atlanta). They are laterally compressed, with a small shell beneath the foot modified into a single ventral fin (i.e. they swim upside down). The female Firoloida possess a permanent egg filament protruding from behind (Figure 8.7I). It is nearly translucent except for the gut and eyes, and feeds on small crustaceans and gelatinous zooplankton. It is typical of tropical, offshore zooplankton. The purple snail Janthina is a large and holoplanktonic gastropod snail that builds a raft of bubbles for a float and can be washed up on the beach during summer (Figure 8.7H).

Frequently there may be many tiny gastropods and bivalves in a zooplankton collection, which have metamorphosed from larvae to juveniles and are ready to settle onto the bottom (Figure 8.7D). When alive, the small bivalves may be observed each extending out their slender mollus-can foot between the shells and flipping themselves around. When dead, they may often be distinguished by the presence of concentric growth lines (Figure 8.7F). Other planktonic molluscs in estuarine samples are rare, but coastal and oceanic samples are rich with squid and cuttlefish larvae or juveniles.

Figure 8.7 Irregular zooplankton showing (A1, A2) actinotroch larva, (B) brachi-opod larva, (C1, C2) planktonic snails (heteropods), (D1-D5) other planktonic snails (shelled pteropods), D6 a shelled pteropod and similar appearance to a larval snail, D7 an echinospira larva - the veliger larva of an unusual gastropod snail, (E) a shell-less planktonic snail or naked pteropod, (F) bivalve larvae, (G) Glaucus, planktonic nudibranch, (H) the unusual prosobranch snail, Janthina with its bubble raft, (I) planktonic snail (heteropod, Firoloida), (J1-J4) larval beach worms and pelagic polychaete worms, (K) marine insect, water strider, Halobates (Sources: Dakin and Colefax 1940; Wickstead 1965).

Figure 8.7 Irregular zooplankton showing (A1, A2) actinotroch larva, (B) brachi-opod larva, (C1, C2) planktonic snails (heteropods), (D1-D5) other planktonic snails (shelled pteropods), D6 a shelled pteropod and similar appearance to a larval snail, D7 an echinospira larva - the veliger larva of an unusual gastropod snail, (E) a shell-less planktonic snail or naked pteropod, (F) bivalve larvae, (G) Glaucus, planktonic nudibranch, (H) the unusual prosobranch snail, Janthina with its bubble raft, (I) planktonic snail (heteropod, Firoloida), (J1-J4) larval beach worms and pelagic polychaete worms, (K) marine insect, water strider, Halobates (Sources: Dakin and Colefax 1940; Wickstead 1965).

The veliger of a beach worm (polychaete - literally 'many chaetae' or small spines) soon begins to grow the many repeated segments characteristic of the true worms (Figure 8.7J). Each segment may have a pair of fleshy limbs (parapodia) with bundles of chaetae extending out. Juvenile polychaete worms may be recognised in plankton samples as they curl up into a ball exposing the many chaetae (like a tiny echidna). Adult polychaetes may also be caught at night when they swim up off the sediments into the plankton, often for breeding. At least one family of holoplanktonic polychaetes are known (Tomopteris, Figure 8.7J2), but are relatively rare in our local zooplankton.

Even rarer are the linguilid larvae of the Brachiopoda, and the actinotrocha larva of the Phoronida (Figure 8.7A). Insects in your plankton samples are usually blow-ins, but there is a remarkable water strider that can be found on the surface of the warm oceans, far out at sea (Halobates, Figure 8.7K). Sea mites are also known.

Was this article helpful?

0 0
10 Ways To Fight Off Cancer

10 Ways To Fight Off Cancer

Learning About 10 Ways Fight Off Cancer Can Have Amazing Benefits For Your Life The Best Tips On How To Keep This Killer At Bay Discovering that you or a loved one has cancer can be utterly terrifying. All the same, once you comprehend the causes of cancer and learn how to reverse those causes, you or your loved one may have more than a fighting chance of beating out cancer.

Get My Free Ebook


Post a comment