Lab 3 – Annelida OEB 51 Lab 3: Annelida

17 February 2015

Earthworms and leeches (Clitellata) were previously thought to be the sister group to polychaetes, but we have known for a while now that they are actually clustered within polychaetes (and leeches nested within earthworms!). This means that polychaetes are not a monophyletic group and that Polychaeta is actually a synonym of Annelida! The same goes for the paraphyletic Oligochaeta, which is actually a synonym of Clitellata!

Before Current

Can you see where the name Polychaeta comes from?

Can you see chaetae in earthworms? What about in leeches?

1 Lab 3 – Annelida OEB 51

Annelid Dissection

Today we will attempt individual/partner dissections of two annelids: one marine ‘Polychaeta’ and one terrestrial earthworm (‘Oligochaeta’).

CHOOSE • Alitta succinea (Polychaeta) (previously Nereis limbata) ONE • Lumbricus terrestris (Oligochaeta)

There are printed detailed lab guides to help you through these dissections. Make sure that you use

anaesthetized specimens – either 5-7% EtOH for the earthworms or 7.5% MgCl2 for the nereidids.

You will want to keep the nereidids especially submerged in MgCl2 during dissection.

Here are some guidelines:

1. Clear a space for yourself on the bench and ensure proper illumination. Get small/medium scissors, and at least one pair of forceps and one probe

2. Before you begin making any cuts of any sort, do a thorough inspection and illustrate the external anatomy of the specimen, also drawing on the side the details of the head of your animal. Some things to look for:

Alitta: prostomium, peristomium, palps, antennae, eyes, pharynx (is it everted? You may attempt to force the animal to evert it by gently pulling by the jaws with fine forceps) Lumbricus: prostomium, peristomium, clitellum, chaetiferous segments, male and female gonopores, nephrostomes

3. Prefer fine scissors to make initial incisions, although you might want to use larger ones to make larger cuts

4. Avoid severing the dorsal blood vessel and most especially the gut. Doing either of these will flood the body cavity with the contents of either vessel, and obscure your dissection. Work slowly and calmly

5. For Alitta you should also remove one of the parapodia with fine scissors and investigate separately in a compound scope to see in more details. Can you make out notopodium/neuropodium, acicula, chaetae?

2 Lab 3 – Annelida OEB 51

Annelid Dissection (continued)

3 Lab 3 – Annelida OEB 51

Annelid Diversity We have a diversity of live polychaetes from Florida, plus several preserved specimens from the Museum of Comparative Zoology. In the next few pages you have specific families assigned, but if you find any others that interest you, let Tauana know so you can substitute one of the mandatory exercises.

• Chaetopteridae (Chaetopterus) – live! Commonly known as “parchment worms”, they inhabit U-shaped tubes and filter feed with a mucous net. Is the segmentation homonomous or heteronomous? How do the different parts help in the feeding strategy? Make a simple illustration of the body indicating what might be the function of specific parts.

4 Lab 3 – Annelida OEB 51

• Sabellidae • Serpulidae Choose one of these sedentary polychaetes (indicate which you chose to draw) and observe it submerged under seawater. Both families have branchial crowns, but sabellids have agglutinated tubes (sediments and mucous), while serpulids have calcareous tubes and an operculum within the branchial crown. How do these polychaetes feed?

If possible, gently remove it from the tube by cutting the tube or with fine forceps. Illustrate and label to the best of your ability. Pay special attention to the ventral/dorsal inversion of the body. Can you see the fecal groove in the ventral side in the abdomen and in the dorsal side in the thorax? Why might that be?

• Cut parapodia of one or a couple of segments and make a slide. Can you see the specialized chaetae for moving/attaching inside tubes (uncini)? How are these parapodia different from those of errant polychaetes?

IF DISSECTED EARTHWORM

5 Lab 3 – Annelida OEB 51

• Nereididae Alitta succinea Choose one errant form. Nereidids are large predatory worms. Observe the unanesthetized specimens closely – how do they move and how do the parapodia work for this purpose? Can you see the nefarious-looking jaws? Draw an overall perspective of this animal, and a detailed one of the head and first segments. IF DISSECTED EARTHWORM

6 Lab 3 – Annelida OEB 51

The following groups were previously thought to constitute their own phyla (Sipuncula, Echiura, Pogonophora, Vestimentifera). However, they are modified/unsegmented annelids!

CHOOSE • Sipuncula – live and preserved in ethanol ONE Themiste alutacea – live; T. lageniformis and Sipunculus polymyotus – preserved from MCZ Commonly known as “peanut worms”, sipunculans live in intertidal zones. We have relaxed specimens; if we’re lucky they may evert the introvert. Where is the anus on these creatures?

• Echiura – live and preserved specimens These largely unsegmented (except for the nervous system) worms live partly buried and use their elongated, spoon-shaped proboscis to draw food particles from the substrate to their mouth by ciliation. They contain two small bristles with homology to chaetae, verifying their position in Annelida.

• Siboglinidae (Oasisia and Riftia pachyptila) – preserved in ethanol from MCZ These highly modified annelids are found in association with hydrothermal vents and cold seeps. Their primary nutrition comes from symbiotic bacteria living inside a trophosome. The bacteria metabolize the sulfide-rich fluids emanating from the hydrothermal vents. Put the tube in a petri dish to observe and draw. Smell it!

7 Lab 3 – Annelida OEB 51

• Oligochaeta These worms have few chaetae and reduced cephalic sensory structures. Most of the segments are homonomous except for the segments that are modified to form the clitellum. Oligochaetes mainly live in limnic and terrestrial environments. Draw an overall perspective and details of head and clitellum area. IF DISSECTED POLYCHAETE

8 Lab 3 – Annelida OEB 51

• Hirudinea (leeches) Leeches lack chaetae and have bodies with fixed number of segments (34) that have superficial annuli; the main coelomic body cavity is largely absent. Their segments are heteronomous and have an anterior and posterior sucker. This group is mostly freshwater but there are some marine and terrestrial forms. They may be ectoparasitic, predaceous or scavengers (watch the video of a carnivorous leech eating a huge earthworm!). Draw a leech, labeling externally visible structures. How does their movement compare to that of the polychaetes and oligochaetes?

9 Lab 3 – Annelida OEB 51

Acoela (Hofstenia miamia, courtesy of Mansi Srivastava) Acoels are triplobastic acoelomates – Bilaterians, but not a deuterostome or protostome. Observe them moving in the petri dish. Does it have a directionality (anterior/posterior)? Put one individual in a regular slide with a drop of the water and, without covering it with a coverslip, observe in more details under the lower power of the light microscope (5x, 10x). Can you observe any structures (mouth, pharynx, pointed tail)? Make a simple representation of your acoel. Cover it with a cover slip with clay feet to see the statocyst (above the mouth) under high magnification. Draw one individual with the structures mentioned above.

10