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> Other marine organisms

Phylum Platyhelminthes - Flatworms

Flatworms are most commonly known for their parasitic members including flukes (Class Trematoda) and tapeworms (Class Cestoda). Free-living flatworms (Class Turbellaria) are found in tropical shallow waters. They are the most spectacular members of the group and hence often confused with the nudibranchs.

Most free living flatworms feed on detritus or small invertebrates. They do not have gills for respiration. Oxygen is absorbed from the surrounding water by diffusion through the skin. Flatworms normally crawl on the bottom but they can swim by undulating their thin and flattened bodies.

 

Pseudobiceros bedfordi
Pseudobiceros  bedfordi 
(Pseudocerotidae)

This distinctively colourful flatworm is one of the most common flatworms seen in Hong Kong and the Indo-Pacific. Sometimes, you may find them swimming in the water column with a beautiful, undulating motion. It mainly feeds on tunicates and crustaceans.

 

 

 

 

Phylum Annelida - Polychaetes and other worms


Phylum Annelida consists of three major groups that include terrestrial (e.g. earthworms), freshwater (e.g. leeches) and marine species (e.g. polychaetes). The distinguishing characteristic of an annelid is the segmented body. 

Annelid is divided into a linear series of similar segments from the head to the tail. Growth involves formation of a new segment anterior to the tail. The head bears sensory organs for processing environmental information.

Most of the polychaetes are marine dwellers. They are extremely abundant and widespread in the ocean. Many of them are benthic (e.g. bristle worms), while some are pelagic and tube-dwelling (e.g. fan-worms). With different body forms, polychaetes feed in different ways, such as deposit feeding, filter feeding and raptorial feeding.  

Polychaete species usually seen by divers are those that reside in a calcareous tube and have colorful tentacle crowns. The tentacle crowns extend from the tube for gas exchange and ciliary suspension feeding. All belong to the family Serpulidae. 

Spirobranchus tetraceros is a close relative of the christmas tree worm commonly found in tropical waters. They posses two colourful tentacle crowns which are flat rather than spiral-shaped. This species is commonly found as a group of individuals on massive corals such as Porites, Montipora and Platygyra in Hong Kong.

 

 Spirobranchus tetraceros

 

   

 

Protula bispiralis

 

The Coco Worm, Protula bispiralis, inhabiting a large tube of up to 20 cm long and 1 cm wide, has two pinkish spiral tentacle crowns. This species is usually solitary and is common throughout the tropical Pacific reefs. 

   

 

Hydroides elegans, a small worm with a tube of up to 2.5 cm long and 0.1 cm wide, is the most common species on shallow hard structures throughout the tropical Pacific. The photo shows the tube, tentacle crown and operculum that can close the tube following withdrawal of the tentacles.

 

Hydroides elegans

 

   

 

Sabellastarte japonica

 

Fan worms belong to family Sabellidae. Like serpulids, they have two tentacle crowns, but their tube is parchment-like and covered with mud debris. They usually live on dead corals or in coral sand, with their tentacle crowns extending into the water column. The feather duster Sabellastarte japonica is a common sabellid in local coral communities. This species is usually solitary, found in rock crevices or sand.

   

Instead of living inside a tube, some polychaetes bore into live corals directly. One of such species associated with living massive corals in Hong Kong is the flabelligerid worm Pherusa parmata. This species has a cephalic cage composed of about 30 long chaetae, which can be extended into the water to capture food. The surface of the chaetae is often coated with fine mud.

 

Pherusa parmata

 

 

Some polychaetes are common inhabitants of corals, but they are cryptic and are normally not visible unless being extracted from coral sand or broken coral skeleton. Among them are highly mobile nereidids, eunicids, phylodocids and polynoids.

 

 

Perinereis cultrifera

 

Perinereis cultrifera, the photo shows the anterior part with an everted pharynx. The head possesses eyes, sensory palps and tentacles. Each segment of the body bears a pair of parapodia.

   

The ventral view of Eunice antennata shows the chitinized jaw plates of the partly everted pharynx.

 

Ventral view of Eunice antennata

 

   

 

Dorsum of Lepidonotus squamatus

 

The polynoids, commonly known as “scale worms”, have an arched back that is covered by a series of overlapping leaf-like “scales”. Scale worms are predators of small animals. Some species are commensals of corals, sea stars and sea anemones. Others are commonly found in crevices or under stones on the sea bottom. The photo shows the dorsum of Lepidonotus squamatus. Each segment is covered by a pair of “scales”. On the sides are dosal cirri and chaetae.

 

Acknowledgment: The above information and photos are provided by Dr. Jian-Wen Qiu, Department of Biology, Hong Kong Baptist University

 

Chloeia flava
 Chloeia  flava  (Amphinomidae)
The brown markings on each segment of the body distinguish this fire worm from the others. The bristle feet are filled with venom, which will cause a burning pain when touched. It can be seen crawling along the sandy sea bottom in search of prey, or occasionally, swimming in the water column.

 

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Last Review Date : 22 March 2017