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Loimia medusa (Savigny in Lamarck, 1818) 

Spaghetti-Worm or the Red-Spotted Worm


John McLaughlin (2014)

 


Fact Sheet

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Summary


Brief Summary


Classification


Names


Physical Description


General Body Plan


The Tentacles


Colouration


Ecology


Habitats


Crypsis


Larvae


Behaviour


Overview


Feeding


Predatory Defense Mechanisms


Survival Mechanisms


Reproduction


Reproductive Characteristics


Reproductive Strategy


Case Study


Ecological Role


Overview


Secondary Production


Sediment Processing


Biogeographic Distribution


Life History


Larval Development


Building of Sandy Tube


Early Benthic Development


Conservation and Threats


References & Links

Larval Development



Loimia medusa larva are a common inclusion of major seasonal planktonic gatherings. Immediately recognised by its bulky gelatinous case, its general body shape is fusiform, but one that is constantly changing as the larva constantly move around. Continuous dilations and constrictions repeatedly pass throughout the body in both directions as the larva elongates and shortens whilst twisting and turning. When shortened, it appears as a large cylindrical gelatinous mass much larger than the size of the larva itself. Rather hard to view due to its perfect transparency, sticking of debris to its outer surface is almost the only way to distuish the organism from the water around it. Contained in the mass runs a tunnel from one end through to the other, in which the larva itself lives, poking out its head and stretching forth its tentacles whilst it floats about the water. The mass itself increases in size as the larva grows, or in some cases the larvae desert their temporary homes and simply create another. The secretion of this gelatinous mass is rapid, and takes less than 24 hours for the larva to be once again completely encased. When free-living in the water column, the animals can swim forward through rapid, vigorous lateral undulations of the body with is tentacles thrown out to both sides. The larvae display little to no pigments and is predominantly transparent with its internal organs clearly in view. The prostomium however is speckled with dark brown which later appear on the ventral shields with some of the specks also being viewed on the tentacles themselves and the upper lip. The gut of L. medusa is greenish yellow or brown, with colour being dependent on its contents. It is this colour being displayed through the transparent body-wall which gives the larvae its complexion.
At 1mm L. medusa is readily recognisable as a terebellid. The medium tentacles begin budding in front of the prostomium, over the great arch of the upper lip. At this stage seven pairs of bristle-bundles are already present, and below each of these excluding the first is a long muscular process which bears a single uncinus of a larval type, continuing posteriorly. A small number of short ‘sensory’ cilia are present on the tips of the tentacles at later stages. The upper lip begins to form a half-tunnel at the base of which the mouth begins to open. Within the mouth itself a prominent tongue-like process, which is actually a thickening of the floor of the buccal cavity, is frequently rolled forwards and backwards before being rotated once again. This organ is most likely used in constructing the tube later in life, and is called the buccal organ. The oesophagus of the organism passes backward to invaginate into a stomach, correlating to an increase in the gut. The initial segment of the stomach is thick-walled followed by a thin-walled portion which eventually passes into the intestine. The gut itself is much longer than the body and hence is looped into folds within the body cavity in which it is suspended by threads of tissues spanning from the body wall. By this stage both the cerebral and dorsal glands are visible behind the prostomium. At this time the developing glands of the ventral shields, the heart and muscles for moving the bristles, and a peculiar larval nephridum anterior to each statocyst are also viewable. 
As development progresses the larvae increase in both width and girth, whilst the upper lip continues to grow. The medium tentacle elongates whilst the buds of others begin to show on each side, giving rise to tentacles in an alternate fashion. These tentacles become deeply grooved both ventrally and dorsally whilst new segments are added posteriorly. At this point an important change in development is noted. Bristles above the statocysts are lost and simultaneously the uncinigerous processes below what was initially the second, becomes the first, whilst the corresponding pair of bristle bundles is lost. 
Development then proceeds progressively with fresh segments being added posteriorly, whilst new tentacles appear and the old ones elongate. Bristle bundles continue to appear until a total of 17 is reached, at which point the parapodia change their character and develop into the abdominal type. At some point prior to this the first set of branchii appear. Following these events folds grow outwards from each side just posterior to the statocyst but between it and the first thoracic chaetae. At a similar time a ridge rises ventrally behind the mouth, passing up a little way on either side which gradually grows into a forwardly directed lamella in order to form a large post-oral platform so characteristic of the species. At this point uncini of the adult type are beginning to develop on the tori, and the first branchii begin to branch. In later stages all branchii, headfolds and papillae are all prominent, whilst all thoracic parapodia are present with the exception of the first pair and are now provided with tori of which the uncini of the adult type have developed. The abdominal parapodia, of which there are approximately 14, are also present with posterior pairings still rather undeveloped. The animals ventral shields are by now far more prominent, and the tentacles longer and in greater numbers. At this stage of development the total body length, inclusive of tentacles, is approximately 7.4mm.
Throughout this entire development the larvae has been encased within its gelatinous transparent mass. The timing of this development period is uncertain, as laboratory conditions may result in slower growth rates than those in natural systems, and growth and development appear heavily reliant on food availability. 

Classification

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Kingdom
Animalia
Phylum
Annelida
Class
Polychaeta
Order
Terebellida
Family
Terebellidae
Genus
Loimia

Synonyms

Terebella medusa (Savigny, 1822)

Common Names

medusa worm
spaghetti worm
Medusenwurm
Spaghetti-Wurm
Kingdom
Animalia
Phylum
Annelida
Class
Polychaeta
Order
Terebellida
Family
Terebellidae
Genus
Loimia
Species
medusa