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CLASS HOLOTHUROIDEA
(SEA CUCUMBERS)By:
Christian Jay Rayon NobBS-Marine Biology
Mindanao Sate University-Naawan Campus
Sea cucumbers are an abundant and diverse group of worm-like and usually soft-bodied echinoderms. Bony fragments along the animal's skin give the appearance of bumps and warts similar to the vegetable from which the sea cucumber derives its name.
They are distinguished from other echinoderms in having the polar axis greatly lengthened, which results in the body having an elongated cucumber shape.
Natural history
HISTORY ON SYSTEMATICS
The name Holothuroidea is derived from the term holothourion named by Aristotle. He based many of his accounts of plants and animals, including marine forms, on first-hand observations.
He refers this organism from the sponges, which is without feeling and motionless, as well as being free and unattached and separated from the ground but plant-like.
This is an unexpectedly inaccurate description of holothuroids from the father of western observation.
WORKERS ON SYSTEMATICS
The term holothurion was also applied to cnidarians, tunicates and priapulid worms until the late 18th century.
Belon (1553) was the first to recognize the resemblance of holothuroids to other echinoderms by the similarity of their tube feet in 19th century.
Holothuroids, because of their soft body and bilateral, vermiform appearance, often served as a connection in the linearly arranged classificatory arrangements between echinoderms .
Validity of the Sea cucumber
Alexander Agassiz in an 1865 letter to Fritz Muller wrote that Darwinists would undoubtedly interpret the similarities of echinoderm.
Holothuroids, because of their soft body and bilateral, vermiform appearance, often served as a connection in the linearly arranged classificatory arrangements between echinoderms.
Holothuroids were often seen as the most advanced echinoderm because they were the least radially symmetrical .
Carolus Linnaeus which originally referred most holothuroids to Fistularia, a name preoccupied by a fish.
Holothuria was formally assigned to the class of echinoderms in 1924.
Bohadsch (1761) and Pallas (1766) provide the first anatomical accounts of holothuroids.
Cucumaris marimus is the first discovered species of sea cucumber.
As for most soft-bodied animals, holothuroids have a poor fossil record.
Published accounts exist of body fossils for about 19 species.
Most ancient holothuroids are known from fossils of isolated ossicles.
Entire or isolated elements of the calcareous ring are also known.
FOSSIL HISTORY
Isolated pieces of the calcareous rings of fossil holothurians
Holothuroids probably evolved by at least the Lower Silurian, most likely from a little known group of extinct Palaeozoic echinoderms called ophiocistioids.
The oldest reported body fossil of a holothuroid is from the Lower Devonian, while the oldest undoubted ossicle is from the Upper Silurian.
BIODIVERSITY OF
SEA CUCUMBER
More than 1500 species of sea cucumber have been described worldwide, with the highest number in the Asia Pacific.
Over 170 sea cucumber species are recorded in the Philippines, majority of them can be found in Mactan, Cebu.
Stichopus is the largest species that is found in the Philippines that may attain a length greater than 3 ft. and a diameter of 8 in.
Sea cucumbers are not only important on aquatic organisms but also on economically valued.
They are often dried to preserve them and are used as food and in herbal and pharmaceutical medicines.
Sea cucumbers contain several vitamins such as: ◦Mucopolysaccharides◦ chondroitins◦ protein◦ vitamins A & C◦ riboflavin◦ niacin◦Calcium◦ iron◦ magnesium◦zinc◦sodium ◦carbohydrates
Living holothuroids are divided into six orders. Each order is described according to its taxonomic diversity and major diagnostic features:
ApodidaFootless sea cucumbers, contains about
269 species in 32 genera and three families. Tentacles are digitate, pinnate or, in some small species, simple. Respiratory trees are absent. Tube feet are completely absent.
Six orders of sea cucumber
ElasipodidaDeep-sea sea cucumbers, contains about 141
species in 24 genera and five families. Tentacles are shield-shaped and used in shoveling sediment.
Aspidochirotida Shield-tentacle sea cucumbers, there are about
340 species in 35 genera and three families. Tentacles are shield-shaped. Respiratory trees are present.
Molpadiida Rat-tailed sea cucumbers, there are about 95
species in 11 genera and four families. Tentacles are simple. Respiratory trees are present.
DendrochirotidaContains about 550 species in 90 genera
and seven families. Tentacles are highly branched and extended to filter material from the water column. Respiratory trees are present.
DactylochirotidaContains about 35 species in seven
genera and three families. Tentacles are simple or with a few small digits. Respiratory trees are present.
SOME SEA CUCUMBER SPECIES
Parastichopus californicus
California Sea Cucumber The California Sea Cucumber grows up to 50cm long. They range in colors
from red in juveniles to brown or mottled brown, with all white individuals occurring rarely. They are covered on the dorsal side with flesh colored papillae and on the ventral side by tube feet. A circle of peltate feeding tentacles surround a sub terminal mouth, directed ventrally at the anterior end.
Cucumaria miniata
Burrowing Sea Cucumber The Orange sea cucumber received its Latin
name, Cucumaria because it resembles a cucumber. Although it seems to be completely soft and fragile, it actually has bone-like plates in the body wall called ossicles. To stay attached to the holes between the rocks, the cucumber uses tube feet that you can see in the image above in 5 rows around the circumference of the body.
Eupentacta quinquesemitaWhite Sea Cucumber
Eupentacta quinquesemita is stiff to touch due to abundant calcareous ossicles in the skin and tube feet. The body grows 4-8 cm in length. The non-retractile tube feet give it a spiny look. The two ventral feeding tentacles are smaller than the other eight. This character is useful for identifying this species when only the tentacles are visible. The expanded tentacles are creamy white with tinges of yellow or pink at the bases.
Holothuria scabra
Sandfish The sandfish is grayish-black on the upper side with dark-
colored wrinkles but paler on the underside. It grows up to 4cm long, is broader than it is high and has a pliable skin. It is covered by calcareous spicules in the form of tablets and button. This species is found in shallow water on soft sediments throughout the Indo-Pacific region that range into the depth of low tide to 30 meters.
Stichopus vastus
Brown curryfish They are found in Indo-Pacific Ocean. Inhabits sandy and rocky
inner and outer reef bottom areas near heavy growths of coral. Can reach a length of about 26 inches (70 cm). Feeds on detritus and organic matter that is sifted from the substrate. Rarely found in the trade and too large for most aquaria.
Thelenota ananas Prickly redfish
They can grey, orange, to red sea cucumber, often with a purple cast. It is a very large species (to 600 (750) cm), square in cross section with prominent 'cockscomb' papillae over its upper surface. It has a thick but pliable body and a smooth tegument. Depth range is 1–35 m.
Thelenota anax
Amberfish T. anax is a cream or brown sea cucumber mottled with red
or brown. Inhabits sandy and rocky inner and outer reef bottom areas. And can reach a length of about 40 inches (100 cm). Feeds on detritus and organic matter that is sifted from the substrate.
Holothuria whitmaei
Blackteatfish Holothuria whitmaei is a large, solid holothurian that is
always black in color and often has a thin coating of sand clinging to the upper surface. There are often several protrusions around the edge of the body where the flat underside meets the steeply-rising sides, giving the species its common name of Black Teatfish.
Holothuria atra
Lollyfish Holothuria atra is a sausage-shaped sea cucumber that
can grow to a length of 60 centimetres (24 in) but 20 centimetres (7.9 in) is more common size. It has a smooth, pliable, entirely black skin which often has sand adhering to it, especially in smaller individuals. The mouth is on the underside at one end and is surrounded by a fringe of 20, black, branched tentacles. The anus is at the other end.
Stichopus horrens
Dragonfish (Selenka’s sea cucumber) Stichopus horrens is a variable, grey to green or black sea
cucumber. It is often variegated with dark patches. It is a medium-sized species (to 300 mm) with a smooth tegument but large and irregular papillae. The big tubercles and irregular body form give an "irregular, soft and almost repulsive" appearance. S. horrens may be found on reefs, below rocks on flats.
a. anal teeth:
usually five in number; each a hard (calcified) triangular structure embedded in the anal wall.
b. anus:posterior (rear) opening of the gut.
c. cuverian organs: consisting of sticky white thread- or ribbon-like structures which are thrown out from the anus of some species; a special defene mechanism.
d. eviscerate: to throw out its guts (through the anus). e. papillae: similar to tube feet but smaller (commonly a few
millimeters up to more than a centimeter in length), they are conical and pointed. Occurring chiefly on the back, they are also often seen on processes of the body wall.
TERMS IN SYSTEMATICS
f. pedicelsadapted from tube feet, they are small and tubular, as distinct from the conical papillae. Occurring on the back, they are commonly only a few millimeters long.
g. Podia Tube-like feet, generally ending in flat disks, which enable the sea cucumber to adhere to the substrate when moving.
h. water vascular system It provides hydraulic pressure to the tentacles and tube feet
allowing them to move. i. Tentacles
Tentacles around the mouth are used to gather food. j. Bivium the dorsal part of the body in the pentaradial symmetry, with 2
radii and 3 interradii. k. Calcareous ring internal collar of plates, generally 10, surrounding the pharynx.
l. Cloacaanal cavity where the intestine ends.
m. Digitations finger-like structures, used as descriptive term for the
shape of tentacles. n. Dorsal upper surface of the animal. o. Lateral
At the side of the animal. p. Ossicles
or “spicules”, are microscopic carbonate skeleton particles in the body wall, tentacles, podia, papillae, and other body parts, useful for species identification; they come in various shapes.
q. Peltate:Describing a structure that is circular or lobed with a stalk in the middle; used to describe the shape of the end of tentacles in Aspidochirotida that are used for deposit feeding on the sea floor.
r. Posterior At the rear ends of the structure or animal.
S. Respiratory tree Arbore scent organ (1 pair), opening in the cloaca, which
fills with water to enable the animals to respire. t. Subdorsal Appearing near, but not quite on, the very upper surface
of the animal; half-way between terminal and dorsal; mostly used here to indicate the position of the anus.
U. Teats large papillae at the border of the ventral surface of the animal.
v. Tegument: The outer tissues of the animal, including the cuticle and epidermis.
w. Terminal Occurring at the very posterior end, facing directly posterior.
x. Tranverse Across the body, perpendicular to the main axis of the body.
y. Trivium The ventral surface of body in the pentaradial symmetry, with 3 radii and 2 interradial areas.
z. Ventral On the bottom, or under surface, of the animal.
EVOLUTIONARY TREES AND
ROOTS
Phylogenetic tree of Sea Cucumber
Sea cucumber belongs to phylum Echinodermata. This soft-bodied marine-dwelling echinoderm from class Holothuroidea is unique due to the existence of evolved skeleton and ancient-looked respiratory system called respiratory tree possessed by few species.
Sea cucumbers possess pentaradial symmetry. However, because of their body bearing, they have secondarily evolved a degree of bilateral symmetry.
Most sea cucumbers reproduce by releasing sperm and ova into the ocean water. Depending on conditions, one organism can produce thousands of gametes. Sea cucumbers are typically dioecious. The reproductive system consists of a branched gonad.
CHARACTERS OF SEA CUCUMBER
The body of the sea cucumber lacks arms, instead the body is covered in tube feet. Surrounding the mouth is eight to thirty modified tube feet called tentacles.
Sea cucumbers do not have eyes, many of them are light sensitive.
Each tube foot has tiny suction cup on the end, allow the sea cucumber to feel and move.
Have a calcerous ring that encircles the throat and serves as the attachment point for muscles operating the oral tentacles.
Sea cucumbers have an orally-aborally elongated body The pentamerous symmetry is sometimes recognizable by the presence of 5 meridional ambulacra bearing podia. Sea cucumbers live on the substrate of the sea floor with their ventral surface. This creeping sole bears the locomotory podia, while on the dorsal surface, the podia are often displayed by papillae.
EXTERNAL MORPHOLOGY OF THE SEA CUCUMBER
External anatomy of sea cucumber
Types of tentacles
INTERNAL MORPHOLOGY OF SEA CUCUMBER
TYPES OF OSSICLES
Ossicles are the spicule constitutes one characteristics of the class and are of primary importance for identification, which is done from alcohol-preserved specimens. Spicules are calcareous bits of microscopic size. There variety of shapes that can be seen in the microscope. Their shapes are one of the sources to identify a specific species.
The calcareous ring is comprised of a series of plates, usually ten, joined side by side like a collar around the esophagus.
Each plate may be a solid piece, or in some species, a mosaic of smaller segments. Being one of the few hard structures in a sea cucumber, the calcareous ring is often the only part that fossilizes, thus providing a way of relating extinct and living forms.
Types of calcareous rings
1. a. Podia are present on both surfaces of the body………………………Thyone, Cucumaria b. Podia have completely disappeared dorsal and lateral part of the body…………………………………………………………….....Holothuria
2. a. Podia are more or less restricted to the five ambulacral areas………………………………………………………………….....Cucumaria b. Podia are scattered over the entire body surface…………………………….……….Thyone
3. a. Tentacles are irregularly branched………………………………………..…………….Psolus b. Tentacles are regularly branched……………………………………Synapta, Leptosynapta
4. a. Surface dwellers…………………………………………..……………Cucumaria, Holothuria b. Surface burrowing species……………………………………………….……………….Thyone
5. a. Species burrow completely beneath the surface…………………..………….Leptosynapta b. Species that are relatively sedentary and excavate a U-shaped burrow…………………………………………………………….Thyone, Cucumaria
DICHOTOMOUS KEY OF SEA CUCUMBER
6. a. The pharynx leads into a slender esophagus…………………… ………Thyone
b. The pharynx appears to open directly into the intestine…………….Cucumaria,
Holothuria 7. a. Evisceration involves the rupture of the cloaca and the
expulsion of one or both respiratory trees ,digestive tract and the gonad…………..………..Stichopus,
Actinopyga b. Evisceration involves falling out of pharynx and associated organs, and at least part of the intestine are
expelled……………..…Thyone
1. a. Podia absent; body vermiform; body wall thin, often translate; dominant spicules in form of anchors with associated with anchor plates tentacles pinnate; pharynx without retractor muscle; no respiratory tree..………………………………………………………………………Apodia.
b. Podia presents; body wall moderately thick; body wall with dominant spicules in form, of tables, perforated plates, buttons, rods, or rosettes…….………………...……………………………….………… →2.
2. a. Tentacles peltate or pelyo – digitate; anterior end of body not introverted and associatedwith retractor muscle…………………………Aspidochirotida (The only order with commercial species in the area).
b. Tentacles branched (dendritic); anterior end of body introverted, associeated with retractor muscles ………………….…..…………………..Dendrochirotida.
Key ID for the shallow water orders of class Holothuroidea
1 a. Body with trivium (sole) usually flattened and dorsal bivium convex; gonads forming a single tuft appended to the left dorsal mesentery; cnvierian organs present or absent; dominant spicules of form tables, buttons (simple or modified), and rods (excluding c – and s – shaped rods)…………………………………………………….Holothuridae.
b. Body square – shaped or trapezodial in cross section; cuvierian organs always absent;
gonads forming 2 tufts appended on each side of the dorsal mesentery; dominant spicules
in form of branched rods c – and s – shaped rods …………………………………………………….……Stichopodidae.
Key ID for the shallow–water families of Aspidochirotida:
1 a. Tentacles 20-30; body wall very thick; podia
short, more or less regularly arranged on bivium and trivium
……………….Parastichopus
b. Tentacles leaf shape, 18-20; body wall moderately thick; podia irregularly arranged
on the bivium, and scattered papillae in 3 rows on trivium…………………………………….…….Stichopus
Key ID for the shallow –water genus of Stichopodidae
1. a. Body wall ossicles anchors and anchor plates (Family Synaptidae)…………................................... 2
b. Body wall ossicles wheels, most common near posterior of body (Family Chiridotidae)……Chiridota ferruginea
2. a. Anchor plates narrowed posteriorly to form a handle................................................... Labidoplaxbuskii
b. Anchor plates rounded posteriorly; no handle.....................................................................................3
3. a. Anchor plates more than 140 μm long……...................................................Leptosynapta tenuis b. Anchor plates less than 110 μm long...........................................................Epitomapta roseola
Key to members of the Order Apodida
EVALUATION OF TREES
Phylogenetic relationship among animals (A) and echinoderms (B), the tree(A) implies that Echinoderms (Phylum Echinodermata) are placed in the group of deuterostomes, with other phyla (Chordata, Xenoturbellida and Hemichordata. Not all phyla are included in this tree. And Protostomes comprise the groups of lophotrochozoans (phyla Platyhelminthes, Annelida etc.) and ecdysozoans (phyla Arthropoda, Nematoda etc.). Porifera and Cnidaria is basal of all phylum. They are five present classes of echinoderms. The position of class Holothuroidea is at the bottom most of the tree (B).
The methods of modern comparative biology had not been applied to these problems until quite recently. Then Littlewood et al. (1997), in an effort to resolve class-level relationships within echinoderms, sequenced two ribosomal genes from a total of four orders. Their analyses consistently supported a close relationship between Dendrochirotida and Aspidochirotida, but they could not resolve the phylogenetic position of Elasipodida and Apodida (Figure 19: A, B). Smith (1997) subsequently argued that the Elasipodida are more closely related to (Dendrochirotida + Aspidochirotida) than the Apodida . This hypothesis recalls an early speculation (Semper 1868) whereby Apodida is sister to the remaining holothuroids.
Phylogenetic tree of sea cucumber according to its order
MOLECULAR SYSTEMATICS
ON SEA CUCUMBER
Molecular systematic is the use of molecular genetics to study the evolution of relationships among individuals and species. The goal of systematic studies is to provide insight into the history of groups of organisms and the evolutionary processes that create diversity among species.
MOLECULAR SYSTEMATICS
A molecular phylogenetic analysis of Holothuroidea was undertaken in order to clarify the systematics and taxonomy of this class. DNA sequence for portions of 16S mitochondrial genes was obtained from the species of sea cucumbers. The resulting molecular phylogeny using maximum likelihood methods of the Holothuroidea conflicting existing taxonomy, which is based largely on the morphology of calcareous parts. In particular, evolutionary relationships among the unclear species were clarified. The results confirmed that some species of sea cucumber such as Holothuria fuscogilva, Actinopyga mauritiana, Holothuria nobilis and Sea cucumber leaves, were identical to Cucumaria frondosa, Pentacta pygmaea, and Cucumaria pseudocurata,Psolus fabricii.
HISTORICAL CONTEXT
Phylogenetic analysis based on mitochondrial DNA (mtDNA) is a powerful technique , particularly for closely related species, due to high rate at which nucleotide substitutions accumulate in this molecule. As the complete mitochondrial genome sequences of more organisms have been determined, it has become increasingly clear that gene rearrangements have occurred in many lineages, and it has therefore been suggested that mitochondrial gene order may also serve an informative phylogenetic role at these deeper levels.
BASIC TECHNIQUES
Accordingly, sea cucumber mtDNA has been proven useful in addressing questions of population genetic structure, taxonomic status, conservation, zoogeography and geographic variation.
In general, molecular systematic aims to:
1) obtain partial sequences of COI mtDNA of selected sea cucumbers
2) study the phylogenetic relationship of sea cucumber species by using COI mtDNA gene sequences
3) update the species validity between and among present species of sea cucumbers
using COI mtDNA gene as an alternative to morphological studies 4) study the usefulness of COI mtDNA gene in phylogenetic
analyses of sea cucumbers 5) suggest further and future recommendation in molecular study
of indigenous sea cucumber in terms of sampling sites, sample size, methodology and concentration based on this preliminary study.
The source of all species variation of sea cucumber is the genome.
DNA has become a powerful systematic tool to investigate evolutionary relationships within and among species of sea cucumber.
Investigating three aspects of molecular systematics: (1) data collection , in this case the analysis of
DNA sequence data from an automated DNA sequencer (2) analysis of molecular evolution: patterns of mutational change between more and more distantly related creatures, and
(3) DNA systematics, the use of DNA sequence variation to establish phylogenetic relationship among species of sea cucumber.
IMPACT ON PHYLOGENETICS
Molecular study shows that the phylogenetic relationship of sea cucumber and higher level was not resolved by using partial sequences of COI mtDNA gene.
The findings also did not conform to the morphology or taxonomic classification of sea cucumbers.
Thus, more intensive studies are needed to get better view of the phylogenetic relationship and also to validate the species records of sea cucumbers. More samples of sea cucumber species from various genera and families are also required in the future in order to get more reliable and up-to-date data, morphologically and genetically. Furthermore, other molecular approaches such as microsatellites analyses must be considered.
LIMITATIONS OF MOLECULAR PHYLOGENETICS
CASE STUDIES TO ILLUSTRATE
MOLECULAR AND MORPHOLOGICAL PHYLOGENETICS
Molecular Phylogeny of Egyptian Sea Cucumbers As
PredictedFrom 16s Mitochondrial rNA
Gene Sequences
By:Atif M. El-Naggar, Neveen A. Ashaat, Hussein I. El-
Belbasi and Mohamed S. Slama
A molecular phylogenetic analysis of some Holothuroidea was undertaken in order to clarify the systematics and taxonomy of this class in the red sea. DNA sequence for portions of 16S mitochondrial genes was obtained from 4 species of sea cucumbers. The resulting molecular phylogeny using maximum likelihood methods of the Holothuroidea conflicting existing taxonomy, which is based largely on the morphology of calcareous parts. In particular, evolutionary relationships among the brooding species were clarified. The current results confirmed that the species Holothuria fuscogilva, Actinopyga mauritiana, Holothuria nobilis and Sea cucumber leaves , were identical to Cucumaria frondosa, Pentacta pygmaea, and Cucumaria pseudocurata Psolus fabricii , respectively with Query coverage 100%, Zero E- value and Max identical 100%.
It was suggested that an in-depth phylogenetic analysis of sea cucumber families will require examination of additional, more slowly evolving, regions of the genome(s).
The current results comprise the first phylogenetic test of the classification of the Holothuriidae. This phylogeny corroborated some aspects of the currently used classification; morphologically there is no database for identification and classification of the selected samples. The classical methods using for example spicules appear to be not enough to the explicit taxonomy.
However, the systematic of sea cucumbers based on morphology particularly in Egypt is still unclear thus, requiring molecular methods as alternatives to solve the problem. It is clear from the previous and present results that there is a large gap between the morphological and molecular phylogeny.
RESULTS AND FINDINGS
It could be concluded that molecular phylogeny is more distinct than morphological approach. Using 16S rDNA is an easy way to identify any Sea cucumber even it is a sea cucumber leaves to species level. Molecular phylogeny for Sea cucumber of the Red Sea needs further studies to establish a reliable database which can depend on it.
Phylogeny of the Apodan Holothurians
(Echinodermata) inferred from morphology
By:ALEXANDER M. KERR
The Apodida is an order of littoral to deep-sea, largely infaunal sea cucumbers with about 270 extant species in 32 genera and three families, Synaptidae, Chiridotidae and Myriotrochidae. In this study, the author perform the first phylogenetic test of the taxonomic and palaeontological hypotheses about evolutionary relationships within Apodida by using cladistic analyses of 34 morphological characters. He introduce several previously unconsidered synapomorphic characters, examine the relationships between all recognized suprageneric taxonomic groups and assess the assumptions of monophyly for each family. Maximum-parsimony analyses of type species from 14 genera and use of three rooting methods recovered identical topologies at the subordinal level and subfamily level within Synaptidae.
Overall, the current higher-level classification of Apodida was well corroborated. Within Synaptidae, the relationships (Synaptinae, (Leptosynaptinae, Rynkatorpinae)) are well supported. The monophyly of Chiridotidae was not supported and appears paraphyletic at the subfamily level. Calibrating the phylogenetic hypothesis of Apodida against the fossil record indicated that most higher-level divergences occurred within the Palaeozoic, unlike that of extant non-holothuroid echinoderms, which radiated in the early Mesozoic. Synaptidae appears to have radiated during the Lower Cretaceous. Alternatively, and if one discounts the considerable ghost lineage duration that this hypothesis requires, they may have radiated during the Eocene.
TAXONOMIC IMPLICATIONS◦The results of this study constitute the first phylogenetic classification of Apodida.
◦He recommends subdividing the apodans into two orders, Synaptina and Myriotrochina.
RESULTS AND FINDINGS
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