Sepiella inermis is a small shallow water cephalopod, they can be found along the coasts of Singapore, from the intertidal (Changi beach, East Coast Park, Tanah Merah) to depths of 40 metres (Pulau Hantu, Tuas, in Seagrass meadows in the North of Singapore). Its common name is Spineless cuttlefish as they are differentiated from other cuttlefish genuses due to a lack of a structure on the cuttlebone that resembles a spine.
A: Diagram of basic cuttlefish features, with spine, B: Sepia zanzibarica, with spine, C: Sepiella inermis, without spine1 .
In Singapore it is also known as the Glittering cuttlefish due to the glittering iridescent spots on the edges of its mantle2 .
They are distributed very widely, from Singapore all the way to Mozambique, and they have an internal calcified shell called the cuttlebone which helps maintain buoyancy (allowing them to hover midwater). They are also able to change their skin color and patterns due to the presence of chromatophores on their skin. However, like other cuttlefishes, Sepiella inermis is harvested as sea food using trawls at night and there have been efforts to aquaculture these cephalopods.
Note that the vernacular Spineless cuttlefish will be used throughout the page instead of glittering cuttlefish for more clarity.
The spineless cuttlefish was first recorded as Sepia inermis by Van Hasselt in 1835 in Ferussac & d’Orbingy 1834-18483 . The scientific name has been changed to Sepiella inermis since 1884, to account for the group of species that did not have a spine on the cuttlebone and had a posterior gland. Therefore, as a new genus (Sepiella) was added into Family Sepiidae, the species name (inermis) was retained but the genus name was changed.
Some of Sepiella inermis's synonyms are: Sepia affinis (Souleyet, 1852), Sepia microcheirus (Gray, 1849), Sepiella maindroni (Rochebrune, 1884)4 .
Original description of Sepia inermis3
Although the location of the type specimen is currently unknown, it is known that the type locality of the specimen was India5 . Earliest specimen that is still in collection is a specimen from Naturalis Biodiversity Museum collected from the Java Sea in 1903 by Van Hasselt. The specimen just consists of the cuttlebone, and it can be seen from the picture below that it lacks a spine6 .
Specimen at Naturalis Biodiversity Centre, creative commons licensed
The next oldest specimen collected is situated in the Smithsonian Invertebrate Zoology collection, collected in 1909 by Palmer Bryant7 .
This species has been speculated to be a species complex, consisting of Sepiella cyanea1 , a similar species. It is also similar to Sepiella japonica, the Japanese spineless cuttlefish found in Japan only, with slight variation in cuttlebone morphology the only way to classify the two.
However, one study has suggested that cuttlebone morphology might not be a good indicator of species, given that cuttlebone morphology varies too much within species8 .
Sepiella japonica cuttlebone
Sepiella inermis cuttlebone, taken from Reid, Jerep and Roper, 2005
Due to the chromatophores and the changing external colors and patterns, it is hard to differentiate between Sepiella inermis and Sepiella japonica through external morphology alone.
Sepiella inermis sequences can be found at any of the links below:
Photo by Ria Tan
Spineless cuttlefishes can range from 4.5 cm to 12 cm, with the ones found in Singapore being smaller, ranging from 5cm to 8cm2 . The body of Sepiella inermis is oval in shape2 and dorsoventrally compressed1 .
Dorsal view of Sepiella inermis1
Like all other cuttlefish, Sepiella inermis have eight arms that are stout and approximately equal in length. The arms are laterally compressed and become slender towards the distal end. Similarly, the suckers on the arms progressively reduce in size as they move away from the head. The two tentacles are long and slender, expanding into clubs. A layer of thin membrane that covers the clubs, also known as the swimming membrane, are slightly shorter than the club itself9 . The suckers on the club are small and are arranged transversally in 12 to 24 rows1 .
The beak consists of the upper and lower beak. The beak of Sepiella inermis is highly pigmented, with the colour ranging from dark-brown black to light brown. The reason for the pigmentation is unknown.
The beak of Sepiella inermis specimen10
The beak is essential when it comes to feeding in the cephalopods. It is chitinous, consists of the upper and lower beak and is used for biting, crushing and tearing the prey. Sepiella inermis also has the radula which is a chitinous ribbon, likened to a tongue in molluscs, with minute teeth arranged in rows for scrapping while feeding9
Radula of Sepiella inermis under electron microscope magnification10 .
The head of the spineless cuttlefish is short and broad, connected to the mantle through a narrow neck.
Photo by Ria Tan
The eyes of this species are large and convex in shape11 . In all cuttlefishes, their pupil is in a striking W-shape in bright light and circular in darkness, much like how we dilate our pupils to adjust to the amount of light. This unique shape has been attributed to providing a better image contrast, helping illuminate the darker parts of their visual field. The structure of their eye is also such that they can see in front and behind of them 12 13 .
W-shaped pupil in Sepiella inermis, photo by Rene Ong, pending permission
W-Shaped pupil of a cuttlefish (Sepia officinalis). Photo by William Warby, creative commons license
Posterior gland in genus Sepiella1
It is greyish-brown or purple, depending on the chromatophores present , with distinct 8 to 9 reddish, iridescent patches along the base of each fin . The ventral surface is pale grey or pale brown .
Photos by Ria Tan
Photo by Ria Tan
The mantle also consists of the cuttlebone, which help with maintaining the buoyancy of the animal. The cuttlebone is broad, oval in shape (the width of the cuttlebone is 33% to 43% of the cuttlebone's length) and when viewed laterally, the cuttlebone can be seen to be largely convex. This species also lacks a spine at the end of the mantle, which distinguishes it from other groups in the Sepiidae family1 .
Sepiella inermis cuttlebone1 .
The females have been consistently found to have attained larger sizes than the males2 . This has been speculated to account for females higher feeding intensity as a result of the higher metabolic strain during the spawning season. Additionally, in the males, the left ventral arm is prominently hectocotylized. The hectocotylus is a specialized arm in the male that is used to store and transfer the spermatophores to the female during reproduction1 .
A hectocotylized arm in a male, taken from Reid, Jereb & Roper, 20051
Taken from Integrated Taxonomic Information System, but modified to include recent developments.
Van Hasselt, 1835
At the species level, we currently know that, based on the mitochondrial genome sequencing, Sepiella inermis separated from Sepia officinalis and then Sepiella japonica very recently.
"Chronogram of Cephalopoda based on complete mt genomes (protein-coding genes analysed at amino-acid level plus rRNA genes) and using a supertree. A Bayesian uncorrelated relaxed lognormal clock with four fossil-based calibration priors (A–D, above each node, see Material and Methods) was used in BEAST. Horizontal bars represent 95% highest posterior density intervals for time estimates; dates are in Ma. Abbreviation: Neo, Neogene"15 .
Furthermore, there is strong evidence that Sepiella inermis and Sepeilla japonica are indeed sister species (monophyletic) and form a bigger monophyly that includes Sepia officinalis15 . Other species that are relatively well-studied and abundant in the Sepiella genus are Sepiella japonica, Sepiella ornate and Sepiella weberi
"Phylogenetic relationships of Cephalopoda based on complete mt genomes (protein-coding genes analysed at amino-acid level plus rRNA genes). The tree is an ML phylogram using Nautiloidea as outgroup. Numbers at nodes are statistical support values for ML and BI analyses (BP and PP, respectively). Asterisks indicate species for which no complete mt genomes are available. Scale bar = 0.3 substitutions/site."15
At the genus level, Sepiella is characterized by autapomorphies, having traits that are not shared with any other taxon. For example, the posterior gland, thought to have a defense role and the lack of a spine on the cuttlebone8
Decabrachia and Sepiidae
The superorder of Decabrachia contains 29 families of squids and cuttlefish. They can be distinguished from other groups as they have no external calcified shells. The cuttlebone, an internalized form of the shell can be found in the cuttlefishes, a derived trait that distinguishes them from their squid relatives as well as other cephalopods. While the superorder is relatively well established within the phylogenetic tree in relation to the subclass Coleoidea16 , phylogenetic resolution is low in the middle to lower ranks. This could be explained by the recent study done on mitochondrial genome in Decabrachia which showed that unlike, Octopodiforms, that retain ancestral molluscan order, Decapodiformes have gone considerable gene rearrangements through tandem duplication and random loss15 . Mitocondrial gene orders from the study can be found at this link.
The tree below, a result of various molecular and morphological analyses and a combination of known data and new data, shows the further positioning of family Sepiidae. In their analysis, they used "all publicly available molecular markers (nuclear: 18S rRNA, Histone H3, octopine dehydrogenase, pax-6, rhodopsin, actin; mitochondrial: cytochrome c oxidase subunit I [COI], 12S rRNA, 16S rRNA) and all the 18 available full mitochondrial genomes of cephalopods available as of November 2015 from GenBank and the Barcode of Life Database." It has strong support from Maximum Likelihood analysis, with Sepiella and Metasepia being closely related to each other.
"Partitioned maximum likelihood (ML) analysis was performed using RAxML v.8.2.4 on the computational cluster with 1,000 bootstrap replicates under the GTR model of evolution with Gamma distribution (GTR+Gamma). Above tree is the Maximum-likelihood tree of the Decabrachia under the GTR + Gamma model with the morphological character set mapped onto the tree. Taxa highlighted in red represents discrepancy to previously published studies16 ."
Biology and Behavior
Photo by Ria Tan
Cuttlefish, in general, are slower swimmers than other more streamlined cephalopods1 and they move by undulating the fins along the edge of their bodies, with the help of the cuttlebone and the funnel (for jet propulsion). Therefore, movement in the spineless cuttlefish involves these primary organs: the cuttlebone, the fins and the funnel.
The cuttlebone is made up of many chambers9 , and the regulation of gas and fluid in these chambers allow the cuttlefish to attain neutral buoyancy1 . The quantity of the fluid is increased or decreased according to the external pressure, while the amount of gas remains mostly constant: the ability to change internal pressure relative to theh external pressure allows them to occupy a large range of depth (0 to 40m)9 . As a result, they are able to hover in midwater if needed, using their fins for balance1
A semi-circle funnel is located below the eyes on the ventral side. Water enters through the funnel into the mantle cavity. Through the rhythmic movement of muscles in the mantle, the water is expelled out of the funnel, allowing the animal to move in any direction it wants9 . This is called jet propulsion. In the cuttlefish, jet propulsion is used mostly to escape from oncoming predators or to seize prey17 .
Funnel (labelled FU) in Sepiella inermis on the ventral side9 .
Camouflage and Predation
Like other cuttlefish, Sepiella inermis has chromatophores on its skin. The dorsal surface changes colours from grey brown to purple and back due to the action of the chromatophores11
They are able to change their colours through contracting and relaxing the muscles around the chromatophore cells, allowing different pigments to be revealed and reflected18 . This ability can be used for camouflage from predators, essentially breaking up the outline of the cuttlefish's body and blending themselves into the substrate, and by allowing the predator to pick out the cuttlefish only through brightness or luminance rather than through the chromatic aspect19 .
Under the chromatophores, are also iridophores. The iridophores provide structural coloration and are capable of polarizing light. While this has not be well studied in the spineless cuttlefish, it has been speculated to serve as a communication tool with other conspecifics or may take on the role of eyespots, which can be used to scare the predatory fishes and other cephalopods away20 .
The most surprising thing is that the cuttlefish are colour blind, yet they are able to camouflage! They are able to do so and match their substrate through the leucophore cells that are present even deeper in their skin. Leucophores are structural reflectors, reflecting the same wavelength of light that is given to them. Studies show that this could be assisting the chromatophores in helping match the colours on the skin to the background substrate21 .
There are several ways that the cuttlefish avoid predators: they camouflage to the background and reflect polarized light, making it hard for the predators to pick them out from the field. Secondly, they use jet propulsion to get away from the predators.
While it is well known that most of the cephalopods store and release ink to obstruct the predator's view and escape, there is no specific research on Sepiella inermis22 .
From Biodiversity of Singapore, by CMBS photography team, permission pending
The hatchlings, also known as paralarva, hatch after an average incubation period of 12.6 days and have a survival rate of over 90 %. The hatchlings mantle length is approximately 4.3 mm23 . The hatchlings were also planktonic in nature swimming in a slanting position. They drift and are dispersed by ocean currents. From the time of hatching, they start hunting their prey, which include larval shrimps and copepods24 . They then go into the benthic phase after 5 days of age and become juveniles23 . Since growth varies according to environmental conditions, the sizes of males and females at maturity varies in different locations1 . Several studies also found that length-weight relationship of the spineless cuttlefish were highly correlated to each other, inferencing that this species follows allometric growth25 .
A successful rearing in the laboratory found that the spineless cuttlefish were sexually mature at 60 days, mating occurred at 70 days, and spawning at 87 days26 , with females attaining sexual maturity faster than males25 .
During mating, the males exhibit dark coloration, while females are paler in comparison. The males also have an hectocotylized arm for holding spermatophores. Mating begins by taste-by-touching between the male and the female, where the arms of both the cuttlefish are locked onto each other for 2 to 4 minutes. This is due to cuttlefish having sensitivity in their suckers that help them distinguish between various objects, substrates, other cuttlefish and cephalopods27 . The male also performs various displays to attract the female while swimming adjacent to her24 .
While mating, the males are dominant24
. During copulation, the males insert the hectocotylized arm into the mantle cavity of the female, and releases the spermatophores into the cavity where the fertilization will occur29 .
The number of eggs laid range between 216 to 850 for every female. The variation has been attributed to the difference in female size (bigger females lay more eggs) or increased fishing pressures leading to lesser number of eggs25 . The eggs are laid in clusters, and they look like shiny single black capsules23 , attached to the hard surface of a substrate2 . The black colour of the egg might be attributed to the cuttlefish adding a coat of ink to her eggs to camouflage them from predators1 . Each egg can be distinguished only under the microscope and is translucent in colour with a reticulate surface11 .
This species was thought to be semelparous, as both sexes were thought to die after spawning at an average of 116 days of age1 . However, more recent studies have shown that breeding is prolonged, with four spawning seasons: January, April, July-August and October in India, suggesting that females might spawn more than once25 .
The spineless cuttlefish hunts fishes, crustaceans and occasionally cephalopods1 . In Singapore specifically, they Sepiella inermis preferred Acetes sp. (prawn), both Stolephorous sp. and Poecilia reticulata (fish), xanthid crabs Sphaerocius sp and Porcellana sp., and soldier crabs Dotilla sp.17 . The kind of food they hunted showed that it depended on the availability and season of that particular animal. For example, crabs were found in their stomachs only when they were in season (May) in Singapore17 . It was also observed that feeding intensity was higher in the month just before spawning, particularly in the females as they had a higher reproductive and metabolic output than the male25 .
The spineless cuttlefish’s predatory behaviour consists of three parts: attention, positioning and seizure17 .
- Attention: Eye resting half-buried in the sand, rises and focuses on the prey, following it for a few seconds.
- Positioning: Elevates itself out of the sand bed, approaches prey slowly. A darkening of the body is observed. Positions itself in line with the prey about its own body distance away.
- Seizure: some attacked as soon as they were positioned, while others waited, moving with the prey as it moved. In healthy cuttlefish, tentacles were used with great precision, whilst in unhealthy cuttlefish the tentacles were not precise, or the arms were used in instead.
Currently, there is not information regarding whether Sepiella inermis uses cephalotoxins in predation, or whether they even have it like other cuttlefish species, such as Sepia officinalis or Sepia latimanus.
The spineless cuttlefish has a very wide distribution, rannging approximately between the Tropic of Cancer and Tropic of Capricon. It is native to eastern and western Indian ocean and the northwest and western central of the pacific. It is especially prolific on the coasts of India, and most of the research that has been done on Sepiella inermis originates from there. But it can also be found in various other places, such as the red sea, southern Arabian sea, Mozambique, Egypt, Pakistan, etc .
Distribution of Sepiella inermis around the world1 .
The full list of places can be found on the IUCN red list page of Sepiella inermis.
In Singapore, it is called the glittering cuttlefish. It is sometimes seen in the seagrass meadows, especially on the Northern shores, at places like Changi. It has been spotted by divers at Pulau Hantu and it has been found near Tuas, East Coast Park and Tanah Merah as well2 .
Human use and threats
Sepiella inermis is one of the main commercial fisheries in India and Sri Lanka. It is popular because it is easy to catch (using trawls at night) and it is easy to harvest. It is in fact so popular that a decline in annual landings and average stock was seen in India in the late 1970s and early 1980s1 . Although it has been mentioned that this decline led to proposals and rules for a sustainable fishery, it is unclear what exactly were the rules that were implemented. Given that IUCN has marked this species as data deficient and that the current data on this species is more than a decade old, it is hard to predict what the population status will be like in the near future if the fisheries are still unsustainble.
However in Singapore, there are no known efforts to harvest this species. It has been mostly spotted on surveys (both public and scientific)2 .
A study has also found that collagen can be extracted from the outer skin of Sepiella inermis. The collagen obtained can be used for various things, ranging from food, cosmetics and biomedical materials30 . However, harvesting for biogenic substances has only been suggested in some research studies. There has been no known harvesting for biogenic substances for this species specifically.
Allometric growth The regular and systematic pattern of growth such that the mass or size of any organ or part of a body can be expressed in relation to the total mass or size of the entire organism.
Aquaculture Also known as aquafarming, is the farming of fish, crustaceans, molluscs, aquatic plants, algae and other organisms.
Benthic An ecological region or zone at the very bottom of water bodies.
Biogenic substances A biogenic substance is a product made by or of life forms. The term encompasses constituents, secretions, and metabolites of plants or animals.
Cephalopod A member of the molluscan class which includes squids, octopuses, cuttlefish and natilus.
Cephalotoxin A substance, present in the salivary glands of cephalopods, that can be toxic to various organisms.
Chromatophore Pigment-containing and light-reflecting cells, or groups of cells, found in wide range animals including cephalopods, amphibians, crustaceans and reptiles.
Collagen The main structural protein found in skin and other connective tissues for various organisms.
Cuttlebone An internalized calcified shell only found in Cuttlefishes that helps them maintain neutral buoyancy.
Cuttlefish Marine invertebrates of the order Sepiida. They are part of cephalopod group which consists of squids, octopus and natilus. They have a unique internalized shell, the cuttlebone.
Distal end A point that is away from the body or the structure.
Dorsoventral Including both the top and the bottom of the species body.
Hectocotylized When the arm of a cephalopod is converted into a hectocotylus that stores spermatophores and transfers them during spawning.
Iridophore A group of chromatophores that reflect structural colors and look iridiscent.
Leucophore A group of chromatophores that reflect white (mostly) or whatever color of light illuminates it.
Planktonic Small organisms that float or drift in large number in bodies of salt or fresh waters.
Polarized light Light waves that vibrate or oscillate on a single plane.
Posterior Towards the back of the specimen, away from the head.
Reticulate Pattern resembling a net or network.
Semelparous Characterized by a single reproductive episode before death.
Species complex A group of closely related species that are very similar in appearance to the point that boundaries between them may be unclear.
Spermatophore A protein capsule containing a mass of spermatozoa, transferred during mating in various insects, arthropods, cephalopod molluscs, etc.
Structural colors Coloration that is produced as a result of the reflecting-material interfering with the light instead of pigments.
Trawl or Trawling Method of fishing that involves pulling a fishing net through the water behind one or more boats. The net that is used for trawling is called a trawl.
Type specimen The specimen on which the description and name of a new species is based.
Undulate moving with a smooth wave-like motion
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|2||Tan, R. (2016). Glittering cuttlefishes (Sepiella inermis) on the Shores of Singapore. Retrieved fromhttp://www.wildsingapore.com/wildfacts/mollusca/cephalopoda/cutglittering.htmAccessed 15 Nov 2018. [ a b c d e f g h ]|
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|24||Samuel, D. (2014). SPINELESS CUTTLEFISH – Sepiella inermis (Van Hasslet, 1835). Retrieved from https://marinelifeindia.wordpress.com/2014/08/30/spineless-cuttlefish-sepiella-inermis-van-hasslet-1835/ Accessed 15 Nov 2018 [ a b c ]|
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Last curated on 2nd December 2018