specimens : six species of decapodiform cephalopods representing 5 families 
 and 2 different lifestyles ( pelagic and benthic ) were used in this study ( table 1table 1.specimens used in this studyspecieshabitatml * ( cm)body weight ( g)todarodes pacificuspelagic22.8289.222.1231.820.6195.319.3145.8loligo bleekeripelagic18.1210.117.3184.316.5156.116.6154.2loligo edulispelagic22.5262.521.3255.420.6241.320.4236.6watasenia 
 scintillanspelagic3.62.43.32.33.32.33.12.2sepia lycidasbenthic22.1822.120.8754.520.5744.418.2671.3euprymna morseibenthic2.57.22.36.32.25.02.25.0*mantle length . ) . pelagic species included todarodes pacificus ( n=4 ) , 
 loligo bleekeri ( n=4 ) , loligo edulis ( n=4 ) and 
 watasenia scintillans ( n=4 ) , all of which have elongated bodies and swim 
 actively . benthic species included sepia lycidas ( n=4 ) 
 and euprymna morsei ( n=4 ) . s. lycidas has a flattened body 
 with a cuttlebone , and e. morsei possesses a pouched body with a round fin . each specimen was fixed in a straight body 
 position using 10% formalin and preserved in 70% ethanol solution . observation and quantification of digestive organs : observation and 
 measurement were conducted at the university museum , the university of tokyo , in july 2014 . images of the digestive organs were obtained 
 using a single - lens reflex camera . to determine organ weight , the following digestive organs were examined : stomach , cecum , digestive gland and 
 digestive duct s appendages . the contents of stomach were removed by dissection to obtain 
 the stomach weight without the contents . each digestive organ was weighed using an 
 electronic balance ( shimadzu auw220d or shimadzu ux420h , shimadzu corporation , kyoto , 
 japan ) . the percentage ratio of each organ s weight to total body weight was calculated for 
 the 6 species based on their respective body masses . statistical tests were conducted with 
 the jmp pro 9 statistical analysis software ( sas institute inc . , the holm 
 method was used to compare the weight ratios for the digestive organs from the 6 species 
 included in this study . observations : the images of the digestive organs of 6 species of 
 decapodiform cephalopods are shown in fig . 1fig . 1.ventral view of digestive organs of ( a ) todarodes pacificus , ( b ) 
 loligo bleekeri , ( c ) loligo edulis , ( d ) 
 watasenia scintillans , ( e ) sepia lycidas and ( f ) 
 euprymna morsei . e , esophagus ; s , stomach ; c , cecum ; cs , cecal sac ; 
 dg , digestive gland ; dda , digestive ducts appendages ; i , intestine . scale bar=1 
 cm .. the shape of the pathway of the digestive duct was similar among all 6 species . the 
 esophagus lay on the dorsal side of the digestive gland and opened into the stomach . ventral view of digestive organs of ( a ) todarodes pacificus , ( b ) 
 loligo bleekeri , ( c ) loligo edulis , ( d ) 
 watasenia scintillans , ( e ) sepia lycidas and ( f ) 
 euprymna morsei . e , esophagus ; s , stomach ; c , cecum ; cs , cecal sac ; 
 dg , digestive gland ; dda , digestive ducts appendages ; i , intestine . evident differences among species were observed in the shape of the digestive gland . in 
 t. pacificus , digestive duct s appendages that were 
 not well developed were observed between the cecum and digestive gland . weight ratios : the ratios of each digestive organ to body weight are 
 presented in table 2table 2.digestive organ weight ratios ( % ) speciesstomachcecumdigestive glanddigestive ducts appendagestodarodes pacificus0.42  0.04a0.82 0.06a4.98  0.33a0.34  0.06aloligo bleekeri0.39  0.05a1.14  0.18b2.25  0.26b0.26  0.04aloligo edulis0.33  0.06ab0.96  0.07a1.06  0.11c0.26  0.05awatasenia scintillans0.46  0.03a0.58  0.03c2.71  0.30d0.33  0.05asepia lycidas0.54  0.12ac0.35  0.04d6.04  0.12e0.56  0.07beuprymna morsei0.51  0.05ac0.56  0.04c7.03 different superscript letters indicate 
 significant differences ( holm method ) ( p<0.05).table 3.morphological trend models of digestive organs weight ratiosdigestive organspelagicbenthicstomachsmalllargececumlargesmalldigestive glandsmalllargedigestive ducts appendagessmalllarge and fig . different superscript letters indicate significant 
 differences ( holm method , p<0.05 ) .. considerable variations were observed in the weight ratios of the digestive organs 
 of these species . though no significant differences were found , there was a weak tendency 
 for the stomach to be larger in benthic species than in pelagic species . significant 
 differences were found between l. edulis and s. lycidas 
 and between l.edulis and e. morsei species in weight ratio 
 of the stomach . the weight ratio of the stomach was lower in l. edulis 
 ( 0.33% ) , and it was significantly smaller than that of benthic species 
 ( p<0.05 ) . in contrast , weight ratios of the stomach for s. 
 lycidas and e. morsei were more than 0.51% . for t. 
 pacificus , l. bleekeri and w. scintillans , for pelagic species , the cecum represented larger 
 percentages of body weight than in the benthic species , and there were significant 
 interspecific differences . in s. lycidas , the weight ratio of the cecum was 
 0.35% , and this value was significantly smaller than those of the other species . the weight ratio of the digestive gland 
 was significantly larger in s. lycidas and e. morsei than 
 in the other species , which had weight ratios of less than 4.98% . significant differences 
 were observed among species in weight ratio of the digestive duct s appendages . the weight 
 ratios of the digestive duct s appendages of pelagic species were significantly smaller than 
 those of benthic species . pelagic species showed weight ratios of less than 0.34% for the 
 digestive ducts appendages , while benthic species had weight ratios of more than 0.52% . a comparison of digestive organs showed interspecific variations in shape ( fig . 1 ) and weight ratios ( fig . a weak tendency for the stomach to be larger in 
 benthic species than in pelagic species was found in this study ( fig . 2 ) . mangold and young   mentioned that variations of size and shape of the stomach exist but are often 
 due to the degree of fullness . in this study , the weight ratios of  empty  stomachs showed 
 variations among species . in contrast , a previous study showed that the 
 functions of the stomach are mechanical and enzymatic breakdown of food . since the stomach is a fleshy muscular organ , mechanical breakdown of food may be conducted by 
 movement of the wall of the stomach . though significant differences were not found between 
 pelagic and benthic species , the weak differences found may be due to the following reasons . since digestion lasts longer in benthic species than in pelagic species , the structure of the stomach of decapodiform 
 cephalopods may be related to digestion time . it has been suggested that highly active 
 swimmers feed more or less continuously or at least several times a day . since pelagic species always need to swim , they may 
 be able to increase the speed of digestion by holding less food in the stomach and passing 
 quickly through absorptive organs , including the cecum . in contrast , benthic species may 
 accumulate more food in the stomach and pass digestive food to cecum at a slow speed . since 
 they take advantage of their ability to store food in their larger stomachs , they can digest 
 more food not only while swimming but also burrowing in the sand . this is likely why pelagic 
 species possessed smaller stomachs and benthic species possessed larger stomachs . the cecum was larger in the pelagic species than in the benthic species in this study 
 ( fig . a large cecal sac was also observed in 
 l. bleekeri and l. edulis ( fig . 1 ) . our observations support the results of the previous studies 
 by mangold and young   and garri and edith r 
  describing a large cecal sac in loliginidae . the cecum produces enzymes , absorbs nutrients and eliminates indigestible particles , and the 
 cecal sac stores nutrients at later stages of digestion . the main absorptive organ in the cephalopod has long been considered to be 
 the cecum , and the large cecal sac and intestine 
 are thought to be the main organs of absorption in the loliginidae [ 1 , 2 ] . the pelagic species may 
 absorb nutrients from their stomachs via their larger cecum , and their larger cecum may 
 enable them to digest food rapidly , particularly in the cecal sac in the case of l. 
 bleekeri and l. edulis . among pelagic species , the size of cecum 
 showed differences . l. edulis and l. bleekeri live along 
 the coasts , and t. pacificus lives along coasts and in offings , and 
 w. scintillans lives in the mesopelagic zone . there may be more food 
 along coasts than in offing and the mesopelagic zone . if there is an abundance of food , 
 cephalopods may develop a large cecum for rapid digestion of food . therefore , t. 
 pacificus and w. scintillans may require more time for digestion 
 than the other 2 pelagic species . futhermore , w. scintillans may require 
 more time for digestion than other pelagic species , since the mesopelagic zone may contain 
 less food than coasts . the relative sizes of the cecum of w. scintillans 
 and e. morsei were nearly the same . since benthic species can spend more 
 time on digestion than pelagic species , their cecum may not need to be as large as those in 
 the pelagic species . therefore , pelagic species had larger cecum than in benthic 
 species . in this study , the digestive gland was larger in benthic species than in pelagic species 
 ( fig . 2 ) . in particular , l. 
 bleekeri and l. edulis possessed a withered - shape digestive 
 gland ( fig . pelagic species possessed a single 
 digestive gland ; in contrast , benthic species had paired glands ( fig . the digestive gland acts as an important digestive and 
 synthetic organ [ 4 , 6 ] . it is also considered a storage organ because of its high lipid content [ 5 , 7 , 15 ] . furthermore , in sepia , the 
 digestive gland is suggested to be the main absorptive organ . regarding other functions , the digestive gland of sepia 
 officinalis is thought to be an important storage organ during short - term 
 starvation , but not long - term starvation . fluckiger et al .   showed that 
 the digestive gland may not be a long - term storage organ for dietary lipids , but instead 
 rapidly turns over and potentially excretes lipids in sepia officinalis . swift et al .   suggested that the 
 major roles of the digestive gland are secretion of digestive enzymes and excretion of 
 metabolic wastes . lipids are suggested to be excreted from the digestive gland in two 
 tropical loliginids species . in 
 loligo , it has been suggested that food does not reach the digestive 
 gland . since l. bleekeri and 
 l. edulis possessed a withered - shape digestive gland in this study , the 
 digestive glands of these 2 loliginids may not store many lipids but instead excrete lipids . it has also been suggested that pelagic species have a single smaller digestive gland to 
 decrease body weight and that excretion of lipids may occur in the digestive gland to enable 
 faster swimming . among pelagic species , since coasts may contain more food than offing and the mesopelagic zone , l. 
 edulis and l. bleekeri do not need to not store a large amount 
 of lipids in the digestive gland . t. pacificus and w. 
 scintillans may absorb nutrients and store lipids in the digestive gland in 
 environments with less food available than along coasts . this is likely why l. 
 edulis and l. bleekeri had smaller digestive glands than 
 t. pacificus and w. scintillans . in contrast , benthic species have larger paired digestive glands , since they may absorb 
 nutrients and store and excrete lipids effectively . since pelagic species need to swim and 
 drift more actively than benthic species , a lighter and smaller digestive gland may be 
 suitable for swimming and drifting . the digestive ducts appendages were larger and more developed in the benthic species than 
 in the pelagic species ( figs . 1 and 2 ) . in sepia , the digestive ducts 
 appendages have a role in absorption of nutrient . the function of the digestive ducts appendages in the other species of decapodiform 
 cephalopods has not been clarified . since the pelagic species may absorb nutrients in their 
 larger cecum for rapid digestion , it is possible that the digestive ducts appendages are 
 not developed . on the other hand , it is also possible that benthic species possess larger 
 digestive duct s appendages because they absorb nutrient via them , in addition to the cecum , 
 more slowly than pelagic species . the morphology of the digestive organs of decapodiform cephalopods reflects 
 their speed of digestion and main mode of life in different ecological habitats .
the digestive organs in decapodiform cephalopod species morphologically vary by 
 individual lifestyle . we examined the following six species of adult decapodiformes 
 cephalopods representing different habitats : todarodes pacificus , loligo 
 bleekeri , loligo edulis , watasenia 
 scintillans ( pelagic ) , sepia lycidas and euprymna 
 morsei ( benthic ) . l. bleekeri and l. edulis 
 possess a bursiform cecal sac connected to the cecum . 
 pelagic species have a single 
 digestive gland smaller than in benthic species . 
 t. pacificus has an oval 
 digestive gland larger than that of l. bleekeri and l. 
 edulis , which possess withered - looking and smaller digestive glands . in 
 contrast , 
 the digestive glands in benthic species are paired . 
 s. lycidas 
 and e. morsei have well - developed and larger digestive glands than those 
 of the pelagic species . 
 well - developed digestive duct appendages are found in benthic 
 species . in qualification of the mass of digestive organs , 
 pelagic species have smaller 
 stomachs , digestive glands and digestive ducts appendages than benthic species . 
 because 
 pelagic species need to swim , they may possess smaller stomachs and larger cecums for more 
 rapid digestion . 
 a smaller digestive gland may have the advantage of reducing the body 
 weight in pelagic species for rapid swimming . 
 in contrast , since benthic species require a 
 longer time for digestion than pelagic species , they compact more food in their stomachs 
 and absorb nutrients via more organs , such as the digestive grand and digestive duct 
 appendages , in addition to cecum .