A Guide to Frequently Used Characters, Character States and MeasuresBy Dr. Steve O'Shea
Last updated: 11/27/02
Frequently cited anatomical characters, structures, organ systems, measures and ratios employed in descriptions of cirrate and incirrate octopods are depicted in figures 1-6:
Figure 1 (click image to enlarge)
top: Opisthoteuthis sp.; bottom: Grimpoteuthis sp. Raw measure delineation points. Legend: AL (arm length), FLIn (fin length inner), FLOut (fin length outer), FuL (funnel length), FW (fin width), HW (head width), ML (mantle length), MW (mantle width), PA (pallial aperture width), TL (total length), WD (web depth).
Figure 2 (click image to enlarge)
top: Graneledone sp.; bottom: Octopus (s.l.) campbelli (Smith). Raw measure delineation points. Legend: A-E (web sectors A through E), AL (arm length), ASe (enlarged sucker diameter), ASn (normal sucker diameter), FuL (funnel length), HW (head width), ML (mantle length), MW (mantle width), PA (pallial aperture width), TL (total length), WD (web depth), 1-4R/L (arms 1 through 4 right or left).
Figure 3 (click image to enlarge)
Argonauta nodosa Solander, 1786 - In conventional incirrate (non-finned) octopuses the distal-most portion of the third right (or rarely third left) arm is hectocotylised. In the male of Argonauta (the Paper Nautilus) the entire third left arm is modified. This individual, fully mature, of mantle length (ML) 13.5 mm shows this grossly enlarged hectocotylised arm. A detached hectocotylus recovered from the mantle cavity of a female is depicted in illustration D.
Figure 4 (click image to enlarge)
The first two images (upper left and centre) are of a species of Pareledone, a genus of octopus that has a single row of suckers only. Many genera of octopus are characterised by a single row of suckers (eg., Pareledone, Eledone, Graneledone, Thaumeledone), although most genera and species that are readily available for purchasing as pets possess the standard two rows.
The image on the upper right is a close up view of the tip (the hectocotylus) of the third right arm of a species of Benthoctopus (a deep-water genus of octopus characterised by possessing suckers arranged in two rows, and in lacking an ink sac): CAL = calamus; LIG = ligula; SG = spermatophore groove.
Figure 5 (click image to enlarge)
The first image (top left) is the alimentary canal of a cirrate (finned) octopus (Grimpoteuthis abyssicola); it lacks an ink sac, and apparent salivary glands. The intestine of Grimpoteuthis is short relative to the length of the anterior alimentary canal (oesophagus and crop), an important character state for differentiating otherwise similar-looking cirrate genera.
The image on the upper right is the male reproductive system of Graneledone challengeri; that on the lower left is the male system of the finned Opisthoteuthis mero; the two differ noticeably.
The female reproductive system in octopuses is generally considered to be conservative. However, marked differences do occur in the relative lengths of the proximal and distal oviducts, the number of oviducts (one in cirrate octopuses, two in incirrate octopuses), in egg size and shape, and in the position that the system occupies in the visceral mass. The two images, lower centre (Tremoctopus robsonianus) and lower right (Grimpoteuthis abyssicola), illustrate some of the differences that can be expected in different types of octopus (and these two illustrations can be compared with the earlier image in Figure 4, Octopus kaharoa).
Abbreviations in illustrations are: AG, accessory gland; AGC, accessory gland complex; AN, anus; BB, buccal bulb; CR, crop; DG, digestive gland; DO, distal oviduct; DVD, distal vas deferens; EGG, egg; INT, intestine; OB, oviducal ball; OES, oesophagus; OS, ovary sac; P, penis; PD, penis diverticulum; PO, proximal oviduct; PVD, proximal vas deference; SC, spiral caecum; SG, spermatophoral gland; SS, spermatophore sac; ST, stomach; SVC, seminal vesicle complex; TS, testis sac.
Figure 6 (click image to enlarge)
One of the few true distinctions between octopus and squid (as both can have fins, internal shells, and 8 arms in the adult) is that the suckers of squid are armed with sucker rings, hooks, or both (the suckers of an octopus are simple suction cups, lacking any armature). Figure 6 depicts the beaks, hooks, sucker rings and general body morphology of some interesting squid (genera Octopoteuthis, Moroteuthis and Lepidoteuthis). Beak shape, and arm and tentacle hook and sucker ring morphology are enormously variable, and are some of the most important characters that are used to identify species.
Because squid have many additional hard parts (as in sucker rings and hooks), in addition to often possessing photophores, the majority of species descriptions detail external and hard-part morphological characters only (as opposed to octopuses, where descriptions essentially include soft-part anatomical detail).
As a detailed, annotated account is presented by Roper & Voss (1983), emphasis here is made on defining points of reference used to measure characters or ratios, and in defining new character states employed in species descriptions. Definitions of indices in bold type are modified from definitions provided by Roper & Voss (l.c.).
ACRONYM DEFINITIONS AND DERIVATIVES
One possible way of defining the size of an octopus, to standardise peoples reference when dealing with size, would be in accordance with the following:
1 Differs in whole arm sucker counts instead of half arm sucker counts.
2 Differs in not employing mantle width.
3 Differs by dividing fin length measures by mantle length as opposed to head width; head width in species of Opisthoteuthis is somewhat indefinable owing to the anterior-posterior compression of the cephalopedal mass.
4 Differs by excluding the terminal lamellae from inner and outer demibranch lamella counts.
5 Differs in head length being indefinable in many instances in octopods.
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