Members of the Decapoda are crustaceans that have five pairs of walking legs which include the chelipeds, three pairs of maxillipeds that function as mouthparts, two pairs of maxillae, a pair of mandibles which could also be described as jaws for eating, and two pairs of antennae. This general organization varies from species to species.
Crabs are benthic decapods that have a highly reduced abdomen and often a large, wide carapace. “True” crabs are members of the Brachyura. Members of the Anomura have also evolved crab like features making them a sister group of the Brachyura. A decapod means ten-legged. Brachyura are true crabs because they have five pairs of visible pereiopods, however an Anomura have only four pairs of pereiopods that are visible and a fifth pair that have been reduced to been hiding under the carapace. Lopholithodes mandtii is a large anomuran crab that lives along the pacific northwest cost from Alaska to California and found in the subtidal areas up to 137m (Cowles 2004). I examined the morphology of one specimen collected subtidally (15 m depth) from Pt. George, Shaw Island.
The most obvious feature of this large crab is that the cuticle of its carapace and legs has large bumps that could make it look or be described as the rough surface of a rock. It seems that this feature of the crab is so that it would be well camouflaged in its rocky habitat. This crab also has a hard, triangular abdomen that is curled under the thorax; it fits into a depression in the thorax but can be pulled away. I examined the appendages of the crab from anterior to posterior. The first two pairs of appendages are antennae. The first pair is unbranched (uniramous) and the second pair has two branches (biramous). Behind the antennae is a pair of mandibles, which were hard and rounded, each with a small branch projecting anteriorly. Posterior to the mandibles are three pairs of appendages that all look similar: they were very thin and leaf-like. These are called the first and second pairs of maxillae, and the first pair of maxillipeds. Posterior to those appendages were maxillipeds 2 and 3, which both are biramous bear appendages, modified to function as mouthparts. The first pair of walking legs, which bear pinching claws, are called chelipeds. The next three pairs of walking legs have pointed dactyls which are used to grip to hard substrate. At first observation, a fifth pair of pereiopods was not noticeable; however closer observation showed that the reduced fifth pair of pereiopods is hidden under the thorax by the abdomen.
The ecological morphology of the Lopholithodes mandtii is not well studied due to the rareness of its species. However in summary, the morphology of the king crab has been adapted to so that it has the ability to survive and function with ease amongst its rocky deep water environment.
Figure 1. Hand drawn diagram of L. mandtii illustrating major morphological features of the organism.
Figure 2. Anterior view of the organism’s mouth parts.
Figure 3. Anterior view of the organism observing its asymmetry and its pereopods/dactyl used for hard substrates.
Figure 4. Ventral view
Figure 5. Dorsal view of the L. mandtii
Figure 6. Ventral view of abdomen/thorax, also showing fifth pereiopod reduced
Literature Cited
Cowles, D. Lopholithodes mandtii (Brandt, 1849). 2004. http://www.wallawalla.edu/academics/departments/biology/rosario/inverts/Arthropoda/Crustacea/Malacostraca/Eumalacostraca/Eucarida/Decapoda/Anomura/Family_Lithodidae/Lopholithodes_mandtii.html
Author
Aleisha Setka, Auburn University
Invertebrate Zoology at FHL 