Gross Anatomy of the Digestive Tract of the Hawaiian Monk Seal, Monachus schauinslandi

Abstract

The digestive tract of a female juvenile Hawaiian monk seal was dissected and described. Intestine lengths were measured for a total of 19 seals ranging in age from 1 day old to over 10 yr old. Small intestine (SI) lengths were measured for 10 seals and ranged from 7.1 to 16.2 m; mean SI to standard ventral length (SVL) ratio was 7.1 ± 0.9 m. Large intestine (LI) lengths were measured for 11 seals and ranged from 0.4 to 1.2 m; mean LI:SVL was 0.5 ± 0.1 m. Total intestine (TI) lengths were measured for 18 seals and ranged from 7.5 to 18.4 m; total intestine length to SL ratio was 7.9 ± 1.3 m. SI and LI lengths both exhibited a linear relationship relative to SVL, whereas stomach weight:SVL showed an exponential relationship. TI:SVL was significantly smaller than ratios determined for harbor, harp, and northern elephant seals, but was not significantly different from those of crabeater, leopard, and Ross seals. No correlation was seen between gut length and body length for seven species of seals, including the Hawaiian monk seal.

The Hawaiian monk seal, Monachus schauinslandi, has a population currently estimated at 1300 individuals with a decline currently occurring at French Frigate Shoals (FFS) (National Marine Fisheries Service [NMFS], unpubl. data), where one of the largest subpopulations exists. The decline at FFS is likely due, in part, to the starvation of juvenile seals (Gilmartin 1993). Because the assimilation of ingested prey is affected by digestive tract morphology in species that have been studied (Lawson et al. 1997), it is essential to understand all aspects of the monk seal's digestive physiology, including gross anatomy of the digestive tract.

Data on intestinal lengths have been compiled for most species of pinnipeds (King 1969), including the rare Mediterranean monk seal, Monachus monachus (Schnapp et al. 1962). In addition, the gastrointestinal tract has been described for several species of pinnipeds (e.g., Weddell seal [Leptonychotes weddelli]: Eastman and Coalson [1974]; crabeater seal [Lobodon carinophagus], harbor seal [Phoca vitulina], and northern elephant seal [Mirounga angustirostris]: Bryden and Erikson [1976], Helm [1983]; harp seal [Phoca groenlandica]: Olsen et al. [1996]); however, the digestive tract of the Hawaiian monk seal has not been described previously. The purpose of this study was to describe the gross anatomy of the seal's digestive tract and also to determine whether differences exist in intestine length between the Hawaiian monk seal and other pinnipeds previously examined.

Materials and Methods

Intestinal lengths were measured from 19 Hawaiian monk seals collected in the main Hawaiian islands and Northwestern Hawaiian Islands (NWHI) that died between 1996 and 1998. Ages of these seals were 1 d (n = 1), 5 d (n = 1), 9 d (n = 1), <1 yr (n = 9), J2 (n = 1), adult (n = 2), >10 yr (n = 1); J2 seals are approximately 2 yr old and adults are approximately >5 yr old. For seals less than 5-6 weeks old (age at weaning), the exact birth date was known. For weaned and juvenile seals, age was determined via plastic identification tags attached to the hind flippers. Age of adult seals was estimated from size, except in the case of the seal >10 yr, where age was estimated by dentition condition. Seals had died from a variety of causes, including drowning by adult males, mobbing of the mother just before birth, perinatal death, shark injury, abscess from a conspecific injury, direct mobbing injury, and possibly old age (NMFS, unpubl. data). Death directly from starvation was not apparent for any of the seals examined.

For 8 of the 19 seals, the entire digestive tract from tongue to anus was collected and fixed in 10% formalin, and lengths and weights of the tongue, esophagus, stomach, small intestines (SI), and large intestines (LI) were recorded. Formalin-fixed organs exhibited a decrease in elasticity, but not overall

Figure 1.

Tongue of Hawaiian monk seal showing bifurcation at tip.

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